Digital display keeps blowing

[mikee04]

I have a remote control box to operate a power unit with a diesel unit in it (about a 100m apart).
The remote has two Digital displays DPM700s.
One display gets its signal from inside the remote box and causes no problem. The second display gets its signal from the unit (100 meters away); this works for a few days then keeps blowing.
The signal comes into the box, through a variable rest then to display. There is a zenor diode across the input and cap to try and cope with any noise or spikes.
I think that the unit is because some spikes that are damaging the display, does any one have any suggestion on what I can do to protect the display from these surges

 

[VEBill]

Are the 100m long cables leading into your control adjacent to any other wiring that might be carrying heavy currents?

If so, then relocate those wires away (several feet) from other wires.

 

[Comcokid]

On long cable runs, any nearby cables with high current can couple to you signal cables - overhead power lines, nearby arc welding, etc. Even a lighting strike some distance away may induce enough voltage to destroy the I/O devices. Your cable for your local display should not run in the same conduit with any other power lines.

Also, the presence of a local electronics, and the cable for the remote display could create a large ground or power loop if you have not carefully considered exactly where power and gound comes from, or even how a remote ground and the local ground are or are not connected together.

Your information "DPM700" was not enough to determine what kind of signal interface you are dealing with - RS232, RS422, parallel digital I/O, etc. Knowing this, other means of signal isolation may be suggested.