AFI instruction or XIC?

Dears;

If we use an AFI instruction (in RS Logix) in the start of a rung or use XIC instruction with a bit address are the same function? I want to say that with AFI instruction, system doesn’t scan the rung but if we use XIC instruction with bit condition 0, system also does nothing with output in that rung. Am I correct or mistaking?

AB2005 said:

Dears;

If we use an AFI instruction (in RS Logix) in the start of a rung or use XIC instruction with a bit address are the same function? I want to say that with AFI instruction, system doesn’t scan the rung but if we use XIC instruction with bit condition 0, system also does nothing with output in that rung. Am I correct or mistaking?

Click to expand...

Mistaken. AFI - always false instruction. The output(s) will be false and functions will not execute (moves, math, etc.).

The XIC when false will have the same effect. The processor will treat any of the logic after the false instruction just like it would with the AFI.

AFI is available in the PLC5 but not in the SLC. For that reason, many programmers reserve a bit and call it always false. They'll put an unconditional OTU instruction at the beginning of the program to ensure it stays zeroed, then use that bit for blocking parts of the logic that is not used, reserved for future options, or for testing.

We always put are own Always False Instruction (AFI) and Always True Instruction (ATI) for disabling or bypassing parts of the logic for testing, and troubleshooting etc. Typically B3/0 for AFI, and B3/1 for ATI in AB PLCs, but no address is required for the Logix Platform. One advantage of this using the Symbol when programming, and they can be searched for to see where they are in the program. In some cases, other manufacturers have these bits predifined already. I first saw it quite a few years ago, and have always made that the first thing I do in a new program, and add them to existing programs I am working on. Just my two cents.

Stu.....

sthompson said:

We always put are own Always False Instruction (AFI) and Always True Instruction (ATI) for disabling or bypassing parts of the logic for testing, and troubleshooting etc. Typically B3/0 for AFI, and B3/1 for ATI in AB PLCs, but no address is required for the Logix Platform. One advantage of this using the Symbol when programming, and they can be searched for to see where they are in the program. In some cases, other manufacturers have these bits predifined already. I first saw it quite a few years ago, and have always made that the first thing I do in a new program, and add them to existing programs I am working on. Just my two cents.

Stu.....

Click to expand...

I do the same; symbols are 'FALSE' & 'TRUE' for b3/0 & b3/1 respectively. The remaining bits in b3:0 I'll use for edit markers if needed.

just for the record, the AFI does NOT "disable" or "deactivate" its rung ... but then neither does a FALSE XIC (or similar condition) either ... specifically, the "official" book is wrong about the AFI ...

as others have mentioned above, the AFI will always merely be FALSE – and the rung WILL still be executed ... the "tricky part" is that the rung will be executed with FALSE logic ...

secret handshake: there is a big difference between FALSE logic and NO logic ... many people (even some with years of experience) have serious misconceptions about this concept ... (you might want to take a look at YouTube Lesson #7 on the Sample Lessons page of my website if you find these ideas confusing) ...

anyway ... below is a forum link about the AFI and about what it DOES and does NOT do in a program ... a FALSE instruction (such as an XIC would do the same thing) ...

http://www.plctalk.net/qanda/showthread.php?postid=5992#post5992

example questions to ask yourself:

what does an MOV instruction do when FALSE comes in? .... answer: nothing ...

what does an OTE instruction do when FALSE comes in? ... answer: it goes and writes a ZERO into a bit/box ...

what does a TON instruction do when FALSE comes in? ... answer: it (basically) resets the timer ...

once you start looking at the rung operations THIS way, then the AFI will be a lot less likely to trip you up ...