stopping PLC

Pandiani · Apr 28, 2005

Hi, I wrote a simple program (following your advices) that switch on first motor and when it is finished second motor is switched on. LAD diagram is in atachment. Now there is one problem. I'm testing these programs in school using PLCSIM. When timer starts counting time (assume first motor is running) and I choose stop on CPU motor output remain active. Is this going to happen in "real world". Should I write another org block, say OB 100 to switch all outputs or what? If real PLC is stoped will pupms and motors continue running or this is only on PLCSIM?
Thanks

walters · Apr 28, 2005

what kind of CPU you use? recently i had a S7-200 PLC and this one remebered the status of the program when i putted it in stop. i can't find anything in youre program that should make this happen but if you start again with plc-sim does the engine output still remains high then or does it get resetted then?? cause if it gets resetted by start up there nothing to worry about

Pandiani · Apr 28, 2005

Well, in school we use s7-300 CPU 315-2 DP. I didn't test on real PLC it's not available to me right now. I tested on PLCSIM only and when I choose stop on PLCSIM's CPU (RUN-P, RUN, STOP)while timer no. 1 is working output remain active. When tur PLC into RUN mode timer is reseted and output is still active until timer is finished. Do I need to program higher organization blocks to stop this or what?

walters · Apr 28, 2005

if you want your motor to run when the timer is on you must program an -||- and you have an -|/|- if im wright you have an on delay timer so when the timer is running youre motor is running also if timer stops running the motor will also stop because the timer gets high and makes the output behind it treu because that output is the same as youre-|/|- the motor will stop also

LadderLogic · Apr 28, 2005

As a matter of fact, there is a lot to worry about - if it ever happened. Imagine having something running or energized that you cannot turn off.

Thankfully, most (if not all, but I don't know them all) PLCs will automatically shut all their outputs off when stopped. Some would offer you a configuration choice: to put all the outputs into "safe state" (i.e. to turn them off) vs. to keep them in the last state before the stop occured. There are PLCs that have and extra "Run" relay to be wired in series into the machine E-Stop circuit.

PLCs are not supposed to stop, stopping it for whatever reason is a grave fault. They are just running and running and running...

walters · Apr 28, 2005

i assumed "B" is his stop button so he can stop it all manually

Pandiani · Apr 28, 2005

walters said:
if you want your motor to run when the timer is on you must program an -||- and you have an -|/|- if im wright you have an on delay timer so when the timer is running youre motor is running also if timer stops running the motor will also stop because the timer gets high and makes the output behind it treu because that output is the same as youre-|/|- the motor will stop also

I meant I want motor to running while timer is counting time. When timer output is on (10 sec elapsed) it will shutdown motor.

My hardware configuration on this project used CPU 312. I'll try to create new proj with CPU 315-2 DP to see if PLCSIM will turn off all outputs when stopped. I hope real PLC will turn off outputs when stoped (because of some error). Maybe this behavior is only on PLCSIM?

JesperMP · Apr 28, 2005

Pandiani.

You mean to say you switched the PLC-SIM "CPU" from RUN to STOP, and then the outputs remained in active state.
If that is the same for a real CPU I do not know.

The online help says:
"In STOP mode, the CPU checks whether all the configured modules or modules set by the default addressing actually exist and sets the I/Os to a predefined initial status."
I would guess that "predefined initial status" means OFF for digital outputs, but I am not 100% sure.

I am quite sure that if the CPU goes to STOP because of a fault, then the outputs are turned OFF. If the STOP mode is activated by the user, then I am 99.99% sure that the outputs are also turned OFF.

rsdoran · Apr 28, 2005

I dont know much about Siemens but had 32 machines that used 314IFM Master and 312IFM slaves with an OP7. On those machines if you put the plc in STOP the outputs were OFF.

Pandiani · Apr 28, 2005

Thanks guys, a very valueable infos, I'll check it tomorrow in school on a real CPU 315-2 DP.

Cheers

walters · Apr 28, 2005

for safety you wont neccisarily connect a device to the output card so when you turn plc in stop you can get hurt by some device. the led's on the plc indicates output high or output low. didn't know if you allready knew that but i thought i tell him that can safe him some work. hope its just plc-sim bug that indicates output high

good luck
regard walters

JesperMP · Apr 28, 2005

for safety you wont neccisarily connect a device to the output card so when you turn plc in stop you can get hurt by some device.

Brilliant !
Do not connect the outputs to anything. That will be very safe and nobody will be hurt. I can use that on some other project.

walters · Apr 28, 2005

that was very sarcastic jesper

but the advice was for free:teacher:

Pandiani · Apr 28, 2005

hehe, thank you for free advice walters...

P.S. I searched for PLCSIM fetures and found this:
The simulated PLC provides the following features which are not available in a real PLC:
- The Pause command halts the simulated CPU and allows you to resume the execution of
the program at the instruction where the program was halted.

This should explaine everything (unless pause is not considered as stop)

Cheers!

RMA · Apr 29, 2005

The Pause command halts the simulated CPU and allows you to resume the execution of
the program at the instruction where the program was halted.

The Pause command is the Symbol at the right hand end of the tool bar, conisting of two vertical bars, just like the Pause button on a video recorder. This is to allow you to stop the execution of the program briefly, to check multiple conditions, or, for example, to set several feedback input signals, which with a running program you may not be able to do quickly enough.

It is not the same as the main CPU STOP switch.

If somebody else doesn't come up with the answer in the meantime, I'll have a play next week and see how PLCSIM's Outputs react when you switch to STOP (I've a sneaky feeling that I know the answer, unfortunately).

stopping PLC

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