just for Omron PLC experts

Can't comment specifically about Omron PLCs, but this (regarding increment and decrement) holds for any breed:

Looking at increment:

When the rung conditions are true, the instruction is obeyed. Increment is the same as "add one to the contents of such-and-such register".

If the rung condition is true every time the PLC scans its program, it will add 1 to the register. This could be e.g. for as long as input N is ON. So, every time the program loops round (the cycle is 'read inputs' - 'do logic stuff' - 'update outputs'), the target location will have 1 added to it. As your manual says, if the scan time (time to do the 'input-logic-output' cycle) is 10ms, the target register will be incremented 100 times per second.

I don't know the Omron-speak for this, as I generally use Rockwell/Allen Bradley PLCs (which have a 'do this only once' type instruction), but this is the sort of algorithm you'd have to use to increment (add 1 to) a location each time input N is turned ON:

0. (Initialization only) Set a flag to remember 'Input N was off'

1. Read inputs.

2. Is Input N on? No, -> (7)

3. Was Input N off? No, -> (8) (Comment: it's ON now, but it was ON last time round, so don't add one)

4. Add one to target location (Comment: this is a new OFF-to-ON transition)

5. Clear the 'Input N was off' reminder flag

6. -> (8)

7. Set the 'Input N was off' reminder flag

8. -> update outputs

9. -> (1)

I hope this makes some sense - you may need to print it out & try some example run-throughs.

Remember that PLC operation is a combination of parallel and sequential operations. All the inputs are read at the same time, and remembered, then each of the ladder rungs is processed in the order they're entered, then all the outputs are updated at the same time.

(Before anyone shoots me down - there are all sorts of exceptions to this rule (depending on the breed of PLC), with instructions to read the instantaneous state of an input, or update the state of an output, embedded in the ladder logic section, and jumps and loops within the ladder logic itself).

Thank U Mr JohnDG

Up to now I don't know how to make advantage of both (Inc ,Dec) within the industrial applications , which applications are required to add one to data register ??

Also what's the meaning of @ sign that comes with Dec& Inc???

I hope to receive further participations ,So come on guys

Increment and decrement are just included as short forms of add 1 and subtract 1. You don't have to use them in any application, but sometimes they could be handy.

Example: Sensor 1 detects items going onto a conveyor, sensor 2 detects items coming off it at the end. When sensor 1 triggers, it increments a data register, and when sensor 2 triggers, it decrements it. Thus, the memory location always contains the number of items on the belt. I'm sure you can think of other examples.

Regarding the @ operator - I may be wrong here, but I think it indicates address indirection. Say data register 400 contained the number 123. The instruction inc 400 would add 1 to register 400, while @inc 400 (or whatever the syntax is) would add 1 to data register 123.

Just had a look at the programming manual, and it seems using the "@" prefix, as in "@inc D400", causes the instruction to be executed only on a transition from FALSE to TRUE (0 to 1 for inputs), so it could be used instead of the method I described earlier.

If you put @ in front of increment or decrement they will count only once when the input is switched on. They will not count again until the input goes of and on again. Hope this helps.

Hi to all, am using the Star hitech MX-400 plc's which is merely equal to omron plc's and the Software too the software name is MX-Builder I need to increment the Address of the 'D' Data registers for the iteration loops can help anybody to sort out this problem asap. thank u.