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APPENDIX B: Electrical Interfaces
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 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.