How to read SINT (short signed Integer) with CJ2 PLC

[Nicolas]

I am reading some joystick data over CAN-OPEN, all analog values are 8 bit signed integers or Short Int. I know the first bit is the sign... I can't find an easy way to read that Signed 8 Bit integer value into cx-programmer, using PLC instructions/conversion. I read about the NEG(160) two'd complement but I can't get it to work. Any ideas?

[Michael Walsh]

The easiest way that I can figure to do it would be to use the ANDW function to mask out bits 7-15, leaving the absolute value of the short signed integer. Then look at the sign bit, if it is on, multiply the masked value by -1, if it is off, just move the value as shown below:

[Nicolas]

Thanks for the reply. I tried that approach but that doesn't return the correct value...this is what you must get: for 1111 1111 two's complement value= -1 but using the mask and multiply method you get -127 Unless I am doing something wrong...

[Nicolas]

Here's the final answer attached for anyone interested, easy to put into a little function block: Result:

[Michael Walsh]

Ahh, yes, you are correct. I did not spend enough time on this and made a mistake. At least I got you headed in the right direction!

[Sergei Troizky]

Edited September 14, 2012 by Sergei Troizky

[Michael Walsh]

Ooooh! Ultra elegant! Nice!

[BITS N BYTES]

Structured Text conversion of Single Signed Integer.