Problem using comparator

[Katerina 0]

I am using version 9.2 of Cx-programmer. I want to compare the current value of a reversible counter(CNTR)  with the value 0. I tried to create a comparator by writing

[CMP C7 0 ] and then in the next rung I connect the comparator with a coil ( which should be energised when the comparator is true). But there is an error :

ERROR: Coil is not positioned correctly at rung 155 ( 6, 0 ). 
ERROR: Function 'CMP(020)' is positioned incorrectly. at rung 155 ( 0, 0 ). 
 I have attached apicture which shoes clearly what I have done!

[Mendon Systems 22]

The CMP instruction sets one of the arithmetic flags in the A memory area. You need to use a NO contact of the appropriate flag (<, <=,=,>=,>) in rung 155.

It is probably easier just to use the corresponding symbol math instruction (<, <=,=,>=,>) and forget the CMP entirely.

Edited 3 Mar 2016 by Mendon Systems

[Michael Walsh 268]

Also, note that if you are trying to compare the counter value with 0, you need to put #0, which means a constant BCD (in this case, can mean Hex also) value of 0.  By just putting 0, you are comparing the value of the counter with the value in channel 0 (which may or may not be a value of 0).  Channel 0 is the first input or output channel of the PLC.

[BobB 58]

You need CF113 in front of the compare instruction. I would just use the symbol math instructions as suggested above and then to an output.

[Katerina 0]

18 hours ago, Mendon Systems said:

The CMP instruction sets one of the arithmetic flags in the A memory area. You need to use a NO contact of the appropriate flag (<, <=,=,>=,>) in rung 155.

It is probably easier just to use the corresponding symbol math instruction (<, <=,=,>=,>) and forget the CMP entirely.

Thank you for your answer! What do you mean by saying''NO contact of the appropriate flag (<, <=,=,>=,>)'' ? Is that what you told me to do? (see the picture)

 

[IO_Rack 181]

This is the method you are attempting. NO is 'Normally Open' contact. The Symbol Math Instructions are a different method.

[Mendon Systems 22]

3 hours ago, Katerina said:

Thank you for your answer! What do you mean by saying''NO contact of the appropriate flag (<, <=,=,>=,>)'' ? Is that what you told me to do? (see the picture)

 

The method that you have used in your latest code sample is the simplest way to accomplish what you want. The symbol math instructions (called "input comparison" instructions in the older CJ1/CS1 manuals) will do everything in one instruction.

The CMP instruction requires you to use the processor flags as shown in IO-Rack's example. It does have the advantage of being able to check for all three conditions in one rung, but you must be careful to do the condition tests immediately after the CMP is executed so that nothing changes those flags.

[Katerina 0]

Thank you all for your answers!!

[Michael Walsh 268]

There is a mistake in this ladder code below, make sure to use the = instruction, not the =F instruction.  =F is floating point and the CNTR instruction uses BCD as the format.   An additional note:  you do not need to make a comparison to a counter value.  When the counter reaches 0, the bit C7 will turn on.  Just Use a N.O. C7 wherever you wanted to use the Bm_Free bit.  With a standard counter (CNT) instruction, using the counter done bit makes sense and will function as expected.  It would count down to from 4 to 0 and then you would have to reset it to start again.  When the counter is at 0, the done bit would turn on.  With the CNTR instruction, it is a bit different.  The top input will make C7 count up, the middle will make it count down and the bottom is a reset.  Both the count up and count down will make the counter value wrap around like this:  when counting up: 1, 2, 3, 4, 0, 1, 2, 3, ....  When counting down:  0, 4, 3, 2, 1, 0, 4, 3, 2, 1...  The done bit will be on when C7 is 0.