ETC RS-422

RS-422 and RS-485
Application Note
B&B Electronics Mfg. Co. Inc.
P.O. Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Internet Addresses:
B&B Home Page:
http://www.bb-elec.com
Customer Service:
[email protected]
Technical Support:
[email protected]
 B&B Electronics - Revised October 1997
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Cover Page
Table of Contents
CHAPTER 1: OVERVIEW ........................................................................... 1
INTRODUCTION ............................................................................................. 1
DATA TRANSMISSION SIGNALS ...................................................................... 1
Unbalanced Line Drivers ......................................................................... 1
Balanced Line Drivers ............................................................................. 1
Balanced Line Receivers.......................................................................... 3
EIA STANDARD RS-422 DATA TRANSMISSION .............................................. 3
EIA STANDARD RS-485 DATA TRANSMISSION .............................................. 6
TRISTATE CONTROL OF AN RS-485 DEVICE USING RTS.................................. 9
SEND DATA CONTROL OF AN RS-485 DEVICE .............................................. 11
CHAPTER 2: SYSTEM CONFIGURATION ............................................ 13
NETWORK TOPOLOGIES............................................................................... 13
TWO WIRE OR FOUR WIRE SYSTEMS ............................................................ 13
TERMINATION............................................................................................. 16
BIASING AN RS-485 NETWORK.................................................................... 17
EXTENDING THE SPECIFICATION .................................................................. 19
CHAPTER 3: SELECTING RS-422 AND RS-485 CABLING .................. 20
NUMBER OF CONDUCTORS .......................................................................... 20
SHIELDING ................................................................................................. 20
CABLE CHARACTERISTICS ........................................................................... 20
CHAPTER 4: TRANSIENT PROTECTION OF RS-422 AND RS-485
SYSTEMS.................................................................................................... 23
WHAT DOES A SURGE LOOK LIKE? ................................................................ 23
Surge Specifications............................................................................... 23
Common Mode vs. Differential Mode..................................................... 25
GROUND ≠ GROUND.................................................................................... 26
TRANSIENT PROTECTION USING ISOLATION .................................................. 27
Isolation Theory..................................................................................... 27
Isolation Devices ................................................................................... 28
TRANSIENT PROTECTION USING SHUNTING .................................................. 28
Shunting Theory..................................................................................... 28
Connecting Signal Grounds ................................................................... 29
Shunting Devices ................................................................................... 29
COMBINING ISOLATION AND SHUNTING ....................................................... 29
SPECIAL CONSIDERATION FOR FAULT CONDITIONS ....................................... 31
CHOOSING THE RIGHT PROTECTION FOR YOUR SYSTEM ................................. 31
RS-422/485 Application Note
Table of Contents
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
i
CHAPTER 5: SOFTWARE ....................................................................... 33
INTRODUCTION ........................................................................................... 33
RS-422 SYSTEMS ....................................................................................... 33
RS-485 DRIVER CONTROL .......................................................................... 33
RS-485 RECEIVER CONTROL ....................................................................... 34
MASTER-SLAVE SYSTEMS ........................................................................... 34
Four Wire Master-Slave Systems............................................................ 34
Two Wire Master-Slave Systems............................................................. 34
MULTI-MASTER RS-485 SYSTEMS .............................................................. 35
SYSTEMS WITH PORT POWERED CONVERTERS .............................................. 35
CHAPTER 6: SELECTING RS-485 DEVICES ........................................ 36
CHAPTER 7: SOURCES OF FURTHER INFORMATION .................... 37
APPENDIX A: EIA SPECIFICATION SUMMARY ................................ 38
APPENDIX B: EIA STANDARD RS-423 DATA TRANSMISSION........ 40
ii
Table of Contents
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Chapter 1: Overview
Introduction
The purpose of this application note is to describe the main elements of
an RS-422 and RS-485 system. This application note attempts to cover
enough technical details so that the system designer will have considered all
the important aspects in his data system design. Since both RS-422 and RS485 are data transmission systems that use balanced differential signals, it is
appropriate to discuss both systems in the same application note. Throughout
this application note the generic terms of RS-422 and RS-485 will be used to
represent the EIA/TIA-422 and EIA/TIA-485 Standards.
Data Transmission Signals
Unbalanced Line Drivers
Each signal that transmits in an RS-232 unbalanced data transmission
system appears on the interface connector as a voltage with reference to a
signal ground. For example, the transmitted data (TD) from a DTE device
appears on pin 2 with respect to pin 7 (signal ground) on a DB-25 connector.
This voltage will be negative if the line is idle and alternate between that
negative level and a positive level when data is sent with a magnitude of ±5 to
±15 volts. The RS-232 receiver typically operates within the voltage range of
+3 to +12 and -3 to
-12 volts as shown in Figure 1.1.
Balanced Line Drivers
In a balanced differential system the voltage produced by the driver
appears across a pair of signal lines that transmit only one signal. Figure 1.2
shows a schematic symbol for a balanced line driver and the voltages that
exist. A balanced line driver will produce a voltage from 2 to 6 volts across
its A and B output terminals and will have a signal ground (C) connection.
Although proper connection to the signal ground is important, it isn't used by
a balanced line receiver in determining the logic state of the data line. A
balanced line driver can also have an input signal called an “Enable” signal.
The purpose of this signal is to connect the driver to its output terminals, A
and B. If the “Enable” signal is OFF, one can consider the driver as
disconnected from the transmission line. An RS-485 driver must have the
“Enable” control signal. An RS-422 driver may have this signal, but it is not
always required. The disconnected or "disabled" condition of the line driver
usually is referred to as the “tristate1” condition of the driver.
