S7-300 - increment 16, decrement 1 counter?

[PaddyClarke]

Hi,
I have a stand alone inspection unit with no PLC but I can send 24v input signals to the machine PLC to show a good read and a bad read when products are inspected.

I have 2 inputs, I0.0 for a ‘Bad read’ and I0.1 for a good read from the inspection unit.
Rather than reject every ‘bad read’ I want to allow all inspections to pass through but have a serial check running where the machine will stop if the amount of rejects are excessive so the operator can visually inspect.

I would like the code to do the following:

For every bad read, I0.0 I want a counter to increment by 16 and for every good read, I0.1 I want the same value to decrement by 1. So this value will continually be going up and down depending on good and bad reads. When the value reaches 200 I want to use a memory bit M1.0 to insert into the stop rung for the machine.

Any advice on the best type counter to carry out this operation would be appreciated.
Thanks.

 

[L D[AR2P#0.0]]

Roll your own in a Function block.

 

[PaddyClarke]

Thanks for your reply.

What would that look like in a FB?

 

[L D[AR2P#0.0]]

e.g.

 

[PaddyClarke]

That's great, your help is much appreciated!

 

[PaddyClarke]

That works well, however how can i prevent the count value from going negative?
The value would count down to zero as 99% will be 'good reads' , sit there at zero until a 'bad read' occurs where it increments to 16 or beyond and counts again as normal. If it goes negative it would take many 'bad reads' to reach the 200 cut off value.

 

[drbitboy]

how can i prevent the count value from going negative?

​

 

[L D[AR2P#0.0]]

Should be Greater or equal to zero and I'd put it after the pos edge instruction.

 

[drbitboy]

Should be Greater or equal to zero and I'd put it after the pos edge instruction.

I did think about it and the difference would be minimal, but I agree, although it was easier to modify the JPEG by putting it in front of the edge detector.

 

[PaddyClarke]

Thanks for your help guys.