CommTech Fastcom 422/4-PCIe, Справочное руководство, Страница: 14 / 18

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Fastcom
®
: 422/X-PCIe
RS422 / RS485
Most engineers have worked with RS-232 devices at least once in their career. If you have never worked with an
RS-422 or RS-485 device, you will be pleased to know that working with the FASTCOM: 422/X-PCIe is not much
different from working with a standard 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/X-PCIe features data rates up to 25 Mbits/second. 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 drawback of RS-232 is that no more than two devices can 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 multi
ple units (up to 32) to share the same “twisted pair” of wires (see diagram on next page). 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.