DuraComm PSDA-MU1, Owner'S Manual

Page: 5 / 47

Page 5 of 47
Simplified Theory of Managing an IDC Circuit
R1
10K
R2
10K EOL
GND1
Alarm
Contacts
GND1
Vs
Annunciator
Monitored Circuit
Wiring
Alarm
Initiating Device
GND1
Test Point
(CH-X, Neg)
Test Point
(CH-X, Pos)
VDC
Figure 1: Simplified Circuit for IDC Management
The EOL Resistor must be placed at the Alarm Initiating Device to detect open circuits in the wiring.
A ‘normal’ circuit divides the test point positive voltage (Vs) in half since the test current passes through
two 10K resistors in series. The test point voltage will be Vs *(R2/R1+R2) or Vs/2. This produces a GREEN
LED.
A trouble alarm switch closure will short out the EOL resistor, and the full Vs voltage will be applied to
the test point (across R1 only), so the test point voltage will be equal to Vs. This produces a RED LED.
A Single Fault (broken wire or one of the wires is shorted to GND1) will produce a test point voltage of
Zero (0) volts.
Both wires shorted to GND1 will produce a Fault since the test point voltage will be Zero. Faults are
reported with a BLUE LED.
If both wires are shorted to any other conductive material that is isolated from GND1, it will produce the
same result as a Trouble alarm since this will short the alarm circuit wires together, and the test point
voltage will be equal to Vs