Siemens plc bits dropping out on start up.

The program seems to be ok, when we go online with the plc and watch the rungs, there are bits dropping out with no explaination, no ryme or reason why, there are five I/O's and 16 drives and a couple inverter drives. ac to dc inverters. In short this is all interfaced with a wire drawing machine built in Italy, custom drives ect. In short I'm getting an alarm for something that isn't true, any advice would be big help, thanks guys.

without more info I can only guess, although as the machine comes from Italy, they wouldn't do this.

But a common problem with new users to Siemens is to forget that Siemens is BYTE orientated, that is it the flags are BYTES of 8 bits and to get a WORD they combine two BYTES.

Therefore Memory Word 2 is really a combination of Memory Byte 2 and Memory Byte 3. So if you use a program where you are using M 3.2 as a flag and in that same program you transfer an integer into MW2, then all MB3 bits will be overwritten.

As I say with no real info this is just a guess.

If the problem is only on startup and once up and running, everything is OK, then two possibilities are:

1) There is some initialising being done in OB100 (or 101 or 102 if its a 400 CPU)
2) Some of the data has been declared non-retentive, so that the state is lost following a power down

To get any nearer to the answer, we need a bit more detailed info.