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Parent Directory -
README.html 12-Mar-2003 16:53 2.0K
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gal.jed 12-Mar-2003 16:53 1.1K
When you lift the handset of A, 'phone B rings.
When you lift the handset of B, 'phone A rings.
When both handsets are up, there is an audio path between A and B (in both directions of course).
In short, it's just like a real exchange, except that there's no dialling (what's the point, with only two 'phones?!) and you never get ringing tone or busy tone (ditto!).
I'm grateful to Mick Farmer <mick@dcs.bbk.ac.uk> for scanning the GIF. If you can't read it, send me your postal address by email and I'll send you a paper copy.
The directory also contains a definition, or ``program'', for the GAL (a programmable logic device).
Some points to note: if both A and B are off-hook, and A (say) replaces the handset, then B does NOT start to ring. An off-hook condition on the A side (say) causes a momentary erroneous off-hook condition on the B side, because of the audio coupling capacitor; the circuit ignores this. The circuit is tolerant of ``contact bounce'' in the handset switches. The GAL takes care of all these little matters.
Ringing voltage is applied between the bell wire (blue in the BT cable to the 'phone) and the A wire (white). Ringing voltage is on the low side, but more than adequate for all modern 'phones. It is adjustable, but if you wish to wind it up to 80V rms (which is the standard) you will probably need to wind your own transformer, using a larger core to prevent saturation. The ringing cadence is almost identical to the standard British one, but this can be changed by re-programming the GAL.
A PCB layout is available on request.
Tony Fisher / fisher@minster.york.ac.uk