Control Logix BTD Instruction

After importing a project from PLC-5 I am facing the problem that CLX doesn't have the option to put a tag in the source/destination bit of the BTD instruction. This was available in PLC-5 for the indexing. Any one who has faced a similar problem and solved it?

Can you explain what exactly you are trying to do with the instruction so maybe I can help you find another solution (not using this instruction).

BTD instruction in Control Logix.

In PLC-5 BTD instruction was being used to copy bits from one word to another word in a different bit location. i.e. If I want to move bit no. 3,4 and 5 of Word X to bit no 7,8 and 9 of Word Y. So the starting source bit will be 3 and length will be 3 where as destination bit will be 7. I want this source and destination to be addressed as tags instead of constants as I want to change it continuously during the runtime program execution.

somu,

what you describe is correct: in contrlologix source bit, dest bit and length can't be tag, only direct value and you will have to modify your program to accomodate this.

Why this is done in logix differently? - here is my guess:

- PLC5 uses interpretor: it reads the program on the fly and executes instruction based on the current state, one at the time.

- CLX uses compiler, it converts every instruction to the set of commands before downloading to the PLC. This makes execution much faster. To have 3 additional fields as variables makes compled code very complicated and as result very slow.

Again - this is my guess.