Appendix C - Electrical Interface
Sealevel Systems
ACB-II
Page
14
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. The driver is in a tri-state or high impedance
condition when this occurs. Only one driver may be active at a time and the
other driver(s) must be tri-stated. The output modem control signal Request
to Send (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.