1
The term “tristate” comes from the fact that there is a third output state of an
RS-485 driver, in addition to the output states of “1” and “0.”
RS-422/485 Application Note
1
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Figure 1.1
Figure 1.2
2
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Balanced Line Receivers
A balanced differential line receiver senses the voltage state of the
transmission line across two signal input lines, A and B. It will also have a
signal ground (C) that is necessary in making the proper interface
connection. Figure 1.3 is a schematic symbol for a balanced differential line
receiver. Figure 1.3 also shows the voltages that are important to the
balanced line receiver. If the differential input voltage Vab is greater than
+200 mV the receiver will have a specific logic state on its output terminal.
If the input voltage is reversed to less than -200 mV the receiver will create
the opposite logic state on its output terminal. The input voltages that a
balanced line receiver must sense are shown in Figure 1.3. The 200 mV to 6
V range is required to allow for attenuation on the transmission line.
EIA Standard RS-422 Data Transmission
The EIA Standard RS-422-A entitled “Electrical Characteristics of
Balanced Voltage Digital Interface Circuits” defines the characteristics of
RS-422 interface circuits. Figure 1.4 is a typical RS-422 four-wire interface.
Notice that five conductors are used. Each generator or driver can drive up
to ten (10) receivers. The two signaling states of the line are defined as
follows:
a. When the “A” terminal of the driver is negative with respect to the “B”
terminal, the line is in a binary 1 (MARK or OFF) state.
b. When the “A” terminal of the driver is positive with respect to the “B”
terminal, the line is in a binary 0 (SPACE or ON) state.
Figure 1.5 shows the condition of the voltage of the balanced line for an
RS-232 to RS-422 converter when the line is in the “idle” condition or OFF
state. It also shows the relationship of the “A” and “B” terminals of an RS422 system and the “-“ and “+” terminal markings used on many types of
equipment. The “A” terminal is equivalent to the “-“ designation, and the
“B” terminal equivalent to the “+” designation. The same relationship
shown in Figure 1.5 also applies for RS-485 systems. RS-422 can withstand
a common mode voltage (Vcm) of ±7 volts. Common mode voltage is
defined as the mean voltage of the A and B terminals with respect to signal
ground.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
3
Figure 1.3
Figure 1.4
4
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Figure 1.5
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
5
EIA Standard RS-485 Data Transmission
The RS-485 Standard permits a balanced transmission line to be shared
in a party line or multidrop mode. As many as 32 driver/receiver pairs can
share a multidrop network. Many characteristics of the drivers and receivers
are the same as RS-422. The range of the common mode voltage Vcm that
the driver and receiver can tolerate is expanded to +12 to -7 volts. Since the
driver can be disconnected or tristated from the line, it must withstand this
common mode voltage range while in the tristate condition. Some RS-422
drivers, even with tristate capability, will not withstand the full Vcm voltage
range of +12 to -7 volts.
Figure 1.6 shows a typical two-wire multidrop network. Note that the
transmission line is terminated on both ends of the line but not at drop points
in the middle of the line. Termination should only be used with high data
rates and long wiring runs. A detailed discussion of termination can be
found in Chapter 2 of this application note. The signal ground line is also
recommended in an RS-485 system to keep the common mode voltage that
the receiver must accept within the -7 to +12 volt range. Further discussion
of grounding can be found in Chapter 3 of this application note.
6
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Figure 1.6
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
7
Figure 1.7
8
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
An RS-485 network can also be connected in a four-wire mode as shown
in Figure 1.7. Note that four data wires and an additional signal ground wire
are used in a “four-wire” connection. In a four-wire network it is necessary
that one node be a master node and all others be slaves. The network is
connected so that the master node communicates to all slave nodes. All slave
nodes communicate only with the master node. This network has some
advantages with equipment with mixed protocol communications. Since the
slave nodes never listen to another slave response to the master, a slave node
cannot reply incorrectly to another slave node.
Tristate Control of an RS-485 Device using RTS
As discussed previously, an RS-485 system must have a driver that can
be disconnected from the transmission line when a particular node is not
transmitting. In an RS-232 to RS-485 converter or an RS-485 serial card,
this may be implemented using the RTS control signal from an asynchronous
serial port to enable the RS-485 driver. The RTS line is connected to the RS485 driver enable such that setting the RTS line to a high (logic 1) state
enables the RS-485 driver. Setting the RTS line low (logic 0) puts the driver
into the tristate condition. This in effect disconnects the driver from the bus,
allowing other nodes to transmit over the same wire pair. Figure 1.8 shows a
timing diagram for a typical RS-232 to RS-485 converter. The waveforms
show what happens if the VRTS waveform is narrower than the data VSD.
This is not the normal situation, but is shown here to illustrate the loss of a
portion of the data waveform. When RTS control is used, it is important to
be certain that RTS is set high before data is sent. Also, the RTS line must
then be set low after the last data bit is sent. This timing is done by the
software used to control the serial port and not by the converter.
When an RS-485 network is connected in a two-wire multidrop party
line mode, the receiver at each node will be connected to the line (see Figure
1.6). The receiver can often be configured to receive an echo of its own data
transmission. This is desirable in some systems, and troublesome in others.
