Serial Communication Adapters
Instruction Manual
5
SERIAL COMMUNICATIONS OVERVIEW
RS-485 is a standard interface approved by the Electronic Industries
Alliance (EIA) for connecting serial devices. In EIA terms, the device
(e.g. meter) that connects to the interface is called a Data Communica-
tions Equipment (DCE) and the device to which it connects (e.g. the
computer) is called a Data Terminal Equipment (DTE).
Line drivers and receivers are used to exchange data between two or
more points (nodes) on a serial communications network. Reliable data
communications can be difficult in the presence of induced noise,
ground level differences, and other hazards associated with installation
of a network. The differential data transmission of RS-485 offers superi-
or performance in most applications. Differential signals can help nullify
the effects of ground shifts and induced noise signals that can appear
as common mode voltages on a network.
RS-485 allows multiple drivers and receivers on the same two-wire or
four-wire system. The RS-485 standard specifies up to 32 drivers and
32 receivers on a single bus, but with the introduction of "automatic"
repeaters and high-impedance drivers/receivers, this number can be
extended to hundreds of points (nodes) on a network.
The cabling used for an RS-485 serial communications network should
always be a high quality cable such as Belden 8162 or Alpha 6203C. A
two-wire system requires two twisted pairs, and a four-wire system re-
quires three twisted pairs (the extra twisted pair is needed for the signal
ground).