Analog doorphone with SPA112

[azacamis]

I have a Panasonic KX-T776X doorphone which I am trying to get to work with SPA112. I have no issues with the SPA112 generally as I am able to create a working line to make calls with a normal analog phone. The problem with the doorphone is that the moment I wire it up, it straight away go off hook. If I set some auto dial plan, it will dial non stop as it don't go on hook ever.

Appreciate if anyone has any ideas I can explore. I tried adjusting polarity, voltage, CPC etc. Nothing works.

[Dan Lukes · Accepted solution]

on/off hook state of analog line is matter of loop impedance (loop is wire + end device connected). According specification:

Off-hook threshold (line seizure): Rdc <  1000 ohm
On-hook threshold (line release):  Rdc > 10000 ohm

1kOhm-10kOhm range is "grey zone" with no state defined - it can be either on or off hook.

It seems your's line impedance is bellow 10 kOhm so SPA112 consider it off-hook. It may be caused either by wire itself (full or even partial shortcut between pair's wires) or by device connected (it's impedance in on-hook state is so low).

Use a multimeter to identify exact cause. Even with no multimeter you can use "divide et impera" approach to test:

1. disconnect wire on Panasonic's end, leave just wire connected to the SPA112. If SPA112 claims off-hook then wire pair is (partially) shorted on the path
2. bring SPA112 to Panasonic and connect them by short wire together. If SPA112 claims off-hook (see appropriate LED on it) then Panasonic is guilty. It's on-hook impedance is so low and it's incompatible with SPA112.

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[azacamis]

Thanks Dan. You're right. The door phone's impedance is only 1kOhm. Time to bring out that soldering iron for some experiment.

[Dan Lukes]

Good luck. An 1:10 impedance transformer may help, but I'm rather skeptic.