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Subject: Re: MT8870 with 8051

Date: 06/10/03 at 3:25 PM
Posted by: David Stephens
E-mail: dstephens@inventgineering.com
Message Posted:

In Reply to: MT8870 with 8051 posted by Vidhya on 06/01/03 at 2:23 PM:

Caller ID is sent in a FSK modem format, not DTMF. In the US, the standard is Bell 202 format.
It is sent in a burst between the first and second ring as serial ASCII text. Are you trying to make a project to monitor outbound numbers dialed? This is where the MT8870 would be useful to detect DTMF tones. Interface to 8051 is easy but attaching to a live phone line presents problems. There is usually high common mode AC voltages as well as the DC loop power. It is critical that your receiver circuit be balanced to avoid disturbing the phone line and AC coupled to avoid the DC loop power. Surges from on-off hook can still blow your circuits so be careful to provide adequate protection.
My web page has some information:
http://www.inventgineering.com/DTMF.htm
Good luck.


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