Precision Digital PD8-765 - Manual (Page 27)

PD8 Series Explosion-Proof Instrument Supplemental Manual
27
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 requires three twisted pairs (the extra
twisted pair is needed for the signal ground).
Figure 10 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 10: General Four-Wire Network Connection
Figure 11 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 without the need to externally wire the DO to the
DI and the /DO to the /DI (see the converter section for complete details).
PDA8485
DATA
DATA DO
DO
DI
DI
GND GND
RS-485 DEVICE
Figure 11: General Two-Wire Network Connection