Be sure to check the data sheet for your converter to determine how the
receiver “enable” function is connected.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
9
Figure 1.8
10
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Send Data Control of an RS-485 Device
Many of B&B Electronics’ RS-232 to RS-485 converters and RS-485
serial cards include special circuitry, which is triggered from the data signal
to enable the RS-485 driver. Figure 1.9 is a timing diagram of the important
signals used to control a converter of this type. It is important to note that
the transmit data line is “disabled” at a fixed interval after the last bit,
typically one character length. If this interval is too short, you can miss parts
of each character being sent. If this time is too long, your system may try to
turn the data line around from transmit to receive before the node (with the
Send Data converter) is ready to receive data. If the latter is the case, you
will miss portions (or complete characters) at the beginning of a response.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
11
Figure 1.9
12
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Chapter 2: System Configuration
Network Topologies
Network configuration isn’t defined in the RS-422 or RS-485 specification.
In most cases the designer can use a configuration that best fits the physical
requirements of the system.
Two Wire or Four Wire Systems
RS-422 systems require a dedicated pair of wires for each signal, a
transmit pair, a receive pair and an additional pair for each
handshake/control signal used (if required). The tristate capabilities of RS485 allow a single pair of wires to share transmit and receive signals for halfduplex communications. This “two wire” configuration (note that an
additional ground conductor should be used) reduces cabling cost. RS-485
devices may be internally or externally configured for two wire systems.
Internally configured RS-485 devices simply provide A and B connections
(sometimes labeled “-“ and “+”).
Devices configured for four wire communications bring out A and B
connections for both the transmit and the receive pairs. The user can connect
the transmit lines to the receive lines to create a two wire configuration. The
latter type device provides the system designer with the most configuration
flexibility. Note that the signal ground line should also be connected in the
system. This connection is necessary to keep the Vcm common mode voltage
at the receiver within a safe range. The interface circuit may operate without
the signal ground connection, but may sacrifice reliability and noise
immunity. Figures 2.1 and 2.2 illustrate connections of two and four wire
systems.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
13
Figure 2.1 Typical RS-485 Four Wire Multidrop Configuration
14
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Figure 2.2 Typical RS-485 Two Wire Multidrop Network
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
15
Termination
Termination is used to match impedance of a node to the impedance of the
transmission line being used. When impedance are mismatched, the
transmitted signal is not completely absorbed by the load and a portion is
reflected back into the transmission line. If the source, transmission line and
load impedance are equal these reflections are eliminated. There are
disadvantages of termination as well. Termination increases load on the
drivers, increases installation complexity, changes biasing requirements and
makes system modification more difficult.
The decision whether or not to use termination should be based on the
cable length and data rate used by the system. A good rule of thumb is if the
propagation delay of the data line is much less than one bit width, termination
is not needed. This rule makes the assumption that reflections will damp out in
several trips up and down the data line. Since the receiving UART will sample
the data in the middle of the bit, it is important that the signal level be solid at
that point. For example, in a system with 2000 feet of data line the propagation
delay can be calculated by multiplying the cable length by the propagation
velocity of the cable. This value, typically 66 to 75% of the speed of light (c), is
specified by the cable manufacture.
For our example, a round trip covers 4000 feet of cable. Using a
propagation velocity of 0.66 × c, one round trip is completed in approximately
6.2 µs. If we assume the reflections will damp out in three “round trips” up
and down the cable length, the signal will stabilize 18.6 µs after the leading
edge of a bit. At 9600 baud one bit is 104 µs wide. Since the reflections are
damped out much before the center of the bit, termination is not required.
There are several methods of terminating data lines. The method
recommended by B&B is parallel termination. A resistor is added in parallel
with the receiver’s “A” and “B” lines in order to match the data line
characteristic impedance specified by the cable manufacture (120 Ω is a
common value). This value describes the intrinsic impedance of the
transmission line and is not a function of the line length. A terminating
resistor of less than 90 Ω should not be used. Termination resistors should be
placed only at the extreme ends of the data line, and no more than two
terminations should be placed in any system that does not use repeaters. This
type of termination clearly adds heavy DC loading to a system and may
overload port powered RS-232 to RS-485 converters. Another type of
termination, AC coupled termination, adds a small capacitor in series with the
16
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
termination resistor to eliminate the DC loading effect. Although this method
eliminates DC loading, capacitor selection is highly dependent on the system
properties. System designers interested in AC termination are encouraged to
read National Semiconductors Application Note 9032 for further information.
Figure 2.3 illustrates both parallel and AC termination on an RS-485 two-wire
node. In four-wire systems, the termination is placed across the receiver of the
node.
Figure 2.3 Parallel and AC Termination
Biasing an RS-485 Network
When an RS-485 network is in an idle state, all nodes are in listen
(receive) mode. Under this condition there are no active drivers on the
network, all drivers are tristated. Without anything driving the network, the
state of the line is unknown. If the voltage level at the receiver’s A and B
inputs is less than ±200 mV the logic level at the output of the receivers will
be the value of the last bit received. In order to maintain the proper idle
voltage state, bias resistors must be applied to force the data lines to the idle
condition. Bias resistors are nothing more than a pullup resistor on the data
B line (typically to 5 volts) and a pulldown (to ground) on the data A line.
Figure 2.4 illustrates the placement of bias resistors on a transceiver in a twowire configuration. Note that in an RS-485 four-wire configuration, the bias
resistors should be placed on the receiver lines. The value of the bias
resistors is dependent on termination and number of nodes in the system.
The goal is to generate enough DC bias current in the network to maintain a
minimum of 200 mV between the B and A data line. Consider the following
two examples of bias resistor calculation.
2
Refer to Chapter 7 for information on National Semiconductors Application
Notes.
