11
RS-422/RS485
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 FASTCOM: 422/2-PCI 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 FASTCOM: 422/2-PCI features data rates up to 1.5 Mbaud. 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 TX+ and TX-; the receive signal (RX in RS-232) consists of RX+ and RX-.
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” of wires (see
). RS-485
is often referred to as a "multi-drop" or "two-wire, half duplex" network. In order for an RS-485 system to work,
only one driver (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: Line Circuits, 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
