User`s guide

Appendix C - Electrical Interface
OMG-ISO-COMM Page 16
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. The output modem control
signal RTS controls the state of the driver. Some communication software
packages refer to RS-485 as RTS enable or RTS block mode transfer. 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.