Wired Or Bus - Error Isolation

[ludzinc]

Hi

I need to interface to some old equipment that uses a simple clocking protocol to send status indication of devices connected.

Simply put, each device has an ID, and after the correct number of clock pluses have been seen, the device asserts it's data line during the clock low period.  An idle period at the start of the clocking sequence signals to the devices to start counting pulses.

The data line is drive by open collector PNP devices and this are 'wired or'  (active high).  Any number of devices can assert the line at the same time (this allows for multiple devices to have the same ID).

This system is often used to link various areas together, with different 'busses' simply wired together.  See the attached pic for (hopefully) some clarification.

So far so good - it's simple and pretty robust. 

- however -

If the data line at any point is shorted high (transistor fails - not typical, installation / cabling error - typical!) the whole system is 'stuck high' and it's a bloody nightmare to fault find.

I've connected buses together in the past, using a micro to read the state of one bus, and then 'echo' this on other buses, but I'd love to implement this with simple logic.  However, every scheme that I can come up with ends up in a race condition and I get the system stuck on all the time.

Yes, using a micro works, but  (at least to me) this seems like a simple thing to solve but the answer alludes me.  Any puzzel solvers out there who can help?

Cheers,

Simon