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APPENDIX B: Electrical Interface
B.3 RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for party line
or multi-drop applications. The output of the RS-422/485 driver is capable of
being active (enabled) or tri-state (disabled). This capability allows multiple ports
to be connected in a multi-drop bus and selectively polled. RS-485 has electrical
characteristics that allow 32 drivers and 32 receivers to be connected to one line.
This interface is ideal for multi-drop or network environments. RS-485 tri-state
driver (not dual-state) 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 tri-stated. RS-485 can be cabled in two ways: two-wire and four-wire mode. Two-
wire mode does not allow for full duplex communication and requires that the
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 or a set of modem control signals. RS-485 does not
define a physical connector.
