"TNB 0" What does it do / achieve

Hello ConfusedAussie

the short answer is that TNB 0 does nothing and it makes no sense to have it in the logic as shown in your screendump.

My guess is that this is a a display/substitution issue of S5 for Windows which has been known for not always displaying original Siemens Step 5 logic correctly (generally related to S5 for Windows being able to represent Step 5 STL logic incorrectly in in Ladder).

Question is if you have Siemens Step 5 to check how it displays the offending networks in question?

Alternatively, upload the original S5 program here and let us know which Blocks and Network in question have the TNB 0 in them, perhaps soemone can quickly check it for you how they display in Step 5.