[asterisk-dev] Changing Line voltage of a FXS card

[Pavel Troller]

Hello Ahmet,
  are you sure that FXS cards will be happy with a foreign voltage provided
by the far end ? They are made to provide their own voltage, not to be fed
from the B-side. I don't know exact hardware design of the cards you've
mentioned, but I have some experience with various telco hardware and FXS
cards with current feed turned off obviously don't behave like FXO cards -
namely, they don't maintain a regular loop, which you probably require. When
supplied with external voltage to a sleeping FXS card, it depends on the 
polarity - in the "native" polarity there is no current through the card,
and in the reverse polarity the current may be there, but it's mostly caused
by protection circuits (diodes) and it's not going through the regular paths
speech currents are supposed to flow.
  With regards,
    Pavel

> Greetings,
> 
> I am trying to add a new signalling type to asterisk, the signalling type
> is something like PLAR; well, line has no voltage at all just like a local
> battery line, when you want to make a call you provide ringing voltage on
> line and then the called party provides the line voltage and dial tone. Its
> just like plugging a subscriber line to a FXO but you should send ring at
> first.
> 
> I happen to have X100M, S100M and TDM400. Since FXO cards cannot generate
> ringing voltage (this is an assumption), i have to use an FXS card to
> provide ringing voltage but apart from that this FXS card should work like
> an FXO namely this FXS card should not provide line voltage.
> 
> I know that SI3215 is used on FXS cards and this chip has the line voltage
> setting feature which would be set to 0 volts or by setting DCOF bit at
> Register 14 DC-DC converter could be turned off.
> 
> I am inspecting dahdi souce codes but unable to come up with an simple way
> to do so.
> 
> Any help will be much appreciated. Thank you.
> 
> Ahmet.

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