A vizz Appears on Telephone line when I connect the serial port!

[Shahriar]

Hi

I 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 device
a 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 advance
Shahriar

[audioguru]

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.

[mettula]

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

[Shahriar]

ThanX a lot

So 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 again
Shahriar