That means it took 6 milliseconds to complete the last scan. Every scan takes a slightly different amount of time, depending on what else the processor has to do, including communicate with RSLogix 500.
The rungs you have constructed count every other scan, or in other words twice as many scans have occured as what you have counted. HOWEVER, the scan toggle bit, S:33/9 changes state every time an END, TND, and REF instruction is executed. So if your program contains subroutines, then you may be getting an inaccurate representation of when every other scan of the full program occurs.
Now if you will look at words S:22 and S:23. Note that they are in 10 millisecond units. What that means is that according to S:22, it takes on average 10 milliseconds to complete a scan. But all you can resolve is 10 millisecond intervals, so it could be slightly more or less. S:23 says that the longest scan since the program was started took 20 milliseconds.
You are likely seeing a difference between what the words S:22, 23, and 35 and your own ladder constructed scan timer because of these resolution differences combined with resolution issues with the TON instruction.
If you will set the bit S:33/13, you can see the values in words S:22 and S:23 in milliseconds instead of 10mS intervals
The values in S:22 and S:35 will be more accurate than something constructed in ladder to measure the scan time.
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How the SLC500 TON instruction works: The SLC500 PLC keeps an 8 bit rolling time base with a resolution of 10 milliseconds. That means the 8 bit timer repeats every 2.56 seconds. When a TON instruciton is scanned and the rung has transitioned to true, the /EN bit and /TT bits are set and the time base value is copied into the low order byte of word 0 of the timer memory element. The next time that ton instruction is scanned, if the rung is still true, the value stored in the low order byte of word 0 is compared to the current rolling time base. If a period equal to or greater than the time base of the timer (.01s or 1s) has elapsed, the timer ACC is incremented and the current time base is copied into the low order byte of word 0. Then the ACC is compared to the PRE. When .ACC >= .PRE the /DN bit is set and the /TT bit is cleared. The timer can be late by up to the timer's time base plus the scan time.
