TDM400P Technical Specifications


I’m looking at getting the TDM400P with an FXS and an FXO module to build a PBX to interface with my analog telephone network. On the FXO end, instead of a PSTN from the phone company, I would use a Dock-N-Talk docking station to dock with my cellular handset.

I am using the docking station now and it works, but it struggles emulating the line qualities of a real telephone network. Notably, it has a hard time ringing my Western Electric 302. It can ring the phone, but the signal is not strong enough for both bells to be rung. Furthermore the device only provides a ringing output of 3 REN.

Before purchasing a $250 card, I wanted to make sure that the technical side of the FXS module will correctly emulate a normal POTS.

I’ve found that the REN for each FXS module is 5, but I can’t seem to find further technical specs for them. The ring voltage is the thing I’m most concerned about. 90V at 20Hz is what I’d like to aim for.

If anyone can confirm these specifications or point me to a real technical datasheet that would be great!


The design of that board is GPL’d. Anything you want to know about it, and more, can be found at


the tdm card is not gpl’d. I think the best option would be: “ask the vendor,



i think there is something opensource/public about the TDM400P. i remember a thread recently on the mailing list when the chinese ‘copies’ started appearing for a significantly lower price. someone stated that the design was in the public domain and therefore Digium couldn’t do anything about it.

personally, i wouldn’t buy one as the board is a fairly old revision and i like to support the guys who gave us Asterisk … although i am tempted to try one of the new Sangoma A200 range :smiley:

#5 … ht=chinese … ht=chinese

They seem to be close to the reference design, but not open source :smile:


that was the phrase i was looking for :smiley:

my relationships with clients is too valuable to risk saving a few bucks on too !!