ProtEX-MAX PD8-7000 Explosion-Proof Temperature Meter Instruction Manual
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Serial Communications Overview
RS-232 and RS-485 are standard interfaces approved by the Electronic Industries Alliance (EIA) for con-
necting serial devices. In EIA terms, the device (e.g. meter) that connects to the interface is called a Data
Communications Equipment (DCE) and the device to which it connects (e.g. the computer) is called a
Data Terminal Equipment (DTE).
RS-485 can support multi-point connections per line because it uses lower-impedance drivers and receiv-
ers.
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. When communi-
cating at high data rates, or over long distances in real world environments, RS-232 is often inadequate.
The differential data transmission of RS-485 offers superior 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.
A multi-point network consists of multiple drivers and receivers connected on a single bus, where any
point (node) can transmit and/or receive data. 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 sys-
tem requires three twisted pairs (the extra twisted pair is needed for the signal ground).
Figure 21 illustrates how to connect a general four-wire network (a four-wire network actually contains 5
wires).
PDA8485
DO
DO
DI
DI
GND
DO
DO
DI
DI
GND
RS-485 DEVICE
Figure 21: General Four-Wire Network Connection
Figure 22 illustrates how to connect a general two-wire network (a two-wire network actually contains 3
wires). Note that the PDA7485 and PDA8485 have DIP switches that allow for two-wire connections with-
out the need to externally wire the DO to the DI and the /DO to the /DI (see the converter section for com-
plete details).
PDA8485
DATA
DATA
DO
DO
DI
DI
GND
GND
RS-485 DEVICE
Figure 22: General Two-Wire Network Connection
