Installation, Configuration, and Programming
PN 15842:K 7/23/2002 31
4.6 Shielding the EIA-485 Circuit
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. Terminate the EIA-485 shield at the Fire Alarm Control Panel only.
• When the EIA-485 shield is in conduit: Connect it to system reference
(system common). The shield can enter the cabinet, but must be insulated
from the cabinet (no electrical contact). Between annunciators, wire-nut
multiple shields together (which can be inside of the respective
enclosure, but can not contact the enclosure).
• When the EIA-485 shield is not in conduit: Terminate the shield at the
outside of the FACP backbox (ground). Do not allow the shield to enter
or even touch the cabinet. Between annunciators, wire-nut multiple
shields together outside of the respective enclosures (see Figure 4-11).
4.7 End of Line Resistor for EIA-485 Circuit
For the last device on the EIA-485 circuit, an end-of-line resistor must be
installed or enabled (depending on the type of ACS device).
• For ACM-24AT or ACM-48A, set
SW29 to “ENABLE” to install the
built-in end-of-line resistor (see
Figure 4-12).
• For ACM-16AT or ACM-32A,
install a 120-ohm End-of-Line
Resistor (Notifier Part Number
71244, supplied with the
annunciator) must be installed at the last annunciator on the EIA-485
circuit (see Figure 4-10 on page 30).
All other ACS annunciators should be set to “DISABLE” or the ELR removed.
Power must be turned off when connecting the annunciator to avoid damaging
the equipment.
Figure 4-11 Terminating the Shield
wirecond.pcx
Enclosure Annunciator
RXB TXB
SW2 9
DI SA BLE ENA BLE
ELR TERMINATE
Figure 4-12 SW29 on
ACM-24AT or ACM-48A
ACM-SW29.cdr
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