Shahriar Posted January 7, 2006 Report Share Posted January 7, 2006 HiI have built a device which shows Caller ID on PC via Serial port.that works properly, but the problem is that when as soon as I connect the serial port to my devicea Vizz(bad sound) apears on telephone. the serial port has the same GND with telephone line and other pins of serial port goes directly to RS232 IC.What is the reference of this Parasit and how can i Solve it!Thanks in advanceShahriar Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 7, 2006 Report Share Posted January 7, 2006 Hi Shahriar,Telephone lines are a balanced pair of wires. For audio, when one wire swings in a positive direction, the other wire swings equally in a negative direction. They are balanced to cancel mains hum and interference, that you hear when you connect to ground the wire that measures 0VDC. The other wire is then a very long antenna.All things coupled to a phone line use a transformer or use optical coupling, to avoid upsetting the balance of the two wires. Quote Link to comment Share on other sites More sharing options...
mettula Posted January 8, 2006 Report Share Posted January 8, 2006 Use an audio isolation tranformer before entry in the serial port. it has to be of ratio 1:1 and 600:600 ohms. I found a good one at radio shack http://www.radioshack.com/product/index.jsp?cp=&productId=2103994&tab=summary Quote Link to comment Share on other sites More sharing options...
Shahriar Posted January 8, 2006 Author Report Share Posted January 8, 2006 ThanX a lotSo I am going to test both of them, First I will use two upto Isolator to connect the Serial Port pins to my device and then try a 600 ohm transformer.ThanX againShahriar Quote Link to comment Share on other sites More sharing options...
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