Why Are Latches Considered Bad Practice in Ladder Logic?
Everyone says it. Nobody explains it.
By Bryce Yokom, owner of Plantwide Integration. Published 2025-12-07. Originally posted on LinkedIn.
"My mentor told me, so it must be true."
It's what latches enable
The issue isn't the latch instruction itself. It's what latches enable: disorganized, scattered code.
Latches let you spread unlatch instructions anywhere. Different routines. Different programs. Now every troubleshooting session starts with a cross-reference hunt. That's slow, error-prone, and brutal during code reviews.
A seal-in keeps it in one place
A seal forces discipline. Your OTE, your seal-in contact, and your interlock conditions live on one branch. One location. When the output drops out, the reason is on that rung.
Scan behavior
A reader pointed out the other half of this: prescan. BOOLs in a Rockwell controller hold their value through a power cycle. An OTL isn't touched on prescan, so a latched bit comes back exactly as it was when the controller went down. An OTE only gets reset if prescan executes that rung. Sometimes holding state is what you want. Often it's a machine that restarts in a state nobody expected.
Rockwell documents the prescan behavior for each instruction: OTL and OTE.
Where latches belong
Are there valid use cases? Absolutely. I use latches in every program I write, just not the way most people do. I use them in modular code, not custom logic. They're in my fault handler, for example, where the latch and the reset live inside one tested AOI instead of across the program.
They have their place in ladder logic. It's just not where I see them used most.
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