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I have a customer who is submitting their machine for CE approval. Everything was moving along nicely until they failed the ESD test.

It appears that static discharge passing through the Com port on their V350 causes the PLC to shut down. 24 V DC power is still there but the screen is blank. The guy running the test must then shut off power and re-start in order to make the display come on again. Perhaps a little more background would be helpful. The machine has a USB port on the outside of it for end-customer convenience. Inside the control panel, then, is a USB-to-serial converter wired between that USB port and the Com port on the V350. The test seems to be injecting ESD on that external USB port and then running all the way into the PLC's Com port.

Does anyone out there have experience in this area? I am hoping there is something I can do with software to monitor for this condition and then Reset the PLC. This would be acceptable to CE and we would then pass the test.

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My experience is that Unitronics doesn't have transient noise filtering circuitry built into the PLC. I usually add snubbers or diodes to the outputs and signal conditioners to the COM ports to clamp the noise down.

These guys make a good device specific to the various communication hardware types:

Citel

Joe T.

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These little buggers-

Data Line Protectors

You have to break the wires out and run them through them - easy enough to do when the com cable is flat. I bought 100 feet of six conductor flat phone wire, some six conductor RJ12 plugs, and the tool to put them on. You have to do this for an RS485 connection, and if the run is RS232 and short you can just cut the programming cable that comes with the PLC. Like anything else, some experimentation is required to determine a good configuration.

You didn't state exactly how the ESD test is being done, but I'm guessing they're probing the com cable or port as part of the test.

Joe T.

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