1.89C51 RX and TX lines are connected to an external modem (receiving modem)through level converter to convert TTL signals to RS232 levels.

2. ON power ON 89C51 issues AT command to external modem to auto answer so that when there is an incoming ring, the modem answers automatically.

3.PC software dials through modem (sending modem) and connects to 89C51, through receiving modem.

4.WHen 89c51 receives connected message from receiving Modem,it starts reading bytes from modem, as and when they come, and starts sending the same to port p1 with a strobe so that printer connected to P1 can print it.

5.The problem is, even when PC is not sending anything, the receiving modem ( connected to 89c51) keeps getting some bytes. So printer prints Junk.

6.I have attempted 'packetising' data sent from PC with header, trailer and checksum. This has improved and the unit is working.

7. But I have not understood why any data is coming on the receiving modem at all when nothing is sent from the PC.

Some explanation for this behaviour will be helpful.
regards
K.Ganeshan