Engineering Documentation

Chapter 2 – Network Electrical Systems
RS-485 MS/TP Communications
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Fig. 19: Intra-Building Repeater or Mixed 1pr & 1.5Pr Cable.
3-Wire Network Terminator (550-975P100, Pkg. of 100)
The 3-wire network requires a new network terminator. The new terminator is a 120
ohm 1/2W carbon composition resistor. One terminator must be placed at each end of
the 3-wire network section.
3-Wire Network RS-485 Reference Terminator (550-
974P10 Pkg. of 10)
The nodes that use a 3-wire network interface must have the RS-485 reference wire
(yellow) of the network cable terminated to EARTH GROUND at ONE END ONLY
through an RS-485 reference terminator (shown below). The RS-485 reference
terminator consists of a PTC thermistor (polyfuse device) and wire to allow connection
to earth ground. A PTC was chosen in case the third wire of the network cable, (the
common reference between all 3-wire nodes), is accidentally grounded to earth
ground at a second location that could cause high ground currents to flow, due to a
difference in earth ground potential. The PTC would open during the short condition if
large currents start to flow in the reference wire. Without the PTC or 100 ohm resistor,
sufficient current could flow to damage the cable. The PTC will return to normal
resistance (less than 1 ohm) when the fault condition is removed.