how I can connect 89c51serial port to telephone lineto send data?

you need a one to one audio transformer to isolate the DC voltage . A good pair of clamping diodes to protect the processor from the ring voltage.

P.S. Don't expect to send square wave signals directly. you'll have to device a method of using sine wave audio. this can be done via good voltage comparator on the RX end to decode it.

first of all many thanks for your reply but I ask for this audio transformer I didn't find it in the electronic shop in my village. and I have 2 questions here

-is there is replacement for it with another circuits & if it to isolate could I replace it with opticoupller pc817 for example because it is available here or optodiac to fire triac gate if it is suitable?

-for the voltage comparator is it ok (lm393 Low power dual voltage comparator ) and if it is ok what is the reference voltage value I should use? ( I measured the telephone line voltage & it about 57volt at rest & may be 70-110 at ring (sure I didn't measured the ring voltage)

Sheesh! The silly gets ridiculous.

If sending baseband data over a phone line were this easy then the modem would never have been invented.

The maximum bitrate a plain old telephone line will pass, through the public switched network, is about 50kbps. Can you limit your video signal to this type of bitrate? No, right?

Your only chance is to store up a bunch of video and then send it s-l-o-w-l-y over the phone line (in non- real time) by using a modem. If you can send the digitally-encoded video over a DSL or cable modem then you'll be able to send it in real time. Otherwise fahgettaboudit.

Bob

You are correct if the intended connection is a dial-up.

However, the OP has not stated whether it is, or whether it is a point-to-point connection, perhaps on non-loaded cable.

How about it OP? What are you trying to do?

Don

if you want to got that with the isolator that's fine. in your case it might just work how ever, you should be using a transistor output type that drives a HV transistor with a reverser polarity protection diode. I think it would be simpler for a 1:1 transformer..

For common stores and parts look at Radio shack, I don't know where you are from how ever.. take a look here

P.S. You should also have a 120 Volt suppressor on the line side along with clamping diodes on the uC side. if you only have single project then rip one from an old modem or the like.

If this is going to be a production product then look else where like Mouser.com or Digikey.com for 1:1 isolation signal transformers.

learn about bandwidth.

Bye. Jasen