Loop Sense Measurement
Summary:
Step 1.
Step 2.
Step 3.
Step 4.
Step 5.
Step 6.
Step 7.
This test verifies that a line hybrid ( U 1) in an LLM (TC4 150) can supply enough current for a
phone to operate properly and detect an off-hook condition; that is, when a minimum
amount of current is flowing. It can be accomplished without milli-amp meters or
breaking
connections These
measurements are especially important in key systems since the line card
introduces a small extra voltage drop in the loop due to a series current detector.
Refer to the Programming Section and program the line to be tested as an administration
phone by giving it the A: 1 attribute.
Refer to the Programming Section and record
the value stored in Location Code 64012, then
set it to 0.
Connect a 470 ohm resistor across the T and
R
terminals of the line under test and verify a
measurement of at least 7.2
volts across the resistor. This represents a current
of 15 ma and
should be sufficient for detection by the line hybrid, causing the TCIV to
connect
the line to a
link for dial tone sending and receiving.
If Step 3 was successful, verify a
digital low
on the LLM C terminal, indicating the connection
discussed in Step
3 was
completed.
If Step 3 or 4 failed, replace the line hybrid.
Remove the 470 ohm resistor
installed in Step 3.
Take the phone on the line under test off-hook and verify a DC voltage reading of 1 to
3
volts
less than the voltage observed in Step 3. This indicates the phone is drawing substantially
more current than the 470 ohm resistor. If not, the phone may not be drawing enough
current to be reliably detected by the line hybrid.
Note: The DC current in a phone varies with handset orientation, motion, and when a tone
sending button is pressed.
Step 7 can also be used to determine the actual loop current in the phone by
performing the following steps:
1. Hang up the phone and connect a decade box across the T and R terminals.
2. Adjust the resistance to obtain the same voltage as read when off-hook.
3.
Compute the current as:
=
/
= 25 to
5 0
ma.
