[VoIP] Line reversals for payphones.....

[Martin Harriss]

Andy Greening wrote:
> Gents,
> 
> Here's a little challenge to someone who knows far more about * than I do
> ;-}
> 
> I have a UAX12 here in Blighty (ok it's off the air at the minute, it's just
> moved location at home and I've got to find time to cable it again!) and I
> have a Pay-on-answer payphone which I want to be able to use on Cnet. The
> problem I have is this:-
> 
> For the payphone to work correctly the junction to the * needs to have its
> polarity reversed when the far end answers, and at certain periods
> throughout the call 'silent line reversals' would be applied to the junction
> to give the metering.
> 
> I use a Grandstream handytone ATA as my outgoing junction, and AFAIAA no ATA
> offers line reversal on answer.
> 
> The only way I can see that the line reversals can be done is to have a
> relay card in the * box and when the answer comes from Cnet have some sort
> of code make a relay on the card change over. The silent line reversals
> would be a bonus, (so that the metering comes from the junction) but is not
> critical, as these can be 'spoofed' locally.
> 
> Does anyone out there have the skills to give this a go?
> 
> Thanks in advance
> 
> Andy G

Hi Andy,

Neat!  Do you have a working CFC on your U12?  Even if you can't produce 
the meter pulses, if you have a local call timer, you can have periodic 
pulses to drive the CFC.

Incidentally, I seem to remember that the current reduction in the MOJ 
arrangement was done only by resistors.  There may have been an inductor 
in there somewhere, but for another reason (transmission bridge?)

I used to have the article from the POEEJ that described the 
arrangement, but it's long gone.  You could probably find it in a good 
library.  The Science Reference Library in London (part of the Patent 
Office) was always very good for that, but access may be restricted 
these days...

Martin