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APPENDIX C: Asynchronous Communications
C.3 RS-485
RS-485 is backward-compatible with RS-422; however, it is optimized for partyline
or multidrop applications. The output of the RS-422/485 driver is capable of being
Active (enabled) or Tristate (disabled). This capability allows multiple ports to be
connected in a multidrop bus and selectively polled.
RS-485 allows cable lengths up to 4000 feet (1219.2 m) and data rates up to
10 megabits per second. The signal levels for RS-485 are the same as those defined
by RS-422.
RS-485 has electrical characteristics that allow for 32 drivers and 32 receivers to
be connected to one line. This interface is ideal for multidrop or network
environments. RS-485’s tristate (not dual-state) driver will allow the electrical
presence of the driver to be removed from the line. Only one driver may be active
at a time, and the other driver(s) must be tristated.
RS-485 can be cabled in two ways: two-wire and four-wire mode. Two-wire mode
does not allow for full-duplex communication, so data be transferred in only one
direction at a time. For half-duplex operation, the two transmit pins should be
connected to the two receive pins (Tx+ to Rx+ and Tx to Rx). Four-wire mode
allows full-duplex data transfers.
RS-485 does not define a connector pinout, a physical connector, or a set of
modem control signals.