In general, from memory's point of view...
There is no difference between a Timer and a Counter.
There is no difference between a Current Value and a Preset Value.
They are simply memory locations.
The difference is in the code. That is, the difference is in the way that various instructions use the particular memory locations.
In most PLC's the programmer is warned against using the same reference number for multiple Timer Instructions. He is also warned against using the same Timer reference number for a Counter Instruction (TMR-314 vs. CTR-314).
Some PLC's are designed to provide "X" number of Timers and "Y" number of Counters. In this case, there might be a TMR-1 and a CTR-1. If you need "X+1" Timers, and even though your program does not use any Counters,... too bad.
Does that mean that a program can not have a TMR-314 and a CTR-314?
No... it does not.
There are Timer Instructions and Counter Instructions.
A Timer Instruction performs "Timer-Type" activity on a particular set of Timer/Counter Memory Locations.
A Counter Instruction performs "Counter-Type" activity on a particular set of Timer/Counter Memory Locations.
If your code included a Timer Instruction for TMR-314 followed by a Counter Instruction for CTR-314, then your code would first perform a "Timer-Type" activity using the Current and Preset Values associated with T/C-314 and then perform a "Counter-Type" activity on the same memory locations.
Is this a practical thing to do? Probably not, however (except in the case of being a "morning person"), never say never.
In my "neighborhood", those memory locations are referenced by...
TCC-314
TCP-314
The first two letters, "TC" refer to the fact that the memory location is in the "Timer/Counter" range.
The third letter, "C" or "P", refers to "Current" and "Preset", respectively.
The number "314", of course, refers to the particular Timer/Counter.
So... on the surface, TCC-314 refers to the "Current Value of Timer, or Counter, -314". Likewise, TCP-314 refers to the "Preset Value of Timer, or Counter, -314".
Now, on a more basic level, TCC-314 and TCP-314 simply refer to a pair of particular memory locations.
As such, those memory locations should be accessible to any "read" and/or "write" instruction.
It should also be the case that Timers and Counters DO NOT have a rememberance of what the Preset or Current Values might have been on the last scan.
That is, once a Timer or Counter is turned ON, it simply performs the Timer or Counter type action, then compares the Current and Preset Values. Having compared the two, the instruction then manipulates the output as necessary.
I believe that means that even if a Timer has timed out in some previous scan, if one were to explicitly write a value into the Current Value indicating that the timer has not timed out, then the result of the Timer Instruction would indicate that the Timer has not timed out - even while the Timer enabler-bit has not gone OFF then ON.
The whole point of my rambling being that Timer/Counter memory locations are nothing more than memory locations and should be accessible as such.
Additionally, that Timer/Counter results can be manipulated, in some cases, by means other than the enabler-bit.
Would that be practical? Or desirable?
Using the favorite PLCnet response... "It depends".
