AN_143 “Auto Sensing and Isolation Design for

Future Technology Devices International Ltd.
Application Note
AN_143
Auto Sensing And Isolation Design For
Rs232/Rs422/Rs485 Interfaces
Version 1.1
Issue Date: 2015-08-17
This application note explains how to design auto mode sensing for RS232, RS485 and RS422
interfaces. It also describes how to implement isolation of these interfaces. The examples of
auto mode sensing require two different connectors, one for RS232 and another for RS422/485.
Only one protocol interface can operate at any one time.
Use of FTDI devices in life support and/or safety applications is entirely at the user’s risk, and the user
agrees to defend, indemnify and hold FTDI harmless from any and all damages, claims, suits or expense
resulting from such use.
Future Technology Devices International Limited (FTDI)
Unit 1, 2 Seaward Place, Glasgow G41 1HH, United Kingdom
Tel.: +44 (0) 141 429 2777 Fax: + 44 (0) 141 429 2758
E-Mail (Support): support1@ftdichip.com Web: http://www.ftdichip.com
Copyright © 2015 Future Technology Devices International Limited
Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
Table of Contents
1
Introduction .................................................................... 2
2
Implementing RS232/RS422/RS485 Auto-Sensing ........ 3
RS232/RS422/RS485 Auto-Sensing reference design ....................... 3
RS422/RS485 Auto-Sensing reference design ................................... 7
Termination Resistors ..................................................................... 10
3
Implementing RS232/RS422/RS485 Isolation ............. 11
Isolated RS232 System ................................................................... 11
Isolated RS422 System ................................................................... 13
Isolated RS485 System ................................................................... 15
4
Contact Information ...................................................... 18
Appendix A – List of Figures and Tables ............................. 19
List of Figures ................................................................................. 19
Appendix B – Revision History ........................................... 20
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
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Introduction
This application note explains how to combine RS232, RS485 and RS422 functions into one product and
how to auto sense which interface is being used. Any application should have two different connectors,
only one of which can be used at any one time: one for RS232 and one for RS422/485. It also explains
how to implement isolated RS232/422/485 interfaces in system.
The example schematics are given are for illustration purposes only.
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AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
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Implementing RS232/RS422/RS485 Auto-Sensing
The following example illustrates one possible solution to auto-sense the RS232/422/485 interface. Any
application requires two different connectors: one for RS232 and another for RS422/485. Only one
connector can be in use at any one time.
RS232/RS422/RS485 Auto-Sensing reference design
When designing the RS232, RS485 and RS422 interfaces in one circuit, care must be taken with the
receiver source since it can be from any one of the three interface converter’s output pins. These cannot
be simply shorted together.
In this case, it is necessary that these three interface outputs are maintained at a high state when they
are idle, and use an AND gate to sense the interface sources. This is illustrated in the following diagram.
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
Figure 2-1 RS232/RS422/RS485 Auto Sensing Block Diagram
As shown, it is necessary to add some bias resistors to guarantee that the RS422/485 interfaces output a
high state when the interfaces are idle. The ZT3243, the converter for RS232 interface, has an internal
5KΩ pull-down resistor, which forces the R1OUT output to a high state when the RS232 bus is idle. The
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
AND gate is used to link the three received sources into FTDI RXD pin. The result is that the correct data
is received no matter which interfaces is being used. The limitation is that only one interface can be used
at any one time.
The same method is suitable for the CTS received signal.
Notes.
1. When using FT232R and FT4232H in RS485 mode, it is necessary to program the EEPROM to
enable the TXDEN function.
2. In the reference design, RS485 cannot support echo function since it will cause data overlap
when in the RS422 mode.
3. The jumpers (J1, J2 and J3) are reserved to enable terminal and bias resistors. Mounting the
jumpers (J1, J2 and J3) will enable the 120 Ohm termination and 680 Ohm bias resistors. The
below section (CH2.3) shows the details with termination resistor. The terminator function has to
be enabled when the device is connected at the end of a cable in RS485 mode.
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
A completed reference design example is given in Figure 2-2
Figure 2-2 Reference Design 1 – RS232/RS422/RS485 Auto-sensing
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
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Clearance No.: FTDI# 158
RS422/RS485 Auto-Sensing reference design
If auto-sensing is only required between RS422 and RS485, then the example shown in Figure 2-3 can be
used. This method is similar to the RS232/422/485 sensing method.
Figure 2-3 RS485/RS422 Auto-Sensing Block Diagram
Again, it is necessary to add some bias resistors to guarantee that the RS422/485 interfaces outputs are
high when the interfaces are idle. The AND gate is used to link the two received sources into FTDI RXD
pin. The result is that the correct data is received no matter which interfaces is being used. The limitation
is that only one interface can be used at any one time.
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
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Document Reference No.: FT_000270
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Clearance No.: FTDI# 158
Notes.
1. When using FT232R and FT4232H in RS485 mode, it is necessary to program the EEPROM to
enable the TXDEN function.
2. In this design, RS485 cannot use echo mode since it will cause data overlap in the RS422 mode.
3. The jumpers (J1, J2 and J3) are reserved for terminal and bias resistors enabled. Mounting the
jumpers (J1, J2 and J3) will enable the 120 Ohm termination and 680 Ohm bias resistors. The
below section (CH2.3) shows the details with termination resistor. The terminator function has to
be enabled when the device is connected at the end of cable in RS485 mode.
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
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Document Reference No.: FT_000270
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Clearance No.: FTDI# 158
A completed reference design example is given in Figure 2-4.
