Do cyclic interrupt OBs use the process image to read input signals?

[parky]

Yep that is what the plc does, when updating the image it copies the PW to the IW anyway all you are doing is exactly the same thing but at a rate or time you want, you can update the Image at any time as posted before you could do that a dozen times in OB1 between calls to other blocks although not consistent i.e. if a call to the update is before & again after some program block the time will vary depending on the internmediate block scan time for example
C PB1 (takes 5ms) some code
C PB100 (takes 2ms) the PIW update
C PB2 (Takes 15ms) some code
C PB100 (takes2ms) The PIW update
& so on
So every call to PB100 the delay between the update will vary i.e. first call 5ms
Second call 15ms but if the total scan of the program was 100ms at least you can update as fast as the slowest block scan.

 

[mk42]

Not sure about Step-7, but in PCS7, you can define PIP's (process image partition) and assign each PIP a specific OB, such as OB35. Then when you are laying out your hardware, and assigning addresses, you also assign a PIP for the image updates. The PIP OB and the logic OB should be the same. You can use OB1 for everything, but it is not recommend.

It's a PLC feature, supported in S7400 and 1500, but not 300 or 1200. I'm sure it gets way more use in PCS7 on the 400 than it does in "regular" 400s, but it is still available in step 7.


As JesperMP said, it doesn't really make sense for just a couple bits, but it can make things easier when its gets to be a decent chunk of data, probably all in the same rack.

 

[mk42]

Jesper my friend, this is gold.
So if I do this code in the OB38, every 10ms I will be reading my input and writing it into MB10.2?
Would it work if I use temp byte in order not to waste M area?

Since your question question wasn't directly answered, yes you could. I agree that the other solution of writing it to the process image tag is even simpler.

 

[drbitboy]

If it's critical, then I would go with an external pulse stretcher i.e. essentially build your own high-speed counter/detector; solving a hardware problem with a software solution is non-trivial; PLC scanning is non-deterministic and there is no way to guarantee sampling intervals.