PCC-IV Instruction Manual (rev. 0.2)
Page 2-23
Section 2 - Installation
RS485 Communications Wiring
RS-485 communications wiring (terminals 1-3) is optically isolated and is not electrically connected to any other
circuits within the PCC-IV. This means that an accidental short to the RS485 cable will not affect the operation of
the other controllers (up to 31 units) connected to the cable.
RS-485 cabling may be up to 4000 feet in length. Longer distances can be accommodated by using RS-485
repeaters. RS-485 network cabling should always be arranged in a bus. Do not wire cabling in a star or with stub
connections. See the drawings below for examples of bus and stub wiring layouts.
Belden P/N 3106A (Preferred part number 16284) is the recommended shielded cable for all PCC-IV RS-485
communications data highways. Belden 3106A is specifically designed for RS485 applications and is 3-wire
shielded cable arranged as: 1 twisted pair with a foil shield, plus a separate single insulated conductor that is not
twisted into the main pair. The 3 wires are surrounded by a second braided shield and an overall PVC insulation.
Belden 3106A has a characteristic impedance of 120 ohms, has 22 Ga. conductors, and is rated 300V @ 60C.
Shielding and RS-485 Commons: The shield should be connected to the RS-485 common terminal at the master
station. At each PCC, PLC, RS-485/232 converter and any other devices on the network, the shields should not be
connected to each controller’s RS-485 common. The shields of the network cables should be wired together to
continue the shield to the next controller. The single wire (i.e., not part of the twisted pair) in the cable should be
connected to all the communications DC commons on the RS-485 network. Cut off the foil and braided shields,
insulate the bare shield wire and any exposed shield and connect as shown below.
RS232/RS485 Converters: Typical DCS and SCADA workstations have RS232 communications ports, and require
an external RS232/RS485 converter. RS232/RS485 converters are available from: Telebyte Technology (516-423-
3232) Model 8324. The RS485 cable must have exactly one set of Pull-Up/Pull-Down resistors installed to ensure
that the cable is biased to greater than 0.2 Vdc differential in the idle state (i.e., no nodes are transmitting). The
Model 8324 has internal Pull-Up/Pull-Down resistors that are activated by setting DIP switches.
4000 ft. cable length max.
(Use repeaters if greater than 4000 ft)
PCC
ORG
WHT
BLU
Belden 3106A
no substitutes
ORG
WHT
BLU
ORG
WHT
BLU
D+
COM
D-
PCC
PCC
1
3
2
1
3
2
1
3
2
1
3
2
1
3
2
1
3
2
PCC
ORG
WHT
BLU
Belden 3106A
no substitutes
ORG
WHT
BLU
ORG
WHT
BLU
D+
COM
D-
PCC
PCC
1
3
2
1
3
2
1
3
2