Figure 2-4 Reference Design 2 – RS422/RS485 Auto-sensing
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AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
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Document Reference No.: FT_000270
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Termination Resistors
The recommended arrangement of the wires is as a connected series of point-to-point (multi-dropped)
nodes, a line or bus, not a star, ring, or multiply-connected network. Ideally, the two ends of the cable
will have a termination resistor connected across the two wires. Without termination resistors, reflections
of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors
also reduce electrical noise sensitivity due to the lower impedance, and bias resistors are required. The
value of each termination resistor should be equal to the cable impedance (typically, 120 Ohms for
twisted pairs).
The reference circuit below illustrates a simple method to control the bias and terminal resistors without
having to use jumpers. Normally, the terminator function has to be enabled when the device is connected
at the end of cable in RS485 mode.
Figure 2-5 RS485’s Termination Enable/Disable Control Circuit
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Implementing RS232/RS422/RS485 Isolation
This chapter illustrates how to implement an isolated RS232/422/485interface. It also shows how to
implement the isolated power and ground. The independent power/ground/interface signals can help to
avoid the abnormal coupling of noise back into the system.
Isolated RS232 System
An example of how to isolate the RS232 interface is shown in Figure 3-1. The transformer is used to
isolate the power source and the optical couplers are used to isolate the interface signals. Note that there
are two independent grounds in the system.
Figure 3-1 Block Diagram of RS232 Isolated
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
A completed reference design example is given in Figure 3-2.
Figure 3-2 Reference Design 3 – RS232 Isolation Implementation Example
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
Isolated RS422 System
An example of how to isolate an RS422 interface is shown in Figure 3-3. The transformer is used to
isolate the power source and the optical couplers are used to isolate the interface signals. Note that there
are two independent grounds in the system.
Figure 3-3 Block Diagram of RS422 Isolated
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
A completed reference design example is given in Figure 3-4
Figure 3-4 Reference Design 4 – RS422 Isolation Implementation Example
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
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Clearance No.: FTDI# 158
Isolated RS485 System
An example of how to isolate an RS485 interface is shown in Figure 3-5. The transformer is used to
isolate the power source and the optical couplers are used to isolate the interface signals. Note that there
are two independent grounds in the system.
Figure 3-5 Block Diagram of RS485 Isolated
Notes.
1. When using FT232R and FT4232H in RS485 mode, it is necessary to program the EEPROM to enable
the TXDEN function.
2. RS485 interface shown above has an echo function enabled. The jumper links shown in Figure3-6 can
be used to enable or disable this function. If using an FT232R, it is possible to use one CBUS pin to
connect to RE# pin to configure the echo function. If setting the CBUS pin to TXDEN by EEPROM
programming, it will not support the RS485 echo function. If setting the CBUS pin to PWREN# by
EEPROM programming, it will support the RS485 echo function.
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
A completed reference design example is given in Figure3-6
Figure 3-6 Reference Design 5 – RS485 Isolation Implementation Example
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AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
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Clearance No.: FTDI# 158
Notes.
1. The optical coupler inverts the signal and the data transfer also has a delay time. If this isolated
example is used, then it is necessary to check the delay time and ensure that it has no side
effects on the selected data baud rate.
2. FT232R support the ability to invert signals using the EEPROM setting. This allows removing the
NOT gates used in the isolated examples.
3. When using FT232R and FT4232H in RS485 mode, it is necessary to program the EEPROM to
enable the TXDEN function.
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Contact Information
Head Office – Glasgow, UK
Branch Office – Tigard, Oregon, USA
Future Technology Devices International Limited
Unit 1, 2 Seaward Place, Centurion Business Park
Glasgow G41 1HH
United Kingdom
Tel: +44 (0) 141 429 2777
Fax: +44 (0) 141 429 2758
Future Technology Devices International Limited (USA)
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USA
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System and equipment manufacturers and designers are responsible to ensure that their systems, and any Future Technology Devices
International Ltd (FTDI) devices incorporated in their systems, meet all applicable safety, regulatory and system-level performance
requirements. All application-related information in this document (including application descriptions, suggested FTDI devices and other
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
Appendix A – List of Figures and Tables
List of Figures
Figure 2-1 RS232/RS422/RS485 Auto Sensing Block Diagram ........................................................... 4
Figure 2-2 Reference Design 1 – RS232/RS422/RS485 Auto-sensing .................................................. 6
Figure 2-3 RS485/RS422 Auto-Sensing Block Diagram ..................................................................... 7
Figure 2-4 Reference Design 2 – RS422/RS485 Auto-sensing ............................................................ 9
Figure 2-5 RS485’s Termination Enable/Disable Control Circuit ........................................................ 10
Figure 3-1 Block Diagram of RS232 Isolated ................................................................................. 11
Figure 3-2 Reference Design 3 – RS232 Isolation Implementation Example ....................................... 12
Figure 3-3 Block Diagram of RS422 Isolated ................................................................................. 13
Figure 3-4 Reference Design 4 – RS422 Isolation Implementation Example ....................................... 14
Figure 3-5 Block Diagram of RS485 Isolated ................................................................................. 15
Figure 3-6 Reference Design 5 – RS485 Isolation Implementation Example ....................................... 16
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Application Note
AN_143 Auto Sensing And Isolation Design For Rs232/Rs422/Rs485
Interfaces
Document Reference No.: FT_000270
Version 1.1
Clearance No.: FTDI# 158
Appendix B – Revision History
Document Title:
AN_143 Auto Sensing And Isolation Design For RS232/RS422/RS485
Interfaces
Document Reference No.:
FT_000270
Clearance No.:
FTDI# 158
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Revision
Changes
Date
Version draft
Initial draft
2010-05-04
Version 1.0
First release
2010-05-26
Version 1.1
Modified RS232, RS422, RS485 Isolation
Implementation Example images (figure 3-2,
figure 3-4 & figure 3-6)
2015-08-17
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