Difference between timer variables in RLogix500

ceilingwalker

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ceilingwalker
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Good day all. I have what is probably going to amount to a "silly" question pertaining to timers in RSLogix500. What is the difference between a timer being enabled and a timer timing? Are they not functionally the same thing? I mean, if a timer is enabled, it is timing, yes? If I were using an RTO I could maybe see the signigance but for a TON, for example, they seem the same.
 
I'm sure you'll get a rigorous explanation of this shortly, but my understanding is that the /TT bit is equivalent to "Enabled but not Done".

You could use those two status bits instead.

There is a straightforward explanation of the status bits of an SLC-500 timer on page 2-9 of the 1747-RM001 SLC-500 Instruction Set Reference.
 
The timer can be enabled but not time, for example it could be enabled but DN, so the EN and DN bits would be on.

I think the TT bit was put in so that it would mimic the clutch contact in some mechanical timers.

Alan.
 
What is the difference between a timer being enabled and a timer timing? Are they not functionally the same thing?
Click to expand...
No, not exactly. The Timer Timing bit is fairly unique to Allen Bradley. I find it useful to turn something on for X time, then back off when TT goes off. For many other brands of PLC, you have to use a Timer bit and a seal-in bit to hold something ON until the timer times out.
 
Lancie1 said:
No, not exactly. The Timer Timing bit is fairly unique to Allen Bradley. I find it useful to turn something on for X time, then back off when TT goes off. For many other brands of PLC, you have to use a Timer bit and a seal-in bit to hold something ON until the timer times out.
Click to expand...

I think I see the difference now. Whilst timing, the .tt will have a 1 in the bit box however, will not retain the 1 in the bit box after the timer has finished (for obvious reason). Whereas, the .en will retain that 1 in the bit box to show that the timer was on, at one time. Am I close to what you are describing, as far as a seal-in bit is concerned Lancie?
 
alan_505 said:
The timer can be enabled but not time, for example it could be enabled but DN, so the EN and DN bits would be on.

I think the TT bit was put in so that it would mimic the clutch contact in some mechanical timers.

Alan.
Click to expand...

What is (or was) a clutch contact?
 
Ken Roach said:
I'm sure you'll get a rigorous explanation of this shortly, but my understanding is that the /TT bit is equivalent to "Enabled but not Done".

You could use those two status bits instead.

There is a straightforward explanation of the status bits of an SLC-500 timer on page 2-9 of the 1747-RM001 SLC-500 Instruction Set Reference.
Click to expand...

After reading it I believe your "Enabled but not Done" is pretty accurate. I am trying to think of an application where that would be important. If I didn't turn this mole-hill into a mountain I just wouldn't be me.
 
ceilingwalker said:
I think I see the difference now. Whilst timing, the .tt will have a 1 in the bit box however, will not retain the 1 in the bit box after the timer has finished (for obvious reason). Whereas, the .en will retain that 1 in the bit box to show that the timer was on, at one time. Am I close to what you are describing, as far as a seal-in bit is concerned Lancie?
Click to expand...
To clarify the part highlighted above...

The .EN bit is ON (1) whenever the timer is enabled (hence the name), regardless whether it is currently timing, or is done.

IOW, it is ON whenever the conditions preceding the timer are true. Think of it as an OTE instruction in parallel with (or even in place of) the timer.

🍻

-Eric
 
Where I once worked we had plug in Eagle signal timers (the were about 4" round and 5" deep).
They had a clutch coil that was used to engage the timing gears, they also had a contact you could use.
I cannot remember how we used them but I do remember that they were used a lot on the off delay timers.

Alan.

see here: http://www.galco.com/buy/Eagle-Signal-Danaher-Specialty-Pro/HP55A6

These were very good timers you could change the timing length by changing the gearing, or you could change them from on delay to off delay with small changes.
Plus they were very quick to change out, just undo one screw and lift a lever/handle and pull them out.
 
Last edited:
Using the .TT bit saves memory over using ".EN AND NOT .DN", so it does serve a purpose in spite of being redundant. I used to work with a group of programmers who lovingly referred to this Allen Bradley timer bit as the "Tiny Tit Bit". ;)
 
alan_505 said:
Where I once worked we had plug in Eagle signal timers (the were about 4" round and 5" deep).
They had a clutch coil that was used to engage the timing gears, they also had a contact you could use.
I cannot remember how we used them but I do remember that they were used a lot on the off delay timers.

Alan.

see here: http://www.galco.com/buy/Eagle-Signal-Danaher-Specialty-Pro/HP55A6

These were very good timers you could change the timing length by changing the gearing, or you could change them from on delay to off delay with small changes.
Plus they were very quick to change out, just undo one screw and lift a lever/handle and pull them out.
Click to expand...

Thanks for that info.
 
Bit_Bucket_07 said:
Using the .TT bit saves memory over using ".EN AND NOT .DN", so it does serve a purpose in spite of being redundant. I used to work with a group of programmers who lovingly referred to this Allen Bradley timer bit as the "Tiny Tit Bit". ;)
Click to expand...

I am trying to imagine how using .TT over .EN saves memory. :oops:
 
I am trying to imagine how using .TT over .EN saves memory.
Click to expand...
Here is the main reason that I use TT - as a self-holding contact to keep the timer running until it reaches its Preset. With many other brands, you need a seal-in relay to keep a timer running, if its start trigger goes off before the timer reaches preset.

RSLogix Timer TT Bits.JPG
 
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