RS-422/485 Application Note
17
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Bias Resistor
Bias Resistor
Figure 2.4 Transceiver with Bias Resistors
Example 1. 10 node, RS-485 network with two 120 Ω termination
resistors
Each RS-485 node has a load impedance of 12KΩ. 10 nodes in parallel
give a load of 1200 Ω. Additionally, the two 120 Ω termination resistors
result in another 60 Ω load, for a total load of 57 Ω. Clearly the termination
resistors are responsible for a majority of the loading. In order to maintain at
least 200mV between the B and A line, we need a bias current of 3.5 mA to
flow through the load. To create this bias from a 5V supply a total series
resistance of 1428 Ω or less is required. Subtract the 57 Ω that is already
part of the load, and we are left with 1371 Ω. Placing half of this value as a
pullup to 5V and half as a pulldown to ground gives a maximum bias resistor
value of 685Ω for each of the two biasing resistors.
18
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Example 2. 32 node, RS-485 network without termination
Each RS-485 node has a load impedance of 12KΩ. 32 nodes in parallel
gives a total load of 375 Ω. In order to maintain at least 200 mV across
375Ω we need a current of 0.53 mA. To generate this current from a 5V
supply requires a total resistance of 9375Ω maximum. Since 375 Ω of this
total is in the receiver load, our bias resistors must add to 9KΩ or less.
Notice that very little bias current is required in systems without termination.
Bias resistors can be placed anywhere in the network or can be split
among multiple nodes. The parallel combination of all bias resistors in a
system must be equal to or less than the calculated biasing requirements.
B&B Electronics uses 4.7KΩ bias resistors in all RS-485 products. This
value is adequate for most systems without termination. The system designer
should always calculate the biasing requirements of the network. Symptoms
of under biasing range from decreased noise immunity to complete data
failure. Over biasing has less effect on a system, the primary result is
increased load on the drivers. Systems using port powered RS-232 to RS-485
converters can be sensitive to over biasing.
Extending the Specification
Some systems require longer distances or higher numbers of nodes than
supported by RS-422 or RS-485. Repeaters are commonly used to overcome
these barriers. An RS-485 repeater such as B&B Electronics’ 485OP can be
placed in a system to divide the load into multiple segments. Each “refreshed”
signal is capable of driving another 4000 feet of cable and an additional 31 RS485 loads.
Another method of increasing the number of RS-485 nodes is to use
low load type RS-485 receivers. These receivers use a higher input impedance
to reduce the load on the RS-485 drivers to increase the total number of nodes.
There are currently half and quarter load integrated circuit receivers available,
extending the total allowable number of nodes to 64 and 128.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
19
Chapter 3: Selecting RS-422 and RS-485 Cabling
Cable selection for RS-422 and RS-485 systems is often neglected.
Attention to a few details in the selection process can prevent the costly
prospect of re-pulling thousands of feet of cable.
Number of Conductors
The signal ground conductor is often overlooked when ordering cable. An
extra twisted pair must be specified to have enough conductors to run a signal
ground. A two wire system then requires two twisted pair, and a four wire
system requires three twisted pair.
Shielding
It is often hard to quantify if shielded cable is required in an application or
not. Since the added cost of shielded cable is usually minimal it is worth
installing the first time.
Cable Characteristics
When choosing a transmission line for RS-422 or RS-485, it is necessary
to examine the required distance of the cable and the data rate of the system.
The Appendix to EIA RS-422-A Standard presents an empirical curve that
relates Cable Length to Data Rate for 24 AWG twisted-pair telephone cable
that has a shunt capacitance of 16 pF/ft. and is terminated in 100 ohms (see
Figure 3.1). This curve is based on signal quality requirements of:
a). Signal rise and fall time equal to, or less than, one-half unit interval at
the applicable modulation rate.
b). The maximum voltage loss between driver and load of 6 dB.
20
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Figure 3.1
Losses in a transmission line are a combination of AC losses (skin
effect), DC conductor loss, leakage, and AC losses in the dielectric. In high
quality cable, the conductor losses and the dielectric losses are on the same
order of magnitude. Figure 3.2 is included in this application note to point
out the significant difference in performance of different cables. This chart
shows Attenuation versus Frequency for three different Belden cables. Note
that the polyethylene cables offer much lower attenuation than PVC cables.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
21
Figure 3.2
Another approach to choosing transmission line is the “E-GRADE
Program,” which has been established by Anixter Bros. Inc. Anixter is a
worldwide distributor of wiring system products. Under this program,
Anixter divides data interface cables into four categories as follows:
E-GRADE 1
E-GRADE 2
E-GRADE 3
E-GRADE 4
LIMITED DISTANCE
STANDARD DISTANCE
EXTENDED DISTANCE
MAXIMUM DISTANCE
Simple charts are used to help the user select the proper cable without
any technical understanding of the cable parameters. This program divides
the usage categories into EIA-232-D, EIA-422-A, and EIA-423-A. When
using this literature, use the EIA-422-A charts for choosing RS-485 cable.
22
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Chapter 4: Transient Protection of RS-422 and RS-485
Systems
The first step towards protecting an RS-422 or RS-485 system from
transients is understanding the nature of the energy we are guarding against.
Transient energy may come from several sources, most typically environmental
conditions or induced by switching heavy inductive loads.
What does a surge look like?
Surge Specifications
While transients may not always conform to industry specifications, both
the Institute of Electrical and Electronics Engineers (IEEE) and the
International Electrotechnical Commission (IEC) have developed transient
models for use in evaluating electrical and electronic equipment for immunity
to surges. These models can offer some insight into the types of energy that
must be controlled to prevent system damage.
Both IEC 1000-4-5: 1995 “Surge Immunity Test” and IEEE C62.41-1991
“IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power
Circuits” define a “1.2/50µs - 8/20µs combination wave” surge which has a 1.2
µs voltage rise time with a 50 µs decay across an open circuit. The specified
current waveform has an 8 µs rise time with a 20 µs decay into a short circuit.
