BB ISO485

®
ISO485
ISO
485
Isolated RS-485
DIFFERENTIAL BUS TRANSCEIVER
FEATURES
DESCRIPTION
● RS-485 AND RS-422 COMPATIBLE
● 100% TESTED FOR HIGH-VOLTAGE
BREAKDOWN
The ISO485 differential, isolated bus transceiver uses
Burr-Brown’s capacitively coupled isolation technology to provide high-speed, low cost bus isolation.
The ISO485 is designed for bi-directional data communication on multipoint bus transmission lines and
meets EIA Standard RS-485 as well as EIA Standard
RS-422A requirements.
●
●
●
●
RATED 1500Vrms
SINGLE-WIDE 24-PIN PLASTIC DIP
EASY TO USE
LOW POWER: 180mW typ at 5Mbit/s
APPLICATIONS
● MULTIPOINT DATA TRANSMISSION
ON LONG BUS LINES IN NOISY
ENVIRONMENTS
The ISO485 uses high voltage 0.4pF capacitors instead of the LED and photodetector which are used in
equivalent optocoupler solutions. As a consequence
the part count of the isolated RS-485 channel is
reduced from multiple optocoupler channels, an RS485 transceiver chip and supporting circuitry to one
ISO485. The capacitors in the ISO485 provide a high
voltage barrier, 1500Vrms and greatly reduce current
spikes on the power line.
The ISO485 combines a 3-state differential line driver
and a differential-input line receiver both of which
operate from a single 5V power supply. The driver
differential outputs and the receiver differential input/
output bus ports are designed to offer minimum loading to the bus whenever the driver is disabled or
VS = 0V.
3
2
1
24
VSA
15
VSB
RE
A
R
B
12
13
D
TRUTH TABLE
RS-485
DE RE
BUS
23
DE
11
14
22
GND
B
GND
A
0
0
RX
0
1
HIGH Z
1
0
HIGH Z
1
1
TX
International Airport Industrial Park • Mailing Address: PO Box 11400
Tel: (520) 746-1111 • Twx: 910-952-1111 • Cable: BBRCORP •
• Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
©1994 Burr-Brown Corporation
PDS-1280C
Printed in U.S.A. May, 1995
SPECIFICATIONS
At TA = +25°C, VS = 5V, unless otherwise specified.
ISO485P
PARAMETER
CONDITION
DRIVER DC CHARACTERISTICS
Input Voltage
High MIN
Low MAX
Input Current
High-Level
Low-Level
Output Voltage
Differential Output Voltage
Change In Magnitude of Differential
Output Voltage
Common-Mode Output Voltage
Change in Magnitude of Common-Mode
Output Voltage
Output Current
Short-Circuit Output Current (1 sec max)
DRIVER SWITCHING CHARACTERISTICS
Propagation Delay Time,
Low-to-High Level Output
Propagation Delay Time,
High-to-Low Level Output
Input to Output Propagation Delay Skew
Output Rise Time
Output Fall Time
RECEIVER DC CHARACTERISTICS
Differential-Input-Threshold Voltage
High
Low
Hysteresis
High-Level Output Voltage
Low-Level Output Voltage
High-Impedance-State Output Current
Line Input Current
Enable-Input Current
High
Low
Input Resistance
Short-Circuit Output Current
RECEIVER SWITCHING CHARACTERISTICS
Propagation Delay Time,
Low-to-High Level Output
High-to-Low Level Output
Input to Output Propagation Delay Skew
Output Rise Time
Output Fall Time
TRANSCEIVER SPECIFICATIONS
Maximum Data Rate
Propagation Delay Driver to Receiver
Driver Output Enable Time
Driver Output Disable Time
Propagation Delay Receiver to Driver
Receiver Output Enable Time
Receiver Output Disable Time
MIN
MAX
UNITS
2
V
V
±1
±1
5
µA
µA
V
0.8
VIN = 2.4V
VIN = 0.4V
IOUT = 0
0
IOA – IOB = 0
