Appendix D – Electrical Interface
Omega Engineering
OMG-P232/P422
Page
19
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.
RS-530
RS-530 (a.k.a. EIA-530) compatibility means that RS-422 signal levels
are met, and the pin-out for the DB-25 connector is specified. The EIA
(Electronic Industry Association) created the RS-530 specification to
detail the pin-out, and define a full set of modem control signals that can
be used for regulating flow control and line status. The RS-530
specification defines two types of interface circuits, Data Terminal
Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The
Omega Engineering adapter is a DTE interface.