Open circuit voltages levels from 1 to 6 kV are commonly used in both the
positive and negative polarities, although under some circumstances voltages as
high as 20 kV may be applied. Figures 4.1 and 4.2 illustrate the combination
wave characteristics. In addition, IEEE C62.41 also specifies a 100 kHz “ring
wave” test. The ring wave has a 0.5 µs rise time and a decaying oscillation at
100 kHz with source impedance of 12Ω as shown in Figure 4.3. Typical
amplitudes for the 100 kHz ring wave also range from 1 – 6 kV.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
23
1.2/50 uSecond Voltage Wave
1
0.9
0.8
0.7
V(t) / Vp
0.6
0.5
0.4
0.3
0.2
0.1
0
0
20
40
60
80
100
Time, us
Figure 4.1 Combination Wave Voltage Waveform
8/20 uSecond Current Wave
1
0.8
V(t)/Vp
0.6
0.4
0.2
0
0
10
20
30
40
50
Time, us
Figure 4.2 Combination Wave Current Waveform
24
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
100kHz Ring Wave
1
0.8
0.6
V(t)/Vp
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
0
5
10
15
Time, us
20
25
30
Figure 4.3 100 kHz Ring Wave
Common Mode vs. Differential Mode
Identifying the type of surges that may threaten a system is an important
part of selecting the appropriate levels and methods of transient protection.
Since each of the conductors in a data cable travels through the same physical
space, it is reasonable to expect transients caused by environmental or current
switching to be “common mode” that is, present on all data and ground
conductors within the data cable. In some installations, there may be another
source of unwanted energy to consider. If there are high voltage cables running
anywhere near the data cables, the potential for a fault condition exists as a
result of insulation failures or inadvertent contact by an installer. This type of
surge could contact any number of conductors in the data cable, presenting a
“differential” surge to the data equipment. Although the voltages and currents
associated with this type surge are much lower than the types of surges modeled
by ANSI or IEC, they have a particularly destructive quality of their own.
Instead of dissipating within several milliseconds, they can exist in a steady
state condition on the data network.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
25
Ground Differences
Realizing that transient energy can be high frequency in nature leads to
some disturbing observations. At frequencies of this magnitude, it is difficult to
make a low impedance electrical connection between two points due to the
inductance of the path between them. Whether that path is several feet of
cable or thousands of feet of earth between grounding systems, during a
transient event there can be hundreds or thousands of volts potential between
different “grounds”. We can no longer assume that two points connected by a
wire will be at the same voltage potential. To the system designer this means
that although RS-422/485 uses 5V differential signaling, a remote node may
see the 5V signal superimposed on a transient of hundreds or thousands of volts
with respect to that nodes local ground. It is more intuitive to refer to what is
commonly called “signal ground” as a “signal reference”.
How do we connect system nodes knowing that these large potential
differences between grounds may exist? The first step towards successful
protection is to assure that each device in the system is referenced to only one
ground, eliminating the path through the device for surge currents searching
for a return. There are two approaches to creating this idyllic ground state.
The first approach is to isolate the data ground from the host device ground,
this is typically done with transformers or optical isolators as shown is Figure
4.4. The second approach is to tie each of the grounds on a device together
(typically power ground and data ground) with a low impedance connection as
shown in Figure 4.5. These two techniques lead us to the two basic methods of
transient protection.
Device
Vcc
Isolated Power
Port
Data Lines Out
Optical
Isolation
Figure 4.4 Isolated RS-485 Device
26
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Vcc
Device
Port
Data Lines
Ground line
Local Chassis Ground Connection
Figure 4.5 RS-485 Device with Signal Ground
Connected to Chassis Ground
Transient protection using Isolation
Isolation Theory
The most universal approach to protecting against transients is to
galvanically isolate the data port from the host device circuitry. This method
separates the signal reference from any fixed ground. Optical isolators,
transformers and fiber optics are all methods commonly used in many types of
data networks to isolate I/O circuitry from its host device. In RS-422 and RS485 applications, optical isolators are most common. An optical isolator is an
integrated circuit that converts the electrical signal to light and back,
eliminating electrical continuity. With an isolated port, the entire isolated
circuitry floats to the level of the transient without disrupting data
communications. As long as the floating level of the circuitry does not exceed
the breakdown rating of the isolators (typically 1000 - 2500 volts) the port will
not be damaged. This type of protection does not attempt to absorb or shunt
excess energy so it is not sensitive to the length of the transient. Even
continuous potential differences will not harm isolated devices. It is important
to note that isolators work on common mode transients, they cannot protect
against large voltage differences between conductors of a data cable such as
those caused by short circuits between data and power circuits.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
27
Isolation Devices
Optical isolation can be implemented in a number of ways. If a conversion
from RS-232 to RS-422 or RS-485 is being made, optically isolated converters
are available. Optically isolated ISA bus serial cards can replace existing ports
in PC systems. For systems with existing RS-422 or RS-485 ports, an optically
isolated repeater can be installed. Examples of each of these type devices can
be found in the B&B Electronics Data Communications catalog.
Transient Protection using Shunting
Shunting Theory
Creating one common ground at the host device provides a safe place to
divert surge energy as well as a voltage reference to attach surge suppression
devices to. Shunting harmful currents to ground before they reach the data port
is the job of components such as TVS (often referred to by the trade name
Tranzorb), MOV or gas discharge tubes. These devices all work by “clamping”
at a set voltage, once the clamp voltage has been exceeded, the devices provide
a low impedance connection between terminals.
