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Document 50182 Rev A2 10/28/96 P/N 50182:A2
Appendix A: EIA-485 Shield Terminations
The EIA-485 circuit must be wired using a twisted-shielded pair cable having a characteristic impedance of 120 ohms,
+/- 20%. Do not run cable adjacent to; or in the same conduit as; 120-volt AC service, noisy electrical circuits that are
powering mechanical bells or horns, audio circuits above 25 VRMS, motor control circuits, or SCR power circuits
Note: All enclosures, including the FACP backbox, must be connected to earth ground! Never use the shield for
grounding purposes.
Terminate the EIA-485 shields at both the Out and Return ends of the CCM-1 (or the DIA-1010 or DIA-2020 Display
Interface Board if employing that option), at either the cabinet (when not in conduit) or at system common (when in
conduit) as outlined below:
When the EIA-485 shield is not in conduit: At each respective LCD-80TM enclosure (except the first on the
loop), terminate the shield coming in from the previous LCD-80TM at the outside of the cabinet backbox (earth
ground). Let the outgoing (to next LCD-80TM) shield float (no connection). Shield termination between LCD-
80TMs can only occur at the receiving end - the end connected to P1 Terminals 2 and 4. For the first LCD-80TM
on the loop, let the shield coming in from the CCM-1 or DIA float.
When the EIA-485 shield is in conduit: At each respective LCD-80TM enclosure (except the first on the loop),
terminate the shield coming in from the previous LCD-80TM at system common. Let the shield going out to next
LCD-80TM shield float (no connection). Shield termination between LCD-80TMs can only occur at the receiving
end - the end connected to LCD-80TM P1 Terminals 2 and 4. For the first LCD-80TM on the loop, let the shield
coming in from the CCM-1 or DIA float.
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