Yes, a normally open C1 contact driving Y001 is the right rung. When your reset rung clears the drum the moment it completes, C1 is on for a single scan, a few milliseconds, and the output flash is too short to see. Add a set-reset hold with a timer to make the completion visible.
Yes, the rung you wrote is correct: one normally open contact addressed to C1, feeding the Y001 output coil. It commands Y001 for exactly as long as C1 is on, and the reason it looks dead is time, not logic. The CLICK drum clears its completion bit when the drum resets, and the usual repeating-cycle arrangement uses that same bit to do the resetting. C1 comes on at the end of the last step and is gone about one scan later, a few thousandths of a second in a small program. The LED, your eye, a data view and an HMI will all miss that. Below: the checks that separate a dead rung from a short pulse, and a set-reset hold that turns the flash into a solid indication.
How do you turn on Y001 from a completion bit like C1?
Put one normally open contact addressed to C1 in series with the Y001 output coil on a single rung, and the PLC commands Y001 for exactly the time C1 is on; an internal control relay drives an output the same way a real input contact does.
An internal control relay is a bit in the PLC's memory. A contact addressed to it reads true whenever the bit is 1, so any rung you would write with an input contact works unchanged with C1. The drum instructions default their completion flag to one of those Cx addresses and set it when the sequence finishes its last step. The flag address is assigned in the drum's configuration, so read it there once and match the contact to it character for character.
One limit to know before anything else: the rung has no memory. If C1 is on for one scan, Y001 is commanded for at most one scan, and nothing in that rung will stretch it.
Why does Y001 never look like it switched on?
Because the drum turns its completion bit off when the drum is reset, and your reset fires the instant the sequence finishes, C1 is true for a single scan and Y001 follows it right back down.
The drum instruction documentation describes the reset path directly: resetting the drum clears the complete bit and returns the sequence to its preset step. The standard way to make a drum repeat is to feed the completion bit into the drum's reset, on its own or in parallel with another reset condition. So the chain at the end of every cycle is: last step finishes, C1 sets, the reset acts, the drum clears C1 and jumps back to step 1.
Nothing failed. The completion bit did its job, the contact did its job and the coil did its job. The event is simply shorter than anything downstream can display.
How long is one PLC scan in real time?
About 3.3 ms in a small program, which puts the pulse near 1/300 of a second; the eye reads that as nothing, and a data view, monitor mode or HMI samples too slowly to land inside it.
A scan is read inputs, solve every rung, write outputs, and its length grows with program size and I/O count, so that figure is a small-program number. A large program can run several times slower, and the pulse stretches with it. The arithmetic: 1 s divided by the scan time is about 303 scans per second, so the completion bit lives for 0.0033 s of the cycle it ends.
The same sampling limit is why an R_TRIG pulse never shows on the HMI screen: the edge is one scan long and the display update is not. From the other direction, how many PLC scans an HMI momentary button bit stays on decides what a screen can catch of any short bit in the first place.
Which symptoms point to a short pulse and which to a broken rung?
If the drum keeps restarting on schedule, C1 fired every cycle and your rung is almost certainly fine; if the drum stalls at the last step, the completion or reset path is what needs the work.
- The drum restarts on schedule and Y001 stays dark. The one-scan pulse. The rung is doing its job on a signal too short to see.
- A faint flicker of the Y001 LED at the end of each cycle. Same cause, caught at the edge of what an LED shows in a single scan.
- C1 reads 0 in a data view while the cycle runs. The expected result with a one-scan pulse; the view samples slower than the pulse, so nearly every sample lands outside it.
- The drum sits at the last step and will not restart. A different fault: the reset is missing or never true. Here C1 holds on, so a correct rung should hold Y001 solidly on, which is your clue.
- Y001 is stuck on between cycles. Two rungs writing one output, a double coil; drum outputs that stay off from a double coil walks the search that finds it.
- Y001 is on for the wrong duration, seconds where you expected a flash. A series contact or a branch is shaping the rung. Audit everything feeding that coil.
What checks settle it without guessing?
Read the completion flag address out of the drum's configuration, put a counter on that bit, and run one cycle: a count of 1 says the bit fired, and the pulse is real.