Since this type of device diverts a large amount of energy, it cannot tolerate
very long duration or continuous transients. Shunting devices are most often
installed from each data line to the local earth ground, and should be selected to
begin conducting current at a voltage as close as possible above the systems
normal communications levels. For RS-422 and RS-485 systems, the voltage
rating selected is typically 6 - 8 volts. These devices typically add some
capacitive load to the data lines. This should be considered when designing a
system and can be compensated for by derating the total line length to
compensate for the added load. Several hundred feet is usually an adequate
figure.
To apply these type products correctly they should be installed as close to
the port to be protected as possible, and the user must provide an extremely low
impedance connection to the local earth ground of the unit being protected.
This ground connection is crucial to proper operation of the shunting device.
The ground connection should be made with heavy gauge wire and kept as
short as possible. If the cable must be longer than one meter, copper strap or
braided cable intended for grounding purposes must be used for the protection
device to be effective. In addition to the high frequency nature of transients,
there can be an enormous amount of current present. Several thousand amps
typically result from applications of the combination wave test in the ANSI and
IEC specification.
28
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Connecting Signal Grounds
Since a local ground connection is required at each node implementing
shunt type protection, the consequences of connecting remote grounds together
must be considered. During transient events a high voltage potential may exist
between the remote grounds. Only the impedance in the wire connecting the
grounds limits the current that results from this voltage potential. The RS-422
and RS-485 specification both recommend using 100 ohm resistors in series
with the signal ground path in order to limit ground currents. Figure 4.6
illustrates the ground connection recommended in the specification.
Figure 4.6 Signal Ground Connection between two nodes
with 100 ohm resistor
Shunting Devices
There are two types of shunting devices to choose from. The least
expensive type is single stage, which usually consists of a single TVS device on
each line. Three stage devices are also available. The first stage of a three
stage device is a gas discharge tube, which can handle extremely high currents,
but has a high threshold voltage and is too slow to protect solid state circuits.
The second stage is a small series impedance which limits current and creates a
voltage drop between the first and third stage. The final stage is a TVS device
which is fast enough to protect solid state devices and brings the clamping
voltage down to a safe level for data circuits.
Combining Isolation and Shunting
Installing a combination of both types of protection can offer the highest
reliability in a system. Figures 4.7 and 4.8 illustrate two means of
implementing this level of protection.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
29
Device
Vcc
Isolated Power
Shunting Device
Port
Data Lines Out
Ground line
Earth Ground
Figure 4.7 Isolated node with shunt protection to earth ground
Device
Shunting Device
Vcc
Isolated Power
Port
Data Lines
Signal Ground
Figure 4.8 Isolated port with ungrounded shunt protection
The method shown in Figure 4.7 is recommended, in this case isolation
protects the circuit from any voltage drops in the earth ground connection. The
shunt devices will prevent a surge from exceeding the breakdown voltage of the
isolators as well as handling any differential surges on the cable. Figure 4.8
illustrates a method recommended for cases where there is no way to make an
earth ground connection. Here, the shunt device’s function is to protect the
port from differential surges, a differential surge will be balanced between
conductors by the shunting device, converted to common mode. The isolation
provides protection from the common mode transient remaining.
30
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Special Consideration for Fault Conditions
Data systems that could be exposed to short circuits to power conductors
require an extra measure of protection. In these cases its recommended to add
a fuse type device in addition to shunting type suppression, as shown in Figure
4.9. When a short circuit occurs, the shunt suppression will begin conducting,
but shunting by itself cannot withstand the steady state currents of this type of
surge. A small enough fuse value should be chosen so that the fuse will open
before the shunt device is damaged. A typical fuse value is 125 mA.
Device
Vcc
Data Lines
125 mA Fuse
Signal Ground
Earth Ground
Figure 4.9 Fused port protection
Choosing the right protection for your system
While it is hard to predict what type and level of isolation is correct for a
system, an educated guess should be made based on the electrical environment,
physical conditions and cost of failures in downtime and repair costs. Systems
connected between two power sources, such as building to building, office to
factory floor, or any system covering long distances should require some level
of transient protection. Table 4.1 is a comparison of transient protection
techniques.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
31
Table 4.1 Comparison of Protection Techniques
Optical Isolation
Shunting
Requires no ground reference
Adds no loading to data lines
Higher complexity
Effective on common mode
transients
Not dependent on installation
quality
Requires an external power source
Not affected by long term or
continuous transients
32
Must have low impedance ground path
Presents additional capacitive loading to
data lines
Lower complexity, uses passive components
Effective on both common and differential
mode transients
Can be improperly installed by user
No power required
Subject to damage by long duration
transients
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Chapter 5: Software
Introduction
RS-422 and RS-485 are hardware specifications. Software protocol is not
discussed in either specification. It is up to the system designer to define a
protocol suitable for their system. This chapter we will not attempt to define a
protocol standard, but will explain some of the issues that should be considered
by the system designer, whether writing or purchasing software.
RS-422 Systems
RS-422 system software differs little from the familiar point-to-point RS232 communication systems. RS-422 is often used to simply extend the
distance between nodes over the capabilities of RS-232. RS-422 can also be
used as the master node in a four wire master-slave network described later in
this chapter. When selecting or writing software for RS-422 systems the
designer should be aware of the signals being used by the hardware in the
system. Many RS-422 systems do not implement the hardware handshake lines
often found in RS-232 systems due to the cost of running additional conductors
over long distances.
