Hello all!

I am using two AT89C51 in a circuit and both are communicating to a PC communication port. Switching is done by CD4052 The circuit works very well.

BUT some AT89C51 ics are not working as expected. If I mount the ic in one location (for example U1) it is not transmitting the data to the PC and at the same location if I put another AT89C51 the communication goes very well.. This means that there is no problem in the pcb.

And the problem goes like this .. when the defective(?) ic is mounted on the other location (U2 ofcourse after programing) it is transmitting data to the PC. The IC is working perfectly in other instruments also (some other application). This means that there is no problem in the IC.

have you ever come across this situation? What could be the reason?

thank you.

regards
Murugan AKT