Error Code 21H -Micrologix 1200

Im getting this error whenever im keeping the PLC in Run Mode.Once I have cleared the Memory ,downloaded other program & kept in Run Mode,the same error repeated.Pl. help me if anybody has faced the same problem .

rajaram

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From:
MicroLogix™ 1200 and MicroLogix 1500 Programmable Controllers Bulletins 1762 and 1764 Instruction Set Reference Manual

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I've gotten this error before, there are few possible causes that I've experienced.
1. You have an expansion module in your I/O tree, but it isn't present.
2. The ribbon cable connector is loose, sometimes removing and re-seating will cure this.
3. Faulty expansion module.
4. I've also seen this on ML1200's that are shut down. For some reason the expansion module powers down before the main unit, the fault is detected just before, the main unit shuts down, when the power is restored, it still has the fault bit set. Try setting the fault override at power up bit, S:1/8 to true.

Error code 21H

Thank u 4 replying.
What uve said has happened.We have a analog IO Module which was not identified by the micrologix 1200 thro "Read IO tab".After checking ur replies I have replaced the IO module with other one,same fault is popping up.
I just suspect the commn. port in the PLC is having a problem because the ribbon connector from Anlog IO module is connected reverse in the PLC (after maintenance somebody kept the ribbon connector reverse).Whether any chance is there for the port to fail if the ribbon connector is connected reverse?
Pl.reply

rajaram
India