Allen Bradley Pyramid Control PLC5/250 TON Instruction

I am in the process of converting a PLC5/250 program and the program has a timer on delay (TON) instruction. The time base is set to SCAN. I have only seen a time base of 0.01 and 1.0 in the PLC5. Does SCAN as a time base mean that when the timer is enabled that it accumulates the number of scans in the Preset value and then the Done bit goes high? Does the timer reset when the Enable signal is lost like a regular TON?

Also, any ideas why someone would use this method over just use a 0.01 or 1.0 time base?

Thanks. Any help would be greatly appreciated.

I have never seen the need to use a timebase of SCAN in any project I have ever worked on....

The TON instruction works exactly the same as when using a "proper" timebase, except that the instruction doesn't have to work out how much time has elapsed since it was last scanned, it simply adds 1 to the accumulator each time it is scanned.

It effectively turns the Timer into a Counter, counting the number of scans of the logic into the ACCumulator, and sets the DN bit when the ACC is greater than or equal to the PREset.

Perhaps the programmer wanted a shorter time interval than the smallest timebase allowed, or perhaps there is some iterative code that actually takes XXX scans to complete, and he used a timer to signal that it was complete. Either way it is possible to replicate this in your new machine by using Counter instead. You will have to unlatch the .CU enable bit each time the CTU has been scanned, or it won't progress past a count of 1.