1.5
RLOAD = 100Ω
RLOAD = 54Ω
2
1.5
2.5
2.5
5
V
5
5
V
V
RLOAD = 54Ω or 100Ω
±0.5
V
RLOAD = 54Ω or 100Ω
3
V
RLOAD = 54Ω or 100Ω
±0.2
V
VOUT = 7V, output disabled
VOUT = –7V, output disabled
1
–0.8
mA
mA
VOUT = –7V
VOUT = 0V
VOUT = VS
VOUT = 12V
–250
–150
250
250
RLOAD = 54Ω
RLOAD =
RLOAD =
RLOAD =
RLOAD =
54Ω
54Ω
54Ω
54Ω
mA
mA
mA
mA
60
ns
60
ns
ns
ns
ns
0.2
V
V
mV
V
V
µA
mA
mA
10
10
10
VOUT = 2.7V, IOUT = –0.4mA
VOUT = 0.5V, IOUT = 8mA
–0.2
70
VID = 200mV, IOH = 400µA
VID = 200mV, IOL = 8mA
VOUT = 1.4V
VIN = 12V, other output = 0V
VIN = –7V, other output = 0V
2.4
0.4
±1
0.7
–0.6
VIH = 2.7V
VIL = 0.4V
1
1
µA
µA
kΩ
mA
60
60
ns
ns
ns
ns
ns
12
1 sec max
40
VID = –1.5V to 1.5V, CL = 15pF
VID = –1.5V to 1.5V, CL = 15pF
35
30
10
8
8
RL = 54Ω
RL = 54Ω
20
RL = 110Ω
RL = 110Ω
CL = 15pF
CL = 15pF
®
ISO485
TYP
2
35
75
155
185
13
110
120
200
280
180
185
Mbits/s
ns
ns
ns
ns
ns
ns
SPECIFICATIONS (CONT)
At TA = +25°C, VS = 5V, unless otherwise specified.
ISO485P
PARAMETER
TRANSCEIVER SPECIFICATIONS (CONT)
Supply Voltage
VSA
VSB
Supply Current
VSA
VSA
VSA
VSA
VSB
VSB
VSB
VSB
RECOMMENDED OPERATING
CONDITIONS
Voltage at Any Bus Terminal
High-Level Driver Input Voltage
Low-Level Driver Input Voltage
Differential Receiver Input Voltage
Output Current High-Level
CONDITION
MIN
TYP
MAX
UNITS
3
4.75
5
5
5.5
5.25
V
V
5
0.4
0.4
0.4
55
55
51
51
mA
mA
mA
mA
mA
mA
mA
mA
12
0.8
±12
V
V
V
V
Driver
Receiver
–60
–400
mA
µA
Driver
Receiver
60
8
mA
mA
85
125
°C
°C
DE RE
0
0
0
1
1
0
1
1
0
0
0
1
1
0
1
1
RS-485 BUS
Rx
HIGH Z
HIGH Z
Tx
Rx
HIGH Z
HIGH Z
Tx
(separately or common-mode)
–7
2
Output Current Low-Level
TEMPERATURE RANGE
Operating
Storage
ISOLATION PARAMETERS
Rated Voltage, Continuous
Partial Discharge, 100% Test(1)
Creepage Distance (External) DIP = “P” Package
Internal Isolation Distance
Isolation Voltage Transient Immunity(2)
Barrier Impedance
Leakage Current
–40
–40
50Hz
1s, 5pC
1500
2400
Vrms
Vrms
mm
mm
kV/µs
Ω || pF
µArms
16
0.10
1.6
> 1014 || 7
0.6
240Vrms, 60Hz
NOTES: (1) All devices receive a 1s test. Failure criterion is ≥ 5 pulses of ≥ 5pC. (2) The voltage rate-of-change across the isolation barrier that can be sustained
without data errors.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
®
3
ISO485
ABSOLUTE MAXIMUM RATINGS
PACKAGE INFORMATION
Supply Voltages, VS .............................................................................. 5.5V
Voltage at any bus terminal ......................................................... –10 to 15V
Enable Input Voltage ............................................................ 0 to VCC + 0.5V
Continuous total dissipation at 25°C free-air temp. .......................... 750mW
Lead solder temperature, 260°C for 10s,
1.6mm below seating plane ............................................................... 300°C
Junction Temperature .......................................................................... 150°C
Package thermal transfer, θJA ........................................................... 75°C/W
MODEL
ISO485P
24 D
RE 2
23 DE
VSA 3
22 GNDA
24-Pin Single-Wide DIP
243-1
PIN ASSIGNMENTS
DIP
R 1
PACKAGE DRAWING
NUMBER(1)
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix D of Burr-Brown IC Data Book.
