How do I monitor a shared phone line for disconnects?

A line-status monitor with an isolated relay output gives the CLICK PLC a discrete indication to text when the shared phone line is disconnected; a homemade voltage detector or an untested daily poll does not provide that dependable interface.

Skip quick fixes that only appear to prove the line is healthy

The problem is not simply whether voltage exists on the pair at one instant. The installation needs to detect a usable connection on a buried private line shared by an alarm dialer and a fax, then pass a clear status to the PLC without disrupting either device.

• Do not wire the telephone pair directly to a CLICK input. Telephone lines can carry DC line voltage and a much higher AC ringing voltage. A PLC input is not a telephone-line interface; connect it only through a device with a documented, suitably isolated relay output.
• Do not treat an idle-voltage reading as proof of end-to-end service. A reading near 48 VDC on an idle working line was cited as a typical observation, but it does not prove the buried cable is intact under load or that the dialer can complete a call. The ring condition was cited at about 90 VAC RMS; that is not a signal to apply to PLC hardware.
• Do not add an arbitrary daily call or polling routine. The line is shared with a fax and an alarm system. A test that seizes the line can make it busy or interfere with an alarm call. First confirm whether a proposed monitor senses line status without taking the line, or deliberately takes control of it.
• Do not rely on a generic continuity tester for an in-service line. A continuity check can find an open pair when disconnected, but it does not establish that the installed alarm path is available while the shared equipment is connected.

These are not permanent repairs. If the line is currently down, use the existing backup text/remote-monitoring path to maintain alarm visibility, then isolate and repair the phone-line fault before treating a status relay as a fix.

Separate line supervision from line seizure

The decisive choice is what the monitor does to the telephone line. A monitor may report a condition through an output, or it may take control of the line from another device. Those functions are not interchangeable on a shared circuit.

Observed condition or choice	What it tells you	Action
Idle voltage is present	The pair may be energized at the measurement point; it does not prove a successful alarm call.	Test the complete path and confirm the dialer’s call result.
Monitor detects a disconnect	The monitor can provide a line-state indication, subject to its documented sensing method.	Route its isolated contact to a PLC input and test both normal and disconnected states.
Monitor takes control from another device	It can affect which device has access to the shared line.	Confirm the intended priority and behavior during fax and alarm use before installation.
PLC input changes but no text arrives	Line detection may work while the separate PLC/C-more notification path fails.	Test the input, logic, and message path independently.

Viking Electronics LDB1 and LDB3 were identified as candidate line-disconnect devices. The LDB3 was specifically described as offering line disconnect for taking control of the line from another device. That makes its line-seizure behavior a design decision, not an automatic advantage. Confirm the current device documentation for the exact sensing behavior, contact ratings, wiring, and compatibility before choosing a model; do not rely on historic price or product-page information.

Find whether the fault is in the pair or the alarm path

The reported cable run is about one-quarter mile between the office and buildings, and past connection problems occurred on that private line. A disconnection can therefore mean a physical break, a poor termination, or a fault at either end. A line monitor reports a condition; it does not locate or repair the bad connection.

1. Record the current states of the alarm dialer, fax, monitor (if installed), and PLC text/remote-monitoring backup before disturbing wiring.
2. Check accessible terminations at both ends for loose, corroded, wet, or damaged connections. Inspect junctions along the route where available.
3. With the line isolated from connected equipment as appropriate, test continuity and shorts on the pair. Restore the connections and repeat an operational test with the actual alarm equipment.
4. If the line appears intact but the alarm still cannot report, distinguish a line failure from the dialer, destination, or remote alarm receiver by checking the dialer’s own status and call result.

Do not infer a cable fault solely from a PLC text failure. Text notification is a second communications path and can fail independently. Conversely, a successful test text does not prove the telephone alarm path works.

Select a monitor that matches the shared-line requirement

Write down the required behavior before replacing or adding equipment: detect a disconnected line, avoid disrupting normal fax and alarm traffic, and provide a contact suitable for the PLC. Then check the device documentation against each requirement.

