RSLogix500 Math Overflow Fault

[eric_lau 0]

Hi all, I have a micrologix. And now in this micrologix, i need to write a code (Logix 500) for dividing one 32-bit float to two 16-bit Int. Here is my logic: Let F20:0 is the 32-bit float. Case 1: If F20:0 >= 65536, div F20:0 by 65536.0 =>N7:0 and div F20:0 by 65536.0 => F11:0 then, If N7:0 > F11:0, add F11:0 by 1.0 and then sub F11:0 by N7:0 => F11:0 else sub F11:0 by N7:0 => F11:0 then, If F11:0 > 0.5, sub N7:0 by 1 => N7:0 then, Mul N7:0 by 65536 => F19.0 then, Sub F20:0 by F19:0 => F19:0 then, if F19:0 <= 32767, Mov F19:0 to N7:1 else Sub F19:0 by 65536 => N7:1 Case 2: If F20:0 < 65536, Mov 0 to N7:0 then, If F20:0 <= 32767, Mov F20:0 to N7:1 else Sub F20:0 by 65536 => N7:1 After the implementation, this code works for a period and then Math Overflow Fault occur!! I have no idea which part generate the fault!! Once the Math Overflow Fault occur, micrologix will stop function!! Anyone help?? Thanks a lot!! Edited 16 Dec 2010 by eric_lau

[dmargineau 6]

Any DIV instruction could trigger the Math Overflow Fault. The CPU dumps the division "leftover" produce into a limited capacity virtual bin, the so called Math Overflow Trap! It is a good practice to add at the end of the routine containing the math a rung with a "hot" CLR S:5/0 (Math Overflow Trap) instruction. This way, during every scan, the useless data is being deleted before the bin becomes full, triggering a Processor Fault. Edited 16 Dec 2010 by dmargineau

[b_carlton 61]

Try just using DIV. Read the following paragraph from the help file on the DIV instruction: Set the destination to an integer location (N). Make sure to have S:2/14 set. Check the math registers (S:13, S:14) immediately after the DIV.

[TConnolly 13]

The MicroLogix supports 32 bit integers. Is there a reason you can't use them? Create a new data file select the Long data type.

[eric_lau 0]

Actually, I found out that there is a case which will generate a number smaller than -32768. I believe this is the root cause of the overflow!! After I modified the problem to prevent this case, everything work fine so far!! I am not sure that part too because I was asked to fix the problem. I'm not the original developer.