[Asterisk-Users] TDM400 w/ FXS S110M pinout on RJ11 connector?

[Werner Johansson]

Hi all,

I just got my long awaited FXS modules for a TDM400 card, and was a bit 
puzzled about how my phones and the FXS interface worked together. Here 
goes:

In Sweden (that's where I am located) phones traditionally are connected in 
daisy chain configuration. This means that a phone in on-hook condition 
passes the line through back onto the outer pair of the RJ11, back into the 
wall outlet to the next phone's inner pair. Our phone outlets automatically 
goes into pass-through mode if no phone is attached (otherwise all outlets 
has to be connected to phones for it to work). Anyway when I tried just 
connecting a phone to the FXS module it immediately thought the phone was 
going off-hook, nothing seemed to help to get it back on-hook. Finally after 
disconnecting the phone it goes back on-hook. I realised that something was 
wrong with the pinout when I used a cable with only the inner pair connected 
and everything just works to work. After some more testing I discover that 
I'm actually causing a complete _short_ on the battery voltage by looping 
the inner pair back to the outer pair in the phone! The voltage drops from 
approx. -48V to 0.0V.

To my question: Does anyone know what's actually connected to the outer pair 
on the TDM400 ports? I guess the only workaround is to only use cables where 
only the two wires from the inner pair is connected?

This might be informative to others trying the same thing as I did, I though 
it would be a quick way to see that the ports were alive, only to find that 
they appeared completely broken when I tried.. :| Any educated guesses why 
the ports are wired as they are?

Regards,
Werner