PIN CONFIGURATION
Top View
PACKAGE
PIN #
NAME
DESCRIPTION
1
2
3
10
11
12
13
14
15
22
23
24
R
RE
VSA
NC
GNDB
A
B
GNDB
VSB
GNDA
DE
D
Data Received From Transmission Line
Receive Switch Controlling Receiving Of Data
+5V Supply Pin For Side A
This Pin MUST Be Left Unconnected
Ground Pin For Side B. Also Connected To Pin 14
Data, Driver Out/Receiver In
Data, Driver Out/Receiver In
Ground Pin For Side B. Also Connected To Pin 11
+5V Supply Pin For Side B
Ground Pin For Side A
Driver Switch Controlling Output Of Data
Data To Be Transmitted
ELECTROSTATIC
DISCHARGE SENSITIVITY
NC(1) 10
GNDB 11
A 12
This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and
installation procedures can cause damage.
15 VSB
14 GNDB
13
B
ESD damage can range from subtle performance degradation
to complete device failure. Precision integrated circuits may
be more susceptible to damage because very small parametric
changes could cause the device not to meet its published
specifications.
NOTE: (1) Pin 10 must be left unconnected.
®
ISO485
4
TYPICAL PERFORMANCE CURVES
At TA = +25°C, VS = +5V, unless otherwise noted.
ISOLATION LEAKAGE CURRENT vs FREQUENCY
TYPICAL INSULATION RESISTANCE vs TEMPERATURE
10m
1015
Isolation Resistance (Ω)
1016
Leakage Current (Arms)
100m
1m
VISO = 1500Vrms
100µ
10µ
VISO = 240Vrms
1014
1013
1012
1011
1µ
1010
100n
1
10
100
1k
10k
100k
0
1M
20
40
60
ISOLATION VOLTAGE vs FREQUENCY
2.1k
100
120
140
160
180
PROPAGATION DELAY vs TEMPERATURE
60
Max DC
Rating
Propagation Delay, tPD (ns)
Peak Isolation Voltage (V)
10k
80
Temperature (°C)
Frequency (Hz)
Degraded
Performance
1k
100
10
CL = 50pF
50
40
30
VS = 5.0V
20
10
0
1
1k
10k
100k
1M
10M
100M
–60
–40 –20
0
20
Frequency (Hz)
40
60
80
100 120
140
Temperature (°C)
TYPICAL POWER DISSIPATION vs DATA RATE
NORMALIZED RISE/FALL TIME vs TEMPERATURE
500
1.6
1.5
400
CL = 50pF
Total Power (mW)
Relative tr, tf
1.4
+1σ
1.3
1.2
Normalized to Average
of Many Devices
at 25°C
1.1
Transmit
300
200
NOTE:
Baud Rate = 2 • Frequency
Receive
100
1.0
–1σ
0
100k
0.9
–60 –40
–20
0
20
40
60
80
100
120 140
1M
10M
100M
Data Rate (Mbit/s)
Temperature (°C)
®
5
ISO485
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, VS = +5V, unless otherwise noted.
