CommTech FASTCOM SuperFASTCOM Hardware Reference Manual page 24

RS-422 / RS-485
Most engineers have worked with RS-232 devices at least once in their career. If you have
never worked with RS-422 or RS-485 devices, you will be pleased to know that working with
the SuperFASTCOM is not much different from working with an RS-232 device.
The RS-422 standard was developed to correct some of the deficiencies of RS-232. In
commercial and industrial applications, RS-232 has some significant problems. First, the cable
length between RS-232 devices must be short (usually less than 50 feet at 9600 Baud).
Second, many RS-232 errors are the result of cables picking up normal industrial electrical
noises such as fluorescent lights, motors, transformers, and other EMF sources. Third, RS-
232 data rates are functionally limited to 19.2K Baud. On the other hand, the newer RS-422
standard makes cable lengths up to 5000 feet possible and is highly immune to most industrial
noises. Data rates are also improved -- the SuperFASTCOM features data rates up to 40
Mega Baud. These improvements were made possible by differentially driving and receiving
the data as opposed to the single ended method employed by the RS-232 standard. With the
RS-422 standard, the transmit signal (TX in RS-232) is a differential signal consisting of SD+
and SD-; the receive signal (RX in RS-232) consists of RD+ and RD-.
Another draw back of RS-232 is that more than two devices cannot share a single cable. This
is also true of RS-422, and that's why the RS-485 standard was developed. RS-485 offers all
of the benefits of RS-422 and also allows multiple units (up to 32) to share the same twisted
pair. RS-485 is often referred to as a "multi-drop" or "two-wire, half duplex" network because
the drivers (transmitters) and receivers share the same two lines. In fact, up to 32 stations can
share the same "twisted pair".
(transmitter) can occupy the network at a time. This means that each station on the network
must control the enabling/disabling of their drivers in order to avoid network conflicts. If two
drivers engage the network at the same time, data from both will be corrupted. In RS-485
mode, the receivers are always enabled.
For a more detailed description of RS-422 and RS-485, we recommend the following
references:
LINEAR AND INTERFACE CIRCUITS APPLICATIONS, Volume 2:
Display Drivers. By D.E. Pippenger and E. J. Tobaben. Published 1985 by Texas
Instruments. ISBN-0-89512-185-9
Note: This book may be difficult to find in a bookstore. The best place to get it is
directly from Texas Instruments or from one their component dealers. Publication #
SLYA002.
"Driver/Receiver Family Extends Data-Link Performance", ELECTRONIC PRODUCTS,
January 15, 1985. By Dale Pippenger and Joe Miller
In order for an RS-485 system to work, only one driver
17
Line Circuits,