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[rwskinner]

No, Modbus ASCII LRC Calculation

[Bruce]

Assume LRC = 10.

LRC = 256 - LRC. Which works out to LRC = 246, but returns a warning message.

Now try this approach with LRC still = 10;

LRC = $FF - LRC ' LRC = 255 - 10. Now LRC = 245
LRC = LRC - 1. Now we have 245 - 1 which = 244. Oops! Not what we expected, although it
looked darn good at 1st glance.

Now try this, and see if you can figure out why it works & doesn't return the warning
message;

LRC = !256-LRC ' LRC = NOT 256 - LRC

Here's a short test to verify it;

LRC VAR BYTE
X VAR BYTE

Main:
    FOR X = 0 TO 255
    LRC = X ' We'll work with LRC = 0 to 255 to prove it works
    HSEROUT ["256-",DEC X,"="]
    LRC = !256-LRC
    HSEROUT [DEC LRC,13,10]
    NEXT X
Here:
    GOTO Here ' done    
    END

Food for thought ehh....;o}

A more efficient approach would be to just promote LRC to a word variable,
and use the lower byte for your result, but this is pretty cool.

[skimask]

.....Now try this, and see if you can figure out why it works & doesn't return the warning message;

That's pretty slick...
I almost missed it and was about to reply back that you were wrong in this instance. Good thing I didn't. My foot goes in my mouth enough as it is...

[Bruce]

It certainly wouldn't be the 1st time I was wrong, but it does work..;o}