Specifications

36 RS-422/485 Application Note
 Copyright B&B Electronics -- Revised October 1997
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
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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. 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.