Olin Lathrop wrote

> Assuming you measured this with a normal ohmmeter that puts a volt or less
> accross the device, 35Mohm sounds plausible and 194ohm is most likely 
> fried.

Yes, I just used a normal meter. I was convinced that 194ohms was wrong, but 
the other one didn't really give me any great clues either.

> I guess they are intended to clamp the lines within some acceptable 
> voltage
> of ground.

That was my impression too.

They are probably TVSs.  You should be able to replace them with
> any TVS with a reasonable rating given whatever common mode range your
> circuit has to handle.

This is where I am still a little in the dark, because all I have is a 
faulty board  ... I dont have a circuit or any user manual for any of the 
system, or for any of the parameters, other than a legend on the back that 
shows it runs on 24v dc. The rest I've had to guess.

Other than not having anything to communicate to the board with, I have 
everything else working  ...  so I'll take a stab at a value, fit a couple 
of TVS's, send it back and see what happens.

Thanks to everyone who has replied.

Regards,  Roger 

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