Confirmed symptom and fix
The reported failure was not the whole tire-detect program: up/down counter B057, configured to switch ON at count 2 and OFF at count 3, behaved correctly in LOGO! Soft Comfort v8 simulation but switched ON at the first count and OFF at the second count on a LOGO! 0BA8 during online test. The counter is used to prove that input I3 is sensed exactly twice before enabling the directional count path to B054, or B051 for the opposite direction.
The evidence-supported resolution was to move the sense signal from I3 to I8; after that change, the isolated counter test and the main program worked. Treat an internal input-stage or channel-specific hardware issue as a hypothesis, not as a proven relay failure, until the module is tested with a known-good input source.
Why I3 is the first suspect
Before changing logic, separate simulation timing from real input behavior. If LSC follows the intended sequence but the 0BA8 does not, verify that the physical pulses actually reach the CPU in the same order and width as the simulation. In the reported sequence, the important discriminator is two clean I3 pulses while the column sensors establish and release around them.
| Step | Expected tire pattern | Check online |
|---|---|---|
| Enter | I1 and I2 ON | Both set the enable/flip-flop path |
| Front sidewall | I3 ON then OFF | Must be count 1 only |
| Middle | I4 and I5 ON, I1/I2 OFF later | No extra I3 edge during transition |
| Back sidewall | I3 ON then OFF | Must be count 2, not a reset/re-trigger |
| Exit | I4 and I5 OFF | Counter disables and clears for next tire |
Hardware constraint: fast inputs on DC LOGO! models
The discussion confirms that LOGO! units with pure DC supply have fast counter inputs on I3 to I6. The unit under test was fed with 24 V, and a 12/24 VDC model was specifically suggested in the discussion, so do not assume I3 behaves like a ordinary low-speed digital input in every configuration. If I3-I6 are involved, confirm whether the input is reserved, filtered, or counted by a fast-input path before using it for a slow mechanical beam-break pattern.
Isolation procedure
- Download only the B057 test logic and drive I3 with a controlled switch or sensor; verify ON at second pulse and OFF at third pulse in online test.
- If B057 still triggers on the first I3 edge, move the same field wire to I7 or I8 and update only the input address.
- Repeat the identical pulse sequence; if I8 counts correctly, reconnect the full tire logic and validate both directions through B054 and B051.
- If only I3 misbehaves while I8 works, inspect wiring/terminal contact first, then plan module diagnostics or replacement; do not rewrite the counter to mask a channel fault.
FAQ
Why does LOGO! B057 work in LSC but not on the 0BA8 PLC?
Because the confirmed failure was input-channel behavior, not the counter algorithm: LSC v8 produced the expected count sequence, while the physical I3 path caused early triggering online. Moving the signal to I8 restored correct operation.
Which LOGO! inputs are fast counter inputs?
On LOGO! models with pure DC supply, the fast counter inputs are I3 through I6. Verify the exact 0BA8 variant and input usage before wiring a slow sensor sequence to one of these terminals.
What is the quickest test for a suspected bad LOGO! input?
Run a minimal up/down-counter program, apply the same repeatable pulse to the suspect input, then move that same wire to I7 or I8 without changing logic. If the new input counts correctly and the old one does not, focus on wiring, terminal contact, or the input channel hardware.