Counters: CTU, CTD and CTUD

Three counters

And the pins they share:

CTU · counting up

CV starts at zero and gains one on every rising edge of CU. Q goes TRUE when CV >= PV. R forces it back to zero and wins over counting.

Batches(CU := PartEdge.Q, R := Reset, PV := 12);
Full  := Batches.Q;
Count := Batches.CV;

CTD · counting down

LD loads PV into CV, then every edge on CD takes one away. Q goes TRUE when CV <= 0. This is the "fill 20 drums and stop" shape: load 20, count down, and CV tells the operator how many are left.

CTUD · both ways

For stock: a part in counts up, a part out counts down. Note: the lab on this site supports CTU and CTD, not CTUD. The lab program below therefore uses a CTU and a CTD side by side, which is also what you do on a platform that lacks CTUD.

The golden rule: an edge in front of the counter

A standard counter detects the edge internally, but the signal reaching it is often not clean. Wire CU := LSH straight through while the level hovers at 90% and every wobble counts as a batch. Put an R_TRIG on the logical event that means "a batch finished", not on a raw sensor state:

FullEdge(CLK := LSH);
Batches(CU := FullEdge.Q, R := Stop, PV := 5);

If the sensor itself chatters, pass it through a short TON first and then the edge.

Common mistakes

• Reading Q when you wanted CV for the HMI. Q is a yes/no question; CV is the number.
• A permanent reset: a hard-wired R := TRUE keeps the counter at zero forever.
• Forgetting the ceiling: CV is an INT, so 32767. A daily production total can pass that, so use a DINT in your own counter or reset at the start of each shift.
• Expecting Q to reset the counter. It does not. CV keeps climbing above PV. Resetting is your job.