We are operating a PLC H SERIES specifically H-702. System is working long time ago with alarm 51. Recently we suffered a problem regarding to energize process digital output. Looking through the software we saw umbelieveble situation in a simple programmed rung consisting in three series contacts for examine on signals, all energized but output referenced for internal bit in off. Supposed impossible, ¿Could you tell us what is happening?, Could be relationed with long time oerating with error 51?

First, let's explore Error Code 51. According to Hitachi, Error 51 means "I/O module error (always checked)". This level of error does not stop the CPU. What it means, is " A fuse blew in an output module. (The error module unit or slot number is set in the special internal output WRF005). OR - An error has occured in the advanced function module or communication module."

The remedy is (1) Isolate the module having a blown fuse by checking the special internal input, and replace the fuse. If this doesn't work then, (2) Replace the error module.

Now, as to why you are not seeing the Internal Output being energized when the three contacts in series are all energized. I would initally agree that under normal circumstances, you should see the output energized. You must have settings correct for monitoring, otherwise you would not see the three contacts change state.

You did not state if all the contacts were from external inputs, or if they were also from Internal Coils. One explantion could be if one or more of the three serial contacts are from a Internal Coil which was energized for only one scan (such as from a DIF - or Leading Edge Detection). The monitor mode of the software attempts to show that the "bit" in question was energized - at least at one time (for one scan). This is good to know obviously, but it is confusing if it doesn't then trigger an output, such as in your case.

One other possibility is with regards to scan time. Even 19.2kB is not "real world" time. Especially in big programs, the scan time may be such that conditions may change before they are updated on the PC monitor. Remember that even though you are only viewing a few ladder rungs, the ENTIRE project from top to bottom is updated as fast as the PLC processor, the serial speed, the PC processor, and the software can update.

One programming trick to determin if the output is really firing (even though it isn't shown on your monitor) is to add a "trap". In parallel with the three series contacts, add a contact from the coil (internal output) in question. Some code writers refer to this contact configuration as a "holding circuit".

When the three contacts do all energize, the internal coil will "hold" the circuit in - which you should see as you are monitoring. You may need to add a NC contact before the output, so that you can reset the circuit manually after you have determined that the coil is being energized.

It is always a good item to add comments/remarks in the project, so that you will be reminded to remove the "test" components when you are done. Otherwise, you can just upload the old program - overwriting the modified program which contains the test circuit described above.

Bottom line is, I don't think the Error Code 51 is playing any part in this phenomenon. If you do find a blown fuse as the error indicates, and after replacing it you still observe the same experience, then you will know for certain the error was a seperate incident.

I hope this discussion has been useful.

Timothy Davis