Mitsubishi RST instruction issue

I am writing a simple line of logic using GX Developer. Basically I look at the state of a bit and if it is true, I reset the same bit. I have created a new block and insterted this line at the top using bit M500 and resetting M500. For some reason the bit is not resetting. Does anyone know why this is so? I have very little experience with Mitsi. Do I need to call the block for it to run? I couldnt find where. In the montor mode, I can see the status of the NO contact but not the rst instruction. The NO contact shows 1 constantly and is only set on the pos edge so is not being overwritten

Hi stuart,
First question - which cpu are you using?
assuming it is a Q series, do you mean you have created another program other than main?

If so have a look at this thread:
http://www.plctalk.net/qanda/showthread.php?t=34582&highlight=pscan

When you look at SET or RST instructions in monitor mode, the RST will be highlighted (like it's active) to show the bit address is OFF, and the SET instruction will highlight when the bit is ON. That's a visual indication, since with a contact or coil you can see the status of the bit.

So if the RST instruction looks true, it's because the bit is OFF, no matter if the logic in front of it is true or not.

Assuming the bit is on at the top off your ladder & you turn it off at the bottom monitoring it means you will probably not see it as the comms time for data to be fetched into gx dev et. will be slower than the scan of the plc.
if you put a set mxx (any flag not used) in parralel to the one you are setting then once the set has occured even though your logic has then turned it off before the pc reads the status will will se that bit Mxx is latched on you could use a timer to reset bit Mxx of say 5 secs then you will see if your set/reset is working.
i.e.
A X0
PLS M100 'Create puls from X0 so it does not hold on M0/M1
A M100
SET M0 'Set your bit
SET M1 'Plus dummy
A M1
T0 K50 'delay 5 secs
A T0
RST M1 Reset the dummy this will give you time to see if the trigger worked

... ' rest of program
A M0
RST M0 'rst M0 in same scan if it is on