Hi Jonny,

I have just finished my project of autodialer. I have used MT8880 instead of MT8888. (The only difference is bus compatibility and the price of course which was my concern)


YOU ASKED:
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My question was if it is possible to connect the DTMF/CP input and the DTMF output of the 8888-chip.
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Answer 1:

YES you can connect the output as well as input together for DTMF/CP input and DTMF/Acknowledge out put.

There are two ways to do that (as far as I understand and have done it)

How ??

a) Please see the data sheet page 10/21 Fig. 13. In the single ended input configuration just tie up the input and output boxes in the schematic together, this will make common input/output.

Now use a 600 ohm transformer (pull out one from your modem ...). Ground one leg and the other to the common input/output. Now its ready........

b) Use differential input configuration data sheet page 3/21 Fig. 4. But this is only to input the DTMF/CP tones.

To hook up the DTMF output use the same 600 ohm transformer. One leg to the ground and one to the DTMF out put.

The other side of the transformer, in both cases, is of course parallel to the Tip and Ring of the Line, you would know.

My experience with both styles is that case (b) is better in terms of sensitivity whereas case (a) often misses out while dialing.

YOU ASKED:
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Another question: Is the phone-line-transformer necessary????
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In addition to Isolation the transformer is required to off hook the line. If you don't want to use the transformer then connect your common I/0 thru some suitable mean to the line (a 0.1uF Cap...). But still you need to connect half of the transformer to off hook the line (you may connect a 2 watt resistor also,)


While
If you have an extension on the line already, then you cannot log in to MT8888 by off-hooking thru ring detection in case somebody picks up the extension and line goes off hook before the set number of rings have reached your application. So you use differential input and give DTMF command and turn the off hook relay on to disconnect the extension and remain off hook. relay connects the transformer to the line.

I hope above helps you.

Simon