Help with using 2 encoders to match length

Hi,
I'm using a Micrologix 1500 PLC with a HSC. I have two identical encoders wired differentially. One encoder is downstream from the other. I need a sample code to check both figures are within 2" of each other and set a bit if not. I already have one calibrated to the proper length conversion.
Any help would be greatly appreciated.
Lou

I have attached a copy of the ladder program. You can see the setup in Ladder 3, rungs 2 thru 5. What I need is to keep F8:2 and F8:40 within 2" of each other. I added a some logic in rung 16, but I'm not sure if this will work or if it's correct.

I will not talk about your program ,

1.) Are this new installation/program? Or existing program and equipment?

2.) Are this problem arising because encoder malfunctions?

3.) Are the encoder revolution same with previous?

4.) have you measure the motor speed ?

5.) how you calibrate the encoder ?

Thanks for the reply.
Most of the program is currently running. I originally had a secondary encoder wired to a RedLion counter that the operator had to check and reset before running another wall. Now, my company would like the encoders to "match-up" before allowing a "cut" to occur, thus eliminating user error, etc. I currently only have one encoder doing all the material moved length counts and the secondary(RedLion) count the actual material measured. There are no problems, but the company wants to eliminate user error and rely on a synch'd set of encoders due to calibration errors. Current encoders are identical, brand, and model. Speed isn't measured, it's set at specific speed, however, changes between material moved through corrugation and final output can/will vary.
Lou

check this example


http://www.youtube.com/watch?v=hFRV24y4VGc

Earlier you said you need to "keep (two floats) within 2" of each other", but I think you meant to detect and fault when it goes out of spec, right?

Within 2.0, so find the difference and use it in a LIM instruction with -2.0 and +2.0 as the limits to keep a bit true when things are happy...