Hello all,

I would like to ask you for your help. I have a system composed of a computer with RS232, RS232/RS485 converter and several RS485 devices which can communicate at 57600bps with each other and with the computer as well. Everything works correctly until I disconnect the converter from the computer (the converter is still powered). It makes the devices unable to communicate among each other and this is the problem. I think it is caused by interference between RXD and TXD in RS232 cable (approx 1m long) which was disconnected on the side of the computer. I found out that signal from a RS485 device converted to RXD signal for PC interferes with TXD in the cable and goes back to the converter. HC4538 takes it as a computer’s transmission and set ADM485 to driver function that leads to collision. So I would like to ask you how to avoid this interference. When I put 1k ohm resistor between GND and TXD at the end of the cable it works. But I don’t know if it is the right solution. Could you suggest me any other, please? I also tried to shorten the cable to 0.5m and it works as well but it is undesirable. Thank you very much for your opinion.