Appendix C - Electrical Interface
OMG-USB-SER-2
Page
13
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for
partyline 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
allows cable lengths up to 4000 feet 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 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 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 pin-out or a set of modem
control signals. RS-485 does not define a physical connector.
