Installation manual

D9124 Operation & Installation Manual

Page 57

74-06799-000-D 2/96

Checking Shielded Cable

If improperly installed, shielded cable can create problems rather than solve them. Follow

the procedure below to check shielded cable for proper installation.

1. Remove the drain wire for the shield from terminal 10terminal 10.

2. Measure the drain wire for resistance to terminal 10terminal 10 (earth ground). If there is a

short to ground, find and remove the foreign ground from the drain wire.

3. Reconnect the drain wire to terminal 10terminal 10.

4. Measure the shield at the far end of the cable (last POPIT location on data expansion

loops) for resistance to a ground reference. If there is an open circuit, find and repair

the open in the shield. Solder and tape all connections.

EMI on Long Wire Runs

EMI (Electro-Magnetic Interference) can cause problems on long wire runs for serial

devices (command centers, POPITs, etc.). The use of shielded, twisted-pair cable

reduces the effect of this interference. Some potential sources of noise on a long wire run

include:

• Radio or television transmitter site.

• Ham radio operator’s transmitter site.

• Computer network system.

• Heavy machinery (large electrical motors).

• PBX telephone system.

• High voltage electrical equipment or transformers (arc welders, certain medical and

dental equipment, etc.).

• Public service office using radio communications (fire department, police department,

and similar services).

• Close proximity to electrical lines, telephone cabling, or fluorescent lighting fixtures.

There are many other possible sources of noise. If you suspect that noise may be a

problem, the use of shielded wire is strongly recommended. Connect the drain wire from

the shielded cable to terminal 10terminal 10 on the D9124.

The drain wire must have continuity from the panel to the last serial device on the wire

run. If you cut the cable to install devices between the last device and the panel, be

certain to reconnect the drain wire to insure continuity to the last serial device.

If continuity is not maintained between the panel and the last serial device, the shielded

cable may aggravate potential noise problems rather than eliminate them. Connecting the

drain wire to ground at other than terminal 10terminal 10 on the panel may also produce problems.

Do not connect the drain wire to any other ground source.