Specifications

RS-232/422/485 2-PORT PCI CARD, OPTO-ISOLATED

B.3 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 (1219.2 m) and data rates up to 10

Mbps. 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 pinout or a set of