ABB AC500 flashing an 'ERR' light
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As said above, checking your hardwares is one of the method.
If your IO modules are plugged in properly, then try logging in through the serial or ethernet ports.
If logging in has a problem, then try resetting your PLC. Be sure you have the final backup. Once reset the datas will erase in the PLC. If you have downloaded the final backup into the SD card then you go ahead peacefully.
In order to reset, press the RUN and OK key together during power up for a min or two. Sequentially three lights on the display will blink. this indicates that you have reset your plc.
Pratik is right...more several reason could be cause the ERR flashing.
First of all, you should check the HW configuration, the problem may be depending of a bad connection of a I/O card.....
In any case, the only way to check the number error and the cause is to connect the CPU to Control Builder, just log in and read the error code. The help manual give you some suggestion to solve the mistake.
If you give us more information, we can help you better.