[Asterisk-Users] TDM400P FXO channel remains Offhook after outoing or incoming call / line is parallel with other telephone equipment

[Rich Adamson]

Inline...

> I am preparing to roll out Asterisk setup with TDM400P, 4 FXO modules in 
> a small office. Asterisk will replace legacy system (4 telco lines, 8 
> extensions PBX), but before the new system and ip phones would be 
> installed, the legacy system is still in use. The four telco lines are 
> now connected in parallel both to legacy PBX and the 4 FXO modules in 
> TDM400P. Asterisk is configured not to pick up any incoming calls.
 
<snip>

> I am dialing out via Asterisk and it works fine untill the following 
> situation:
> 
> - one of the telco lines occasionally becomes mute after call is 
> completed, would not provide dial tone, (not sure about ringing on that 
> line) - both via old and new PBX.
> - zap show channel <n> would show that line as 'Offhook', though no 
> telephone is off hook.
> 
> If physical line would be unplugged from TDM card, the line would become 
> normal again.

Others have posted similar type issues relative to the TDM400P card.
Not sure if that issue has been addressed in code or not. I'm not seeing
that problem with current cvs head.

> Sorry, if it is a well known problem, but I did not find any specific 
> information yet.. Please answer two questions:
> 
>  - is it really bad to have parallel connection on TDM400P FXO lines to 
> an additional telephone equipment, does it prevent TDM400P to detect 
> Offhook/Onhook correctly?
>  - will the problems go away when parallel lines would be disconnected 
> (legacy PBX shut down)?

In very general terms, no its not a problem. It certainly can be a problem
when, as an example only, the legacy pbx is actually using a line and *
attempts to use the same line. (eg, * is not going to check to see if
dialtone or voice is present before dialing.) Same in reverse; if * has
a line in use, will the legacy pbx detect that before dialing?
 
The 3050 chip on the TDM fxo module has the capability of sensing whether 
another analog device on the pstn is off hook (ie, much lower line voltage),
but I'm 80% sure that flag is not currently handled by the tdm drivers, etc.

> As you may understand the office personnel has anxiety that this may be 
> a bad Asterisk setup / bad TDM card etc (which I am sure so far that it 
> is not).

Sounds like you really need to go through a pre-cutover test plan without
the legacy pbx attached to validate your config, etc.

The tdm card does have some unusual issues that appear to be driver
oriented, but there are lots of folks using the card in production.