RSLogix500 TON preset - setting from HMI

I'm new to Allen-Bradley PLCs & RSLogix 500 - cue stupid question - and I want to set a timer preset value from an HMI. I was surprised to see I could only put a constant numeric value into the Preset field of the TON instruction

I've googled and I'm MOVing the value to the .PRE value of the timer. I've set the Preset field to 32767 and perform the MOVe on the next rung. I'm guessing that it will work okay but it seems very inelegant. Won't the preset value be written to 32767 briefly on each scan before being written back to the required value?

Is there a better way to do it?

...and what's the best way to post screen shots? That one's barely legible, sorry.

Just point your HMI tag to T4:x.PRE.

When you set up the numeric entry object in the HMI, you should be able to select "T" and the data table (or memory) type, a number for the address of the timer and ".pre" for the FLAG.

If the HMI doesn't have that capability, reference an "N" address with the numeric entry object, then MOV that value into the timer preset in your ladder, like this:

SOR MOV N7:0 T4:0.PRE EOR

As rdrast said, you can write directly to T4:0/PRE and also T4:0/ACC if you wish, the same way you can write to N7:x.

Shawn

Thanks rdrast, that's not a bad solution. So my 32767 in the TON Preset field is good, correct, sensible, needed? As I see it, if I left it at the default of 0, then the timer would timeout every scan, or is that not how it works?

How long do you want this timer to run?

When you drop the TON/TOF instruction on the rung, during programming, you can put a value there and never change it again. You do not need to keep MOVing a value to /PRE in your logic.

rdrast said:

Just point your HMI tag to T4:x.PRE.

Click to expand...

I would not do that, its always a good idea to check the value for limits
to prevent faulting the processor and setting the value to some unwanted setting. So write to an integer, then check it for limits, then move it to the timer's preset.

The limit instruction works well for this. ("LIM" instruction)

shawn_75 said:

How long do you want this timer to run?

Click to expand...

I want the timer to run for as long as the operator requires.

shawn_75 said:

When you drop the TON/TOF instruction on the rung, during programming, you can put a value there and never change it again. You do not need to keep MOVing a value to /PRE in your logic.

Click to expand...

I think you're suggesting hard coding the value into the TON instruction. I need the operator to be able to set the time delay from the HMI at runtime.

Welcome to the forum.

technolog said:

Won't the preset value be written to 32767 briefly on each scan before being written back to the required value?

Click to expand...

To answer your specific question, the very first time your program is run after you download it the preset will be 32767 on the first scan. Then after that it will be the integer value in N10:0. It won't change back to 32767, and if you went online and looked at the preset you would see the new value written from N10:0 as the preset.

Is there a better way to do it?

Click to expand...

Yes, reorder the rungs so that the move instruction is before the timer instruction.


Note that when you enter a value for a timer preset when you program, you are not hard coding the value of the preset to be some particular value. The value you enter is actually being pre-placed in memory in the timer element preset, not actually being set in stone by the ladder code. If you want to verify this for yourself, open the T4 file in the left hand project pane, scroll to the timer, and change the value of the preset in memory. Now go back and look at your rung. You will see the new value reflected on the rung.

I do recommend checking the value entered against limits as Mickey suggested. Whether you have the HMI perform the limit check and then write the value directly to the T4:XX.PRE or whether you have the HMI write the value to an integer and then perform a limit check in your PLC program before MOVing it to the timer preset is up to you.

TConnolly said:

technolog said:

Is there a better way to do it?

Click to expand...

Yes, reorder the rungs so that the move instruction is before the timer instruction.

Click to expand...

Thank you, that makes sense.

I generally write to an integer and then copy it to the .PRE of the timer.

Since I started programming a long time ago, I group all my data to/from my HMI together to minimize the separate number of read/write messages needed. This used to be very important for not slowing down comma. Now you can get away with a lot more with Ethernet comma but I still tend to keep things together out of habit.

I generally reserve the B13 file and N17 file just for HMI values.