I'm building a 25-pin cable tester using a PIC16F877A, and to simplify things I'm using cascade BCD decoders to send the test signal out each wire. On the receiving side, I would like to use cascade BCD encoders so that I don't need to use 25 I/O's (plus to simplify code), but if the test signal returns on 2 wires at the same time (2 wires shorted together), I need to see that. I was hoping to find an encoder that would sum the inputs, but unless I'm reading the datasheets wrong, none seem to be able to do that.

If I use a priority-type encoder, it would report only the higher number input, so one of 2 shorted wires would report normal, and the other (lower number pin) would report an error, so that situation would be covered, but if they built the cable wrong and one pin is shorted to an adjacent pin that should not be used, and that one was a lower number pin, it would not be detected.

Any suggestions?

 

You probably need to arrange to two things to happen on the far end of the cable. The pin under test needs to show that it is connected to ground, and then every other pin needs to show they are not. If the pin tested is isolated, then all 24 others may be lumped together with a single pull-up resistor. Any short will pull all 24 down for a failure condition.