Decoding Pulse Telephone

[Ioannis]

I need to decode 3 digits from a dial telephone.

It pulses at a rate of aprox. 10Hz and as many pulses as the digit dialed.

I need to decode say a number like 100.

The problem is thatthe user does not always dial at the same time space each digit.

Also if wrong digit are dialed, it should reject them (obviously).

The idea of using Pulsin is rejected because it times out pretty quickly. Maybe timer1 is good idea, but I have to consider also the time between digits.

Any idea welcome.
Ioannis

[pedja089]

I had somewhere rotary dialer from old phone, and first pulse should be longer than rest of them(I think that phone with IC do same thing when switched to pulse dialing).
First hook up phone to scope, you need 100R resistor 12V PSU and phone. So dial few number and get timing for first pulse and period for rest of them.
Logic would be wait for long pulse, than count impulses, with timeout period that is twice long as period that impulses are repeated. And reset timeout on each received pulse. Create something like SERIN works. It should be easy.
For measuring time you can use timer, or loop with pause 1 inside and count how many time loop are repeated, or timer with interrupt, and in ISR just increment some variable, etc...

[richard]

most dials have a 66/33 mark/space ratio and fixed impulse rate of 10 impulses / second , there is also a fixed minimum interdigital pause between digits built into the dial mechanism . a pause in dialing longer than the 66/33 ratio will always indicate a new digit or the end of dialing

[Ioannis]

Thanks for the info. Did not knew there is a 66/33 ratio. Soon the scope will reveal the case.

Ioannis

[keithdoxey]

Its a long time since I did my BT apprenticeship (40 years!) and I no longer have any of my notes but this thread triggered something in my brain about the minimum Inter Digit Pause being 450mS as that is the amout of time that was needed for a strowger switch to detect the end of the dialled digit and sweep the bank looking for a free outlet. If its a rotary dial tone you wont be able to get the minimum below about 1 second as thats how long it takes to turn the dial for the next digit but if its Pulse Dial push button phone then the minimum Inter Digit Pause will be determined by the dialling chip in the phone.

[Ioannis]

OK, Thanks Keith.

The first pulse does not seem to be different than the rest.

The pause between digits is between 500-750ms according to the phone.

Each pulse is 1Hz or 600/400msec ratio.

Ioannis