OUTPUT HIGH VOLTAGE vs
DRIVER OUTPUT CURRENT
OUTPUT LOW VOLTAGE vs
DRIVER OUTPUT CURRENT
5
VOH – High-Level Output Voltage – V
VOL – Low-Level Output Voltage – V
5
4.5
VS = 5V
TA = 25°C
4
3.5
3
2.5
2
1.5
1
0.5
VS = 5V
TA = 25°C
4
3.5
3
2.5
2
1.5
1
0.5
0
0
0
20
40
60
80
100
0
120
OUTPUT VOLTAGE vs
DRIVER OUTPUT CURRENT
4
VS = 5V
TA = 25°C
3.5
3
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
60
70
80
90
100
IO – Output Current – mA
®
ISO485
20
40
60
80
100
IOH – High-Level Output Current – mA
IOL – Low-Level Output Current – mA
VOD – Differential Output Voltage – V
4.5
6
120
MODE OF OPERATION
LOADING
RS-485 recommends a maximum of 32 unit loads on any
one line: the unit loading being derived from the 12kΩ input
impedance and the 12V maximum common-mode voltage.
The ISO485 represents 1 unit load. We could, therefore,
connect up to 31 outstations to the master controller and
comply with the specification.
The ISO485 is a differential, isolated transceiver for half
duplex multi-point communication, and complies with the
EIA Interface Standards summarized in Table I. The signals
transmitted across the isolation barrier can achieve transmission rates up to 35Mbit/s typical. The barrier is designed to
perform in harsh electrical environments without signal
degradation, while providing high isolation and good transient immunity.
TERMINATION
When a signal starts to change at the output of a transmitter,
the other end of the line will eventually see this change and
a reflection will occur. If this reflection returns to the
transmitter before the transmitted signal has reached its
maximum value, the line may be considered as a “lumped
parameter” model. In this case no termination is necessary
because the line has a negligible effect on the system.
Referring to the block diagram on the front page, data
present at the D input can be transmitted across the barrier
when the data enable pin DE is a logic high. The data
appears as a differential signal on the outputs A and B and
within the output range 0V to +5V. The isolated side of the
DE logic high also inhibits the isolated side of data read R.
The input NOR gate arrangement prevents attempts to transmit and receive simultaneously. The truth table shows the
conditions on the RS-485 bus for the possible states of DE
and RE.
If the rise of the signal at the receiver TRISE is much less than
the time taken for the signal to go from transmitter to
receiver and back again 2TPD termination of the line is
necessary. It is usual to terminate the line with its characteristic impedance, ZO when the following rule applies:
ISOLATION BARRIER
Data is transmitted by coupling complementary logic pulses
to the receiver through two 0.4pF capacitors. These capacitors are built into the ISO485 package with Faraday shielding to guard against false triggering by external electrostatic
fields.
The integrity of the isolation barrier of the ISO485 is
verified by partial discharge testing. 2400Vrms, 50Hz, is
applied across the barrier for one second while measuring
any tiny discharge currents that may flow through the
barrier. These current pulses are produced by localized
ionization within the barrier. This is the most sensitive and
reliable indicator of barrier integrity and longevity, and does
not damage the barrier. A device fails the test if five or more
current pulses of 5pC or greater are detected.
2TPD ≥ 5TRISE
(1)
For this installation we have selected an Alpha Wire Corporation cable, No. 6072C. The cables characteristics are
shown in Figure 2. The rise time TRISE at the receiver was
measured between the 10% and 90% points.
TRISE = 10ns
(2)
From Figure 1 we can see that the velocity of propagation VP
is given as 80%. Since this is the ratio of the signal speed in
air, to the signal speed in the cable, we have
VP
Conventional isolation barrier testing applies test voltage far
in excess of the rated voltage to catastrophically break down
a marginal device. A device that passes the test may be
weakened, and lead to premature failure.
