S7-400 redundant PLC went crazy

[Kenwei8011 0]

Hi all, I have a set of s7-400 Simatic H station with CPU 412-5 H PN/DP, redundant system. I have done my program in FBD logic and completed all the logic test successfully. then I added some program for AI signals where I use the FC105 scale function and some comparison blocks to generate some alarm bit signals. After I downloaded my program (I created new FC for the the AI signal logic), my existing DO points went crazy as the output keep "On" and "Off" alternately. I am sure that my logic for the AI signals are correct. After further diagnostic, I suspected the error is from the function called "Midline Output". All these Midline output have assigned to "TEMP" address. So at the end I changed all these Midline to memory address, and my system is working fine now. My question is why can this happen? Basically all the program, logic are remain the same, just by changing the temp address (which was successfully tested) to memory M address. Is Siemens have some bugs in their Simatic software? Or just I'm stupid enough to make the PLC went crazy? Thanks. Best regards,

[Groo 7]

Not enough information to be certain. Two things come to mind. 1. From OB1 down, there is a finite number of TEMP words available, if the program was close to max on a leg and an additional block is added with TEMP memory used, it could tip the boat. I'm certain that you get a message from Simatic Manager regarding that stops you saving though, so this is unlikely. 2. This is more likely. The TEMP words were being used as memory, i.e. the programmer expected them to remember the state from the previous scan. Sometimes you can be lucky and the same area of TEMP words are used by the system and it appears as if the program is good, then at a later date a modification is made and now the PLC changes the TEMP area used every scan and what had looked a good program now does not function. TEMP area bits do not remember last state, the golden rule of TEMP is you condition it immediately before you use it.

[Kenwei8011 0]

Yes, I changed all the TEMP addresses to memory M address and it works fine now. Thanks.

[Auu 0]

Hi Kenwei8011, I have a question about s7-400 Simatic H station with CPU 412-5 H PN/DP, redundant system.  I use step 7 version 5.5 SP4 How to setting redundant in software ?  How to download PLC redundant ?   Thanks. Best regards,