Specifications

Appendix C - Testing and Troubleshooting RS-422/485 Systems RS-422 And RS-485 Applications Ebook
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Testing and Troubleshooting RS-422/485
Systems
This section provides resources to assist you with testing and
troubleshooting RS-422 and RS-485 systems.
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. Is the device configured for two-wire or four-wire systems?
2. Is a signal ground connection available?
3. Is the device isolated? Does it contain surge suppression?
4. What value bias resistors (if any) are used in the device? Are they
accessible for modification?
5. Is the device terminated? Is it accessible for modification?
6. What is the device's response time (turn around delay)?
7. What is the programmable address range of the device?
8. 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.
APPENDIX C