Specifications

RS-422/485 Application Note 35
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
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.
36 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
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.