Station 3 on our press line, 1769 L30ER on V30. Didn't write it, but I can read it. The step logic is a latched bit per state, StepClamp, StepPress, StepEject and so on, OTL and OTU, about 15 states.
Twice this week the press cycled and the ejector fired at the same time. I caught the two bits both on online. Nobody hurt, the guard was down, but it bent a part.
I tried adding an OTU for every other state bit into every transition rung, so now its a wall of instructions and it still happened once after that.
Our controls guy's off till next month and my supervisor says no SFC on this site. Is there a cleaner way to do this in ladder without 40 latches? No SFC on this site, so it has to be ladder our lot can follow online, any ideas what that looks like?
What changed since it last ran clean, a sensor swap, an edit, a faster cycle? And when both bits are on, which transition rung set the second one? Look at that rung's own conditions online, not at the bits.
Cycle rate went up about 10 percent in April, that's all we know of. Caught it online today. The clamp to press transition and the press to eject transition were both true in the same scan, because the clamp switch was still made when the eject eye cleared. So two OTLs in one scan and nothing stopping it.
Two latches can both be true because nothing in the program says only one of them may be. An integer holds one value and can't hold two, so the rule enforces itself. Not brand specific either, I've seen the same shape on four runtimes.
One DINT tag, Step. One rung per transition: EQU Step 20, the condition, MOV 30 Step. Outputs in their own rungs, EQU Step 30 drives the press valve and nothing else does. Only the transition rungs write Step, all in one routine, and on V30 that's a half day's work.
The integer is what stops two states being on together. Reverse order, highest step at the top, fixes the other half: when Step goes 20 to 30 the 30 to 40 rung is above it and already evaluated, so you advance one step per scan instead of three. Number them 10, 20, 30 so you can insert later. The HMI shows Step, one number instead of 15 bits. Someone did an article with the state layout in it: https://plctr.com/maximizing-plc-efficiency-in-automotive-manufacturing-systems/