APPLICATION EXAMPLE
Consider an RS-485 network in an industrial area. The
system specifications are:
= 3 x 108 x 0.8
= 2.4 x 108 m/s
therefore
TPD = 1/VP
= 4.2ns/m
For the cable
2TPD = 4.2 x 10–9 x 50 x 2
= 42us
(3)
Equation 1 holds, therefore the line must be terminated with
its characteristic impedance.
• Distance between master controller and the farthest outstation 50 meters.
• System data rate is to be 30Mbit/s.
EYE PATTERNS AND ZO
Eye patterns can be used to assess the signal distortion and
noise on the transmission line. It is also a convenient method
of determining the characteristic impedance of the line. The
term ‘eye’ comes from the shape of the trace on the oscilloscope. See Figures 2 and 3.
• One daisy-chain cable will link the master controller to the
outstations.
The main design considerations in implementing this system
are:
• Line loading and termination
The eye pattern was obtained using the non return zero
pseudo-noise generator circuit shown in Figure 5. Figure 2
shows the effects of the termination resistor for the three
cases: ZT > ZO, ZT = ZO, ZT < ZO with ZT = ZO the eye
• Selection of correct cable for requirements
• Attenuation and distortion of the signal
• Fault protection and fail-safe operation
®
7
ISO485
1 we can see that the specified attenuation figures given
agree with those obtained by measurement; approximately
–1.3db/100ft, at 30Mbit/s (15MHz).
pattern is clear. In practice a precision decade resistance box
was used to determine the exact value of ZT to use.
As the data rate is increased we can see from Figure 3 how
the signal distortion also increases. From the graph in Figure
TYPICAL ATTENUATION VALUES
AT 20°C
10.0
6073 - 6079/27
Attenuation (dB/100ft)
5.0
6072
1.0
0.5
1.0
5.0
10.0
50.0
Frequency (MHz)
TYPICAL ELECTRICAL CHARACTERISTICS
Capacitance
ALPHA WIRE CORP.
NO.
CDR. to CDR.
pF/ft
CDR. to (CDR. AND SHIELD)
pF/ft
pF/m)
(pF/m)
VP.
%
Z0 at 1MHz,
Ω
6072C
8.7
(28, 5)
5.9
(52, 2)
80
150
6073C thru 6079/27C
12.5
(41, 0)
22.0
(72, 5)
80
150
FIGURE 1. Cable Characteristics.
PARAMETER
Mode of Operation
Number of Drivers and Receivers
EIA-232
RS-432-A
RS-422-A
RS-485
Single-Ended
Single-Ended
Differential
Differential
1 Driver
1 Driver
1 Driver
32 Drivers
1 Receiver
10 Receivers
10 Receivers
32 Receivers
Maximum Cable Length (m)
15
1200
1200
1200
Maximum Data Rate (bps)
20k
100k
10M
10M
Maximum Common-Mode Voltage (V)
±25
±6
6 to –0.25
12 to –7
±1.5
Driver Output
Loaded
±5
±3.6
±2
Levels (V)
Unloaded
±15
±6
±5
±5
3k to 7k
450 (min)
100 (min)
60 (min)
30V/µs (max)
External Control
NA
NA
500 to VCC
150 to GND
150 to GND
150 to GND
NA
NA
NA
12k
300
60k
60k
12k
Driver Load (Ω)
Driver Slew Rate
Driver Output Short Circuit
Current Limit (mA)
250 to –7 or 12V
Driver Output
Resistance
Power on
High Z state (Ω)
Power off
Receiver Input Resistance (Ω)
3 to 7
4
4
12
±3V
±200mV
±200mV
±200mV
Receiver Sensitivity
TABLE I. Summary of EIA Interface Standards.
®
ISO485
8
STUB LENGTH
If the outstations are not to act as transmission lines, they too
must meet the criteria determined by equation 1. They must
be seen as a lumped parameter. As a rule-of-thumb, the
transition time of the pulse from the transmitter, TRISE
should be ten times longer than the propagation delay,
pdSTUB down the stub to the outstation.
