[cisco-voip] Fax Machines and ATA's

[Robert Singleton]

On Wed, 2007-08-08 at 11:51 +0100, Johnny Crothers (c) wrote:
> Has anyone had any issues with ATA 186’s and Panafax UF series fax
> machines? Cannot send or receive, although when ringing them, appears
> to be presented just nothing happens on the fax. Analogue phones and
> other models of fax’s work fine.
> 
>  
> 
> Don’t think it’s a baud issue as the fax doesn’t even recognise that
> there is a call being presented.

There is one thing that I actually thought of when I was re-reading this
post and it might explain it better than any of the thoughts below. Is
the RJ11 line cord in the "line" jack or the "phone" jack? Sometimes it
matters.

Though I haven't had *this* problem with any of our faxes, I understand
that some faxes can be sensitive to the ring voltage from the ATA, which
may not be be high enough. This kind of problem usually only presents
itself when using an ATA to drive a house full of bell ringers (that's
where that ringer equivalency rating comes in). Bell ringers draw more
current than electronic ringers and an ATA (and most ISDN modems, BTW)
are much more current limited than your LEC, so the voltage drops, per
Ohm's law, and then electronic ringers don't see ring voltage.

The ringing cadence can be an issue, too... if the ATA is doing two
short rings, a stubborn fax machine may not see it.

Finally, it's incredibly rare, but some devices can be polarity
sensitive. The "standard" RJ11 cord flips polarity from one end to the
other, but many people make their own cords, often without a flip or
maybe without regard to polarity at all and many times, and an RJ11 data
patch cord (a rare bird these days, too) will specifically *not* flip
polarity. Almost nothing modern cares what the polarity is, but it's an
easy thing to try...

Robert