Thecable shield must not be used as the signal ground.
It is tempting to try and reduce the cost of 5-wire cabling by using a 4-wire cable
withthe shield used as the signal ground. DON’T DO IT. The initial cost savings
arealways exceeded by the maintenance costs once the system is operating under
fieldconditions. It is often necessary to completely replace the network com-link
withthe proper cable (5-wire plus shield) to eliminate noise problems in the system.
Thesignal ground must not be connected to the chassis or earth ground.
Thechassis or earth ground is intended as a safety ground for power supplies, EMI
filters,voltage spike protection circuits, 120 VAC neutral returns, and all manner of
ACand DC driven devices. As a result, the chassis or earth ground can carry large
voltage potentials and currents. Connecting the signal ground to chassis or earth
groundcan damage the devices connected to the com-link.

2.4. Set Jumper for 3-Wire RS-422/485

Thisstep is required only if the OIT will be communicating with the host controller
usingRS-485 half-duplex 3-wire communication.

STEPS

1. Removethe protective cover from the rear of the unit.
CAUTION: When the rear cover is removed, the circuitry inside is exposed to
possibledamage by electrostatic discharge. Referto Static Awareness on page 4.
2. Locatejumper “JP4” using Figure 7 as a guide. The jumper is installed on
theupper two pins of a 3-pin header. Thisis called Position “A”.
3. Movethe jumper to the lower two pins of the 3-pin header (Position “B”).
Ensurethat the jumper is pressed fully into place.
4. Replacethe rear cover and the four retaining screws.
INSTALLATIONMANUAL 13
1010-0046A,REV 00
JP4
B
ANormal
A
Position "A", Half-Duplex 3-Wire
Position "B",
JP4
RS485
B
(Display)
(CPU Board)
B
JP4
A
Figure7 MAP450DJumper Installation