• Confirm whether monitoring is passive or requires the monitor to seize the line, place a call, or otherwise interrupt another device.
• Check what the device calls a disconnect and how it signals that condition. Do not infer alarm polarity, reset behavior, or output type from a product name.
• Confirm the output is a relay contact or other interface explicitly compatible with the PLC input circuit. Follow the monitor and PLC wiring specifications for isolation, voltage, and current.
• Check how the monitor behaves during a fax transmission and an alarm-dialer attempt. The intended line priority must not prevent the alarm system from placing an urgent call.
• Verify that the device is intended for the specific kind of private line in this installation. If the provider or telephone equipment has changed, confirm compatibility rather than assuming a device for one line arrangement suits another.

The source identifies the LDB1 and LDB3 as possible products, and says the LDB3 can take control of the line from another device. It does not specify exact output wiring, contact ratings, or current model availability. Read those values from the selected unit’s current documentation. If that documentation does not establish safe sensing and PLC interfacing, stop before wiring it into the operating line.

Wire the relay status into the CLICK input

Use the monitor’s documented output as the boundary between the telephone circuit and the PLC. Do not build a mains- or phone-line sensing circuit from the cited idle and ringing voltages and connect it directly to the controller.

1. Identify the selected monitor’s output type and normal/disconnect states from its wiring documentation. Determine whether the relay is energized or de-energized in each state.
2. Choose a CLICK input and wire the contact using the PLC input module’s specified supply and common arrangement. Keep the telephone pair on the monitor side of the isolation boundary.
3. In the PLC, assign a descriptive input tag and define what state represents healthy line versus line fault. Make the fault state drive the existing C-more text/remote-monitoring notification path.
4. Decide how to avoid nuisance notifications when the contact changes briefly during normal line activity. Use only a delay or persistence requirement supported by the monitor behavior and the alarm response requirement; do not adopt an arbitrary interval.
5. Label the monitor, pair, relay terminals, PLC input, and alarm logic so a shift technician can trace the status without guessing.

Keep the existing alarm dialer and its connection in service unless the selected monitor’s documented installation explicitly requires a change. In particular, a line-seizure feature must not be wired or configured in a way that blocks the alarm dialer from using the shared line.

Test the full alarm and backup paths before release

Prove the monitor independently, then prove the communication paths. A green status or a PLC bit alone is not a complete acceptance test.

1. With normal line service and both shared devices connected, record the monitor output and corresponding PLC input state.
2. Use the monitor manufacturer’s documented test method to create or simulate a disconnect. Confirm the relay changes, the intended PLC input changes, and the PLC/C-more backup sends the expected text or remote alarm.
3. Restore the line and verify that the monitor, PLC input, and notification logic return to their documented healthy states without repeated or latched fault indications.
4. Place a controlled test call through the alarm dialer and separately confirm fax operation. Verify that monitoring has not seized or blocked the line during either use.
5. Record the device model, wiring, normal and fault contact states, PLC input address, and results. Repeat the tests after repairing a connection or changing monitor wiring.

If the relay reports healthy while a controlled alarm call fails, investigate the line and dialer path rather than changing PLC logic to mask the discrepancy. If the monitor detects a fault but no text arrives, troubleshoot the PLC input and backup message path as separate faults.

Keep the repair distinct from the temporary restore

The PLC/C-more text and remote-monitoring setup is a backup notification route; it does not restore the buried telephone circuit. Use it to preserve awareness while the phone line is unavailable, and clearly identify any period when the alarm dialer cannot place calls.

For a recurring disconnect, repair the poor connection or damaged cable section, then repeat the end-to-end dialer, fax, monitor, and PLC notification tests. Do not close the work based only on a restored idle-voltage reading or a successful PLC text. The permanent repair is complete when the physical line is stable and the alarm and backup paths both pass their tests.

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