- Open the drum's configuration and read the completion flag it uses. Time-based and event drums default that flag to a Cx bit; when the drum completes on a count instead, configuring CLICK drum events for CT1 count completion shows where that completion event is set.
- Check your rung against that address, character for character, and remove any extra series contacts or branches you added while testing.
- Cross-reference every rung that writes Y001. There should be one writer.
- Point a counter's count input at C1, reset it to 0, run 5 cycles, and read 5.
- Watch the drum's step value in a data view: it should reach the last step and return to step 1 each cycle.
How do you hold Y001 on long enough to see it?
Set a spare control relay from C1, drive Y001 from that relay, and clear the relay with a 2 s timer: the indication changes from a one-scan flash to a solid light at every completion.
- Rung 1: a normally open C1 contact feeding a Set coil on a spare control relay, C10 in this example.
- Rung 2: a normally open C10 contact feeding the Y001 coil. Your original rung with one address changed.
- Rung 3: a normally open C10 contact feeding a timer with a 2 s preset.
- Rung 4: a normally open contact of the timer's done bit feeding a Reset coil on C10.
C1 pulses one scan at completion, and one scan is enough for a Set coil. C10 then holds Y001 on and runs the timer; when the preset elapses, the done bit resets C10 and the light drops. The next cycle arms it again.
One check before you write this in: confirm what Y001 actually switches. A timed hold on a stack light is an indication; the same hold on a valve or a starter is machine motion. If the output should stay until a person clears it, drop the timer and reset C10 from an operator input or from the first step of the next cycle.
How does the rung set behave in a worked example?
A 10 step time-based drum at 1.0 s per step completes every 10 s, so with the hold timer set to 2 s, Y001 is on 2 s and dark 8 s out of every 10 s.
- Cycle time: 10 steps at 1.0 s per step = 10.0 s.
- Raw pulse: one scan of 0.0033 s, which is 0.0033 divided by 10.0 = 0.00033 of the cycle, or 0.033%. That number is why nothing downstream shows the completion.
- Held indication: 2 s of every 10 s cycle, an on-time fraction of 0.2, with 10.0 minus 2.0 = 8.0 s dark.
- After 6 cycles the counter reads 6 and the light has been on 6 times 2 s = 12 s in total.
- Want the light on for half the cycle? A 5 s preset gives 5 divided by 10 = 0.5.
How do you prove the fix on the machine?
Run one full cycle with the machine in a safe state and watch two numbers: Y001 solidly on for the timer's preset, and the event counter up by exactly 1; that is a pass.
Run a second cycle to see it re-arm: the light on again for the same duration, the counter at 2. If the light never drops, the reset path is at fault; check the timer's done bit contact and the Reset coil address. If the counter counts but the light never comes, the C10-to-Y001 rung or its output address is wrong, or another rung is writing Y001.
Recommendation: Keep your C1 contact rung as the event source and add the set-reset hold with a 2 s timer whenever a person watches Y001. Use the maintained version, reset by an operator or by the first step of the next cycle, when the output must persist between cycles. Leave the raw one-scan contact in place only where the consumer is more program logic, such as the drum reset rung or the counter, because logic in the same program acts on the bit in the scan that sets it.
Questions
Does the completion bit stay on if nothing resets the drum?
It stays on from the end of the last step until something resets the drum, because the reset is what clears the complete bit and returns the sequence to its preset step. With no reset rung at all, a C1 contact holds Y001 on indefinitely. A permanently lit output after completion is a reset problem, not a rung problem.
Can an HMI screen catch the one-scan pulse?
No. At a few milliseconds the pulse is shorter than any HMI poll, so the tag reads 0 on nearly every sample. Count the completions in the PLC with a counter and display the count value, or display the held relay from the set-reset rung. Both give the operator a number or a light that stays put.
What if Y001 has to stay on for the whole cycle, not just the hold time?
Drive it from the step logic, or from a maintained bit with a deliberate reset condition, and leave the completion pulse for counting and resetting the drum. A completion bit reports that the sequence finished, so it is the wrong source for an output that must be on from step 1 through step 10.
Why did the same rung work on another machine?
Because that drum was reset later, or by a separate condition such as an operator input, so its completion bit stayed on for whole seconds and the contact had something to hold. The rung was never the variable. What changed between the machines is the time between completion and reset.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.