Logix 5000 ABS, LES and GRT

[caseymx]

I have been searching and cannot find a reason for why this is happening. This programming was factory installed by a machine vendor and I have duplicated it on my test station in both function block and ladder format with the same results.
When using the ABS instruction preceding a LES or GRT instruction, it seems that the non absolute value is still being used. My past understanding is that the ABS strips the sign from the negative value making it a positive value. So why doesn't the LES and GRT instructions use the result accordingly?

 

[Steve Bailey]

The images you posted look correct to me.

Source A = ABS(-3.6539412 - (-3.6525083)) = 0.0014329

Source B = 0.1

Source A is less than Source B

 

[caseymx]

Well I guess I didn't do my math like I should have. So why does the result show 1.43289 instead?

 

[drbitboy]

I agree with @Steve Bailey: that code is doing exactly what it has been told to do: when the difference between _741_GHZ_1271.Thr_pos_Select and meas, as calculated and output by block SUB_01_01, is between -0.1 and +0.1, exclusive (i.e. when the absolute value of the difference is less than 0.1), then the discrete output of block SETD_07, _741_GHZ_1271.Thr_pos, will be 1 because its Set input pin is 1, and it will stay that way until that difference is outside the range -0.2 to +0.2 (i.e. the absolute value of the difference is greater than 0.2).

How do you want it to behave, and is that any different from how it has been told to behave?

Note also that that the Ladder Logic shown is not equivalent to the FBD logic.

 

[drbitboy]

Well I guess I didn't do my math like I should have. So why does the result show 1.43289 instead?

It shows 1.43289e-003, i.e. 1.43289 x 10^-3; cf. this link.

 

[caseymx]

Thanks. This is the answer I was looking for. Been a while since I've had to deal with exponents. My mind skips a little after 50 years.
Much thanks guys.