FXS Module Power Requirements?

Can anyone tell me what the power requirements are for an FXS module (X100M)? I understand that to use an FXS, you have to hook up a 12V from the PSU to the card (TDM410) … I do, and I am having problems. This is in a big house, and the fax machine is on the second floor. I configured a Zaptel extension (using DAHDI compatibility), and when you call the extension, it rings about 1/2 of a ring, and the line hangs up … I am wondering if this is a “not enough supply power” issue.



Check out Appendix B of the TDM410 / AEX410 manual from the Digium website:

digium.com/en/login/login.p … ocs/TDM410

A single FXS module should draw less than 2.5W on the 12V rail if the REN load is 1 and 2.5W for a REN load of 3.

How many feet of copper separate the end device from the FXS interface? And, do you know the REN load of that end device?

Here’s a bit about REN (en.wikipedia.org/wiki/Ringer_equivalence_number).

What you’re describing, a half-ring and then nothing, sounds like a ring trip, which generally occurs when there isn’t enough voltage to drive the end ringer device.

Do you know the characteristics of your power supply? What’s the amperage of the 12V rail of your power supply?

I am guessing that the copper length is somewhere in the ballpark of 100-200 feet, and its OLD. I don’t know the REN of the device … Any ideas where I might be able to find it?

This is installed in a Mini-ITX box running an Atom CPU and a single 2.5" HD. The box has a 60 watt PSU (well, a 100 watt PSU with a 60 watt DC-DC converter, so effectively a 60 watt PSU) … Some testing (using a “Kill-A-Watt” power consumption meter) shows that the box running under normal load was pulling about 19 watts with the TDM410 installed but not in use.

I’m guessing that it is the wiring … My next course of action is to put a SIP ATA in place at the FAX machine … I don’t foresee having any issues this way. What do you think about the SIP ATA? I was looking at a Grandstream Handytone 286 … Found it at voiplink.com for $35.

Thank you for your assistance.


Depending on the device, it may have a sticker on it that indicates its REN loading.

What does your power consumption meter say when the TDM410 begins a ringing activity?

Is there a reason you need the physical fax machine instead of just using Fax For Asterisk?