RSLogix Log2 Alternative

kekrahulik · Mar 15, 2012

I’m locked in a hotel room trying to do a month’s worth of programming in a few days (called in as the last minute janitor for a mess) and I need a little time saving help with RSLogix5000 programming.
I have programmed each bit of a DINT to represent a state (DINT.0 = State 0, DINT.1 = State 1). One and only one bit is ever true at a time, so the DINT value can only be 1, 2, 4, 8, 16, 32, 64....
I would like to quickly determine the bit position within the DINT and store it as an integer. Ie if the DINT = 32, my integer result would be 5.
A quick way would be to take the Base2 Log of the DINT, but I only see Base10 Log in RSLogix5000. It seems that I should be able to use an FBC to do this, but I need to break the DINT bits out to an array first.
Any ***y solutions out there?

Ken Roach · Mar 16, 2012

If you really wanted to be clever you could try to implement one of these bit hacks:

http://graphics.stanford.edu/~seander/bithacks.html

But I would just do a brute-force method: Thirty-two rungs with an XIC and an MOV each, then embed it as an Add-On-Instruction.

TConnolly · Mar 16, 2012

The fast way to program it: DintTag = LN(StateTag)/0.6931472

However log functions are CPU intensive. What you want it the SLC 500 equivalent of the DCD instruction. Check Rockwell's website for a downloadable AOI.

(8{)} ( .) · Mar 16, 2012

Or a FOR NEXT loop or a recursive (GASP - gotta watch that stack) SUB (Shift right and count until the value is 0).

Good Luck,

(8{)}

.)
(Yosi)

kekrahulik · Mar 16, 2012

I like the AOI approach. I don't mind writing the code - just hate to have something that ugly.

Another one of those simple answers that I should have come up with myself - love this forum.

Thanks!

TConnolly · Mar 16, 2012

In case anyone was wondering how DintTag = LN(StateTag)/0.6931472 was derived:

To find log base 2 of X you use LN(X)/LN(2)

X is variable but LN(2) is a constant equal to .6931472. The floating point computation will be very very close to a whole integer so type casting the answer to a DINT completes the conversion.

khordin · Dec 16, 2015

performance

I know it's an old thread, but I found it useful.
I tested both "brute force" and Log2 methods. Brute force works two times faster.

kekrahulik · Dec 16, 2015

I took Ken's advice on the AOI. I've used that many times since. Glad to hear that the simple, clunky method is the more efficient one.

RSLogix Log2 Alternative

kekrahulik

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(8{)} ( .)

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