I have a customer with a door phone that has been working for over a year and now is playing up it seems to get a bad feed back type noises form the door phone once a call has been hung up at the station end and it doesn’t ring properly through the door phone speaker when pressed.

The DPIM was installed in the Comms Rack and the door station was installed about 20 metres away over cat6 cable. So I tried hooking up the DPIM out at the door location to see if it was an interference issue as the application note says install DPIM as close as possible to door phone this worked better but still not perfectly.

One observation was if I was to put my finger on the P- terminal the thing would work faultlessly weather the DPIM was in the cabinet or at the door location.

So then I decided to take the DPIM and plug it into my system and it works fine here.

So I can only put it down to an interference issue at the customer’s site there is a lot of X10 power line control that may be causing the issues. Are there any other things / or additional grounding that can be done to overcome these sorts of problems?