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Lighthouse PLCs, Inc. : Automation Forum

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PLC and Inverter Forum
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Re: EM dropping out of RUN mode


Dear Robert Nichols


The “EM”-Series was produced from 1987 to 1995. In 1990 an upgraded processor (CPM-E3) was added, making the series “EMII”. The EMII is still being produced, and is a very popular model. There is quite a bit of information on our website on this model, including our FAQ section and our Web Forum.




The first (ladder) line of the program in the EM/EMII requires a minimum of two “contacts” in front of FUN98 (start). Most programmers just use M990 – an internal memory register that is “always on”. One or both of these “contacts” may be replaced – it is not unusual for an input (such as Input 0 that you mentioned) to serve as the replacement. Once both of these “contacts” are TRUE, then FUN98 turns the PLC RUN mode on (Run Light illuminates on the processor).

An example is presented here (hopefully).




The most obvious cause of the RUN light going on and off, as you mention, is a mistake in the wiring. Perhaps your if you have replaced one of the “M990” contacts with some other Input, there may be some problem (like an intermittent open) in that wiring.




You indicate that “…the RUN light blinks on and off with the outputs.” What you may be observing here, is that when the processor goes into “RUN” the output lights will of course come on as the program “processes”, and then they will go out when the processor drops out of “RUN” mode. I suspect you are observing the “effect” rather than the “cause”. A couple of other possibilities that could cause the RUN light to “come and go”, are 1) a possible short in the input wiring – perhaps in the circuit (Input 0?) that controls FUN98. Or, (2) a “glitch” in the program, that is causing a “syntax” error in the processor. At the beginning of each scan, the processor does a “self-diagnostic” check. Unless everything is “go”, the RUN light will not go on (or will stay off).

According to the EMII manual, other possibilities include: Noise (= transient voltage or EMI/RFI) that might be interfering (which could explain the 10 sec. lapses), Program too long (taking more than 100ms to scan) [doubtful], or a damaged processor [possible].

Hopefully we have provided some hints that might help you determine your problem.


Good Luck with your Project!


Best Regards,


Timothy R. Davis

--- --- --- --- --- --- --- --- ---

Replying to:

To Whom It May Concern,


I am having a problem with a PLC that was on a CNC router that was purchased from Japan around 1986. When the power is supplied to the machine, the run light blinks on and off along with the outputs. The input 0, which will toggle you from run to program, stays on all the time. The input and output power of the power supply on the PLC remain constant. It sometimes happens for 10 sec. and then stops and the machine will work fine after this. Sometimes it won't stop until you cycle power again.

If you think it is the CPU- can these still be purchased? Any info would be great. Thanks.






Robert Nichols


Electromechanical Supervisor


Ashley Furniture Ind.


One Ashley Way


Arcadia, Wi 54612


Ph. (60 -323-6205


Fax (60 -323-6013