EIA Standard RS-485:
Document Summary
The physical and electrical characteristics for the TBOS protocol are mandated by CB-149to conform to EIA standard RS-422. A similar standard, RS-485, is often used in multidrop applications to avoid termination problems and contention among multiple transmitters for a "party-line" to a single receiver. As described in the TBOS Overview, TBOS monitoring of multiple network elements is a simple example of such an application.
The electrical characteristics of an RS-485 signal are practically identical to that of RS-422 (particularly at the data rates used for TBOS). Use of a high impedance transmitter that in effect does not interfere with any other transmitters that might be present is the key difference between the recommended standards.
EIA standard RS-485 (dated April, 1983) is entitled "Electrical characteristics of generators and receivers for use in balanced digital multipoint systems". EIA-485-B can be ordered on-line ($42 US) through the Electronic Industries Alliance site.
Unfortunately, this means we can't publish the RS-485 standard on this site! Click here to open the EIA document search and order page, then search on "EIA-485". Be sure and visit TBOS.net again sometime!
This page last updated January, 1999.