Eye Patterns
Z T > ZO
Z T < ZO
TRISE ≥ 10pdSTUB
Z T = ZO
(4)
pd = 1/VP x stub length
From
TRISE ≥ 10 x 1/VP x stub length
Figure 2. Eye Patterns.
16.5 x 10–9 ≥ 10 x
1
x stub length
3 x 108 x 0.8
2Mbit/s
Therefore
START-UP CIRCUIT
Because the ISO485 is a capacitively coupled device, it is
possible to power up an indeterminate state. The circuit of
Figure 4 ensures that the ISO485 powers up in the receive
mode, thus avoiding any conflict on the transmission line.
Signal at
Generator
0% Jitter
Signal at
Receiving
End
stub length = 396mm (15.6") maximum
+5V
4.7kΩ
10Mbit/s
4.7kΩ
DE
DE
Signal at
Generator
330nF
POWER
DE
Signal at
Receiving
End
5% Jitter
10ms
Figure 4. Start-up Circuit.
TRANSMIT/RECEIVE MODE
Because the ISO485 is a capacitively coupled device, indeterminate states can occur when the change from transmit to
receive or, from receive to transmit is initiated. This is easily
overcome by transmitting an edge prior to the data of
interest. The four possible conditions which could happen
are detailed in Figures 5a, 5b, 6a, and 6b. Thereafter, data is
known and correct.
20Mbit/s
Signal at
Generator
Signal at
Receiving
End
50% Jitter
Figure 3. ISO485 Signal Distortion vs Data Rate.
®
9
ISO485
A
A
B
B
R
High
Z
?
R
Don’t Care
D
High
Z
?
Don’t Care
D
RE
RE
DE
DE
Figure 5a. Transmit to Receive.
Figure 5b. Transmit to Receive.
A
High
Z
?
A
High
Z
?
B
High
Z
?
B
High
Z
?
High Z
R
R
High Z
D
D
RE
RE
DE
DE
Figure 6a. Receive to Transmit
Figure 6b. Receive to Transmit.
®
ISO485
10
U3C
8
U1
1
2
8
9
A
B
QA
QB
QC
QD
QE
QF
QG
QH
CLK
CLR
3
4
5
6
10
11
12
13
9
10
74ACT 86
74ACT 164
4
U2
1
2
8
CLK I/P
9
A
B
QA
QB
QC
QD
QE
QF
QG
QH
CLK
CLR
3
4
5
6
10
11
12
13
U3B
5
5
D PR Q
3
CLK
74ACT 86
1
6
CL Q
1 74ACT 74
U3A
12
3
2
74ACT 164
11
O/P
UA4
4
2
6
74ACT 86
U4B
10
D PR Q
9
CLK
8
CL Q
13
74ACT 74
5V
+VCC
FIGURE 7. NRZ Psuedo-Noise Generator.
+12
+12
R1
R3
D1
D2
A
A
PTC
PTC
RT
RT
B
PTC
Z1
B
PTC
Z2
Z6
Z5
R2
R4
Z3
Z4
Z8
Z7
a) R1 = R3 and R2 = R4
For open circuit conditions this biases the line to a logic '1'
b) Z1 = Z3 = Z5 =Z7 = BZX85 = 12V
Z2 = Z4 = Z6 =Z8 = BZX85 = 6.8V
D1 =D2 = IN4048
The zener diode in conjunction with the ptc thermistors limit the current
drawn on the line when taken beyond the +12V and –7V. The ptc
thermistors also current limit on the line being shorted to the ground.

V 
c) R1 = R 2 = 0.5 × Z 0  1+ CC 
V TH 


V 
R T = Z 0  1+ TH 
V CC 

NOTE: PTC = Positive Temperature Coefficient Thermistor.
FIGURE 8. RS-485 Line with Fail-Safe Protection.
®
11
ISO485