Sanity check - is there something wrong with this RS232 receiver design

[daqq]

Hi guys,

There's a device/system that is just a passive receiver for RS232 signals coming out of a remote device (1 to 10 meter cable, baud rate 9600). The schematic for the Rx part is in the attachment.

Apparently, there is the occasional problem with these devices that they go dead for reasons unknown. The rest of the system works well. Is there any way this input setup could kill the system or allow a spike from something to kill the rest of the system or the Rx part?

Edit: The zener is actually a 3.9V, LTSpice does not have its model.

Thanks,

David

[SeanB]

I would suggest another 10k resistor between the zener anode and the base of the transistor, so that you get more overload protection, as in the current diagram the zener is not really doing anything but acting as a diode. Most likely the transistor fails from reverse base current if the zener goes open circuit. You could replace the zener with a regular diode as well, will make no difference but the capacitance will be lower.