RS-485 Driver Control
The principle difference between RS-422 and RS-485 is that the RS-485
driver can be put into a high impedance, tristate mode, which allows other
drivers to transmit over the same pair of wires. There are two methods of
tristating an RS-485 driver. The first method is to use a control line, often the
RTS handshake line, to enable and disable the driver. This requires that the
host software raise the RTS line before beginning a transmission to enable the
driver, then lower the RTS line after the completion of the transmission. Since
only a single RS-485 driver can be enabled on a network at one time it is
important that the driver is disabled as quickly as possible after transmission to
avoid two drivers trying to control the lines simultaneously, a condition called
line contention. Under some operating systems it can be difficult to lower RTS
in a timely manner and this method of driver control should be avoided
altogether.
The second method of RS-485 driver control we refer to as Automatic Send
Data Control. This type of control involves special circuitry that senses when
data is being transmitted and automatically enables the driver as well as
disabling the driver within one character length of the end of transmission.
This is the preferred method of driver control since it reduces software
overhead and the number of potential pitfalls for the programmer.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
33
RS-485 Receiver Control
The RS-485 receiver also has an enable signal. Since RS-485 systems
using a two wire configuration connect the driver to receiver in a loopback
fashion, this feature is often used to disable the receiver during transmission to
prevent the echo of local data. Another approach is to leave the RS-485
receiver enabled and monitor the loopback data for errors which would indicate
that line contention has occurred. Although a good loopback signal does not
guaranty data integrity it does offer a degree of error detection.
Master-Slave Systems
A master-slave type system has one node that issues commands to each of
the “slave” nodes and processes responses. Slave nodes will not typically
transmit data without a request from the master node, and do not communicate
with each other. Each slave must have a unique address so that it can be
addressed independent of other nodes. These type systems can be configured as
two wire or four wire. Four wire systems often use an RS-422 master (the
driver is always enabled) and RS-485 slaves to reduce system complexity.
Four Wire Master-Slave Systems
This configuration reduces software complexity at the host since the driver
and receiver are always enabled, at the expense of installing two extra
conductors in the system. The Master node simply prefixes commands with the
appropriate address of the slave. There are no data echo or turn around delays
to consider. Since each of the slave transmitters share the same pair of wires,
care must be taken that the master never requests data from multiple nodes
simultaneously or data collisions will result.
Two Wire Master-Slave Systems
Two wire configurations add a small amount of complexity to the system.
The RS-485 driver must be tristated when not in use to allow other nodes to use
the shared pair of wires. The time delay between the end of a transmission and
the tristate condition becomes a very important parameter in this type system.
If a slave attempts to reply before the master has tristated the line, a collision
will occur and data will be lost. The system designer must know the response
time or turn around delay of each of the slave nodes and assure that the master
will tristate its driver within that amount of time. B&B Electronics’ Automatic
Send Data control circuits tristate the driver within one character length of the
end of a transmission.
34
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Multi-Master RS-485 Systems
Each node in a multi-master type RS-485 system can initiate its own
transmission creating the potential for data collisions. This type system
requires the designer to implement a more sophisticated method of error
detection, including methods such as line contention detection,
acknowledgement of transmissions and a system for resending corrupted data.
Systems with Port Powered Converters
RS-232 to RS-422 or RS-485 converters that derive their power from the
RS-232 port are becoming more common in data systems. A good
programming practice is to set unused handshake outputs to a high voltage
state in systems using any type of RS-232 to RS-422 or RS-485 converter. This
will assure the best possible operating conditions for all converters used.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
35
Chapter 6: Selecting RS-485 Devices
When purchasing devices for an RS-485 system many pitfalls can be
avoided by determining the device’s communications characteristics before the
system design is complete. Knowing what questions to ask up front can save a
lot of troubleshooting in the field. The following device characteristics are all
things that should be answered in the system design stage.
1.
2.
3.
4.
5.
6.
7.
8.
Is the device configured for two wire or four wire systems?
Is a signal ground connection available?
Is the device isolated? Does it contain surge suppression?
What value bias resistors (if any) are used in the device? Are they
accessible for modification?
Is the device terminated? Is it accessible for modification?
What is the device’s response time (turn around delay)?
What is the programmable address range of the device?
What baud rate, or range of baud rates, is supported?
If possible it is often useful to have a schematic of the serial port of each
device in a system. The schematic can provide additional information that may
be useful in troubleshooting or repairing any problems in the data system.
36
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Chapter 7: Sources of Further Information
EIA Standards and Publications can be purchased from:
GLOBAL ENGINEERING DOCUMENTS
7730 Carondelet Avenue -- Clayton, MO 63105
Phone: (800) 854-7179 -- FAX: (314) 726-6418
GLOBAL ENGINEERING DOCUMENTS
15 Inverness Way East -- Englewood, CO 80112
Phone: (800) 854-7179 -- FAX: (303) 397-2740
Global Engineering Documents web site can be found at http://global.ihs.com.
Related data interface standards are:
a) EIA-232-E
Interface between data terminal equipment and date circuitterminating equipment employing serial binary data
interchange (ANSI/IEA-232-D)
b) EIA-422-A
Electrical characteristics of balanced voltage digital interface
circuits
c) EIA-423-A
Electrical characteristics of unbalanced voltage digital
interface circuits
d) EIA-485
Standard for electrical characteristics of generators and
receivers for use in balanced digital multipoint systems
e) EIA-449
General purpose 37-position and 9-position interface for data
terminal equipment and data circuit-terminating equipment.
f) EIA-530
High speed 25-position interface for data terminal equipment
and data circuit-terminating equipment
g) EIA/TIA-562 Electrical characteristics for an unbalanced digital interface
Manufactures of integrated circuit data transceivers often offer practical
application information for RS-422 and RS-485 systems.
National Semiconductor’s Interface Data Book includes a number of excellent
applications notes. These notes are also available online at
http://www.national.com/. A search engine is provided to search the text of the
available application notes. Entering “422” or “485” as search criteria to get a
current list of related application notes.
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
37
Appendix A: EIA Specification Summary
EIA RS-422 Specification Summary
Parameter
Driver Output Voltage
Open Circuit
Driver Output Voltage
Loaded
Driver Output Resistance
Driver Output
Short-Circuit Current
Driver Output Rise Time
Driver Common Mode
Voltage
Receiver Sensitivity
Receiver Common-Mode
Voltage Range
Receiver Input Resistance
Differential Receiver
Voltage
Conditions
RT = 100 Ω
Min
Max
10
-10
2
-2
A to B
Per output to
common
RT = 100 Ω
100
±150
10
RT = 100 Ω
±3
Vcm ≤ ±7
-7
Units
V
V
V
V
Ω
mA
% of Bit
Width
V
±200
+7
mV
V
±10
±12
Ω
V
V
4000
Operational:
Withstand:
EIA RS-485 Specification Summary
Parameter
Driver Output Voltage
Open Circuit
Driver Output Voltage
Loaded
Driver Output ShortCircuit Current
Driver Output Rise Time
Driver Common Mode
Voltage
Receiver Sensitivity
Receiver Common-Mode
Voltage Range
Receiver Input Resistance
38
Conditions
RLOAD = 54Ω
Per output to
+12V or –7V
RLOAD = 54Ω
CLOAD = 50 pF
RLOAD = 54Ω
Min
1.5
-1.5
1.5
-1.5
Max
6
-6
5
-5
±250
Units
V
V
V
V
mA
30
% of Bit Width
-1
3
V
-7
±200
+12
-7 ≤ Vcm ≤ +12
mV
V
Ω
12K
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
EIA RS-232 Specification Summary
Parameter
Driver Output Voltage Open
Circuit
Driver Output Voltage Loaded
Conditions
3 KΩ ≤ RL ≤ 7
KΩ
-2V ≤ Vo ≤ 2V
Driver Output Resistance,
Power Off
Driver Output Short-Circuit
Current
Driver Output Slew Rate
Maximum Load Capacitance
Receiver Input Resistance
Receiver Input Threshold
Output = Mark
Output = Space
3V ≤ VIN ≤ 25V
Min
Max
25
5
3000
15
Units
V
300
V
V
Ω
500
mA
30
2500
7000
V/µs
pF
Ω
-3
3
V
V
EIA RS-423 Specification Summary
Parameter
Driver Output Voltage
Open Circuit
Driver Output Voltage
Loaded
Driver Output Resistance
Driver Output ShortCircuit Current
Driver Output Rise and
Fall Time
Receiver Sensitivity
Receiver Input Resistance
Conditions
RL = 450 Ω
Min
4
-4
3.6
-2V ≤ Vo ≤ 2V
Baud Rate ≤ 1K Baud
Baud Rate ≥ 1K Baud
Vcm ≤ ±7V
Max
6
-6
6
Units
V
V
V
50
±150
Ω
mA
300
30
µs
% Unit
Interval
mV
Ω
±200
4000
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
39
Appendix B: EIA Standard RS-423 Data Transmission
RS-423 (EIA-423) is another standard used in point to point
communications. RS-423 data transmission uses an unbalanced line driver
that connects to an RS-422 type balanced line receiver as shown in Figure
B.1. The RS-423 line driver is unique to this system. It produces voltage
similar to RS-232 but has a slew rate control input that is used to limit rise
times and cross talk on the data lines. Typical adjustment on the slew rate
control is from 1 to 100 µs. This is done by the proper selection of one
resistor on the wave shape control input.
Figure B.1
40
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
We hope you find this Application Note useful. Your comments on it would be welcomed.
For a Free 52-page Color
Catalog of
Data Communications Equipment
including
Interface Converters
Data Acquisition Sensors and Modules
Repeaters and Boosters
Custom Data Communication Products
Dataline Test Equipment
Software for Data Analysis & Control
Hardwire, Fiber Optic and Radio Modems
Cabling and Connectors
Isolators and Surge Protectors
Smart Data Switches
Fax this form to: (815) 434-7094
or Mail to: B&B Electronics 707 Dayton Road P.O. Box 1040 Ottawa IL 61350
Your Name:
Your Company:
Your Title:
Address:
City:
State/Province:
Zip/Postal Code:
Country/add’l address information:
E-mail:
Phone with Area Code:
Ext:
Fax:
We are a:
¨
¨
¨
Electronic Designer/Manufacturer
Systems Integrator
Production Engineering Department
¨
¨
¨
Software Developer
MIS Department
Student
¨
¨
¨
Installation Company
Value Added Reseller/Dealer
Other:
We are interested in:
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
RS-232 to RS-422 Interface Converters
Data Acquisition Sensors
RS-232 to RS-485 Interface Converters
Data Logger Modules
RS-232 to Current Loop Interface Converters
Sensor Signal Conditioners and Amplifiers
RS-232 to Fiber Optic Interface Converters
Data Line Test Equipment and Software
Data Line Boosters and Repeaters
Wireless Communication Equipment
¨
¨
¨
¨
¨
¨
¨
¨
¨
¨
Smart Switches
Custom Cables and Adapters
Isolators and Surge Protectors
Fiber Optic Cables and Accessories
Modems
Telephone Signal Equipment
Keyboard and Mouse Extenders and Adapters
Ethernet Equipment
Other:
Here is a specific problem we have. Does B&B have a solution?
Multiple Port Cards
Communications Control Software
Multiplexers
Serial Communications Books
AppNotes 6/98