RSLogix5K: Software Manual vs Manual (.SWM vs .MO bit)?

What is the difference between Software Manual and Manual mode? When I set the .MO bit, The Set Output (.SO) value will not write to the output %, as when in SWM mode.

The contents only say that SWM is used to "simulate" manual mode.

The Manual Mode (.MO) has more to do with using tieback. SWM is the correct mode for manually setting the output %. From the manual:

software manual (.SWM) also known as set output mode lets the user set the output % from the software. The set output (.SO) value is used as the output of the loop. The set output value typically comes from an operator input from an operator interface device.​

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manual (.MO) takes the tieback value, as an input, and adjusts its internal variables to generate the same value at the output. The tieback input to the PID instruction is scaled to 0-100% according to the values of .MINTIE and .MAXTIE and is used as the output of the loop. The tieback input typically comes from the output of a hardware hand/auto station which is bypassing the output from the controller.​

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