22 LCD2-80 Instruction Manual — P/N 53242:B1 10/25/2012
Appendix 5: 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. All
enclosures, including the FACP backbox, must be connected to earth
ground! Never use the shield for grounding purposes.
Note on remote power supplies:
When the LCD2-80 is powered from a separate power supply, use a
separate conductor to connect the main power supply common terminal to
the remote power supply common terminal. Disable earth fault detection
on the remote power supply.
When employing the LCD2-80 in Terminal Mode:
Terminate the EIA-485 shields 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 LCD2-80
enclosure (except the first on the loop), terminate the shield coming in
from the previous LCD2-80 at the outside of the cabinet backbox (earth
ground). Let the outgoing (to next LCD2-80) shield float (no connection).
Shield termination between LCD2-80s can only occur at the receiving end
- the end connected to P1 Terminals 2 and 4.
When the EIA-485 shield is in conduit
: At each respective LCD2-80
enclosure (except the first on the loop), terminate the shield coming in
from the previous LCD2-80 at system common. Let the shield going out
to next LCD2-80 shield float (no connection). Shield termination between
LCD2-80s can only occur at the receiving end—the end connected to
LCD2-80 P1 Terminals 2 and 4.
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