AB: RTO timer - TT bit status vs. TON

I'm working with RsLogix v6.10 and notice that AB's documentation states, as it has for many years, that the RTO timer behaves as the TON timer except for the RES and ACC functions. My experience contradicts this, as the TT bit remains set until the ACC reaches the Preset or the timer is Reset, in the emmulator software, if the time rung is true or false. This is NOT like the TON. Is this a result of legacy documentation in need of update due to a change in PLC or software functionality. AB support didn't seem to know or care, when called.

Yes, you are correct in the function of the TT status bit.

What was the real purpose of the post?

Purpose...

We teach PLC programming and all external sources of data on the RTO function, including textbooks and AB's documentation, do not correctly explain, or show in timing diagrams, the TT bit's function.

Greetings GJM,

I’m not 100% sure that I understand your situation ... but this is the way it works in a real-world processor ... I just double-checked it on a spare SLC-5/05 system ...

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are you saying that it works differently in the emulator software? ... if so, I’d just chalk it up to the fact that the emulator doesn’t always function EXACTLY like the real-world hardware ... a lot of people wish that it would ... but ...

Teaching is fun, most times!

Take a look at AB Manuals online and then look for this pub#. 1747-RM001D-EN-P. It is the instruction set/reference manual for the SLC500 family of processors and it does a good job of explaining the TT,EN,DN and other aspects of the timer instructions. The bottom line is how the instructions act in a real PLC processor. The TON and RTO are exactly the same but for one difference. The TON will reset when the rung it is in goes false. The RTO on the other hand will just stop timing and retain the ACC value until a RES instruction is executed to cause it to reset. The TT bits of both timers behave exactly the same way. They are both true while the timers are running or timing and both will be return to the off or zero state when the timer is no longer timing, no matter the reason for the timer having stopped its timing. I teach PLCs also, and specifically the AB systems.


Ron, you and I were doing responses at the same time, but thanks for the visual, and you are exactly right.

Thank you, Ron & Randy, for the reply.

The Emmulator software shows the TT bit as functioning as I described, staying high until the ACC or RES clears, once the timer rung has gone true.

I haven't taken the time to check on hardware, yet. We use SLC500s. But, if there is a difference between reality (hardware) and the virtual (software), then that's an important issue. I'll check on Monday. We spend most of our time teaching through the Emmulator software, with logic developed accrodingly. We then proceed to hardware to verify functionality. Use of the TT bit could be problematic as a result.

Thanks to you both,

Greg

Emulate 500 WILL act differently with some instructions as opposed to the real world PLC program.

I have run into the same problem with the RTO instruction in the past and most recently the RMP (ramp) instruction in the emulator (a few others also).

Emulate is good in a pinch for testing basic instructions but... it's not to be compared with real bench testing. Read the documentation with Emulate, there is a number of instructions that it can not process.

Regards,
Chris

Glad to be of help

GJM,
If you are using software to simulate PLCs, you should know about LogixPro. It is available from Bill Simpson's web site and is quite reasonable in cost. I use it exclusively to begin each new class of PLCs that I teach. It makes the transition to RSLogix much smoother and the simulations that come with the software are first rate. If you have not looked into it, you should. There is also a really good text out that you may already know about. It is the 3rd edition of Frank Petruzella's Programmable Logic Controllers from McGraw-Hill. It is now available with a companion lab manual plus a CD Key version of LogixPro. I have been using the text for the last 7 years but since the updated 3rd edition came out with the lab manual and CD, well that is just the ticket for my classes. So far, the students have accepted it well and are learning to program PLCs which is the goal. Good luck!