Bosch Building Technologies

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    How to troubleshoot a faulted panel zone used for monitoring D125B 2-wire loops.

    The D125B Powered Loop Interface Module provides two 2-wire loops (A and B) for powered devices.  Each 2-wire loop uses a 1.8K ohm end of line resistor for supervision at the end of the loop.  The condition of the supervised loop will determine the voltage across the D125B supervisory zone and common as follows: 

    • 2 - 3Vdc: Normal zone, D125B sees the 1.8K EOL at end of 2-wire loop.
    • 0 - 1.3Vdc: Shorted Zone, there is a short across the 2-wire loop, or a detector (i.e. smoke) has activated.
    • 3.7 - 5Vdc: Open zone, when one of the following conditions exist:
      • The protected loop is open.
      • The D125B has no power.
      • Either the positive (+) side or negative (-) side of the loop is shorted to ground.

    The D125B supervising zone connects directly to a panel onboard zone with no EOL.  The control panel responds to the supervised zone condition using the programmed response for normal, open or short.  Connecting an EOL between the D125B supervisory zone and the panel onboard zone will cause a faulted zone regardless of the normal state of the 2-wire loop.  Additionally, the supervising zone must connect to a device with a common power connection to the control panel. Polling devices such as Popits or multiplex devices will not work. 

    • Note, the 1.5K EOL, originally installed with former D125 or D125A Powered Loop Interface Modules, is allowed for use with the D125B.


    Version history
    Last update:
    ‎06-27-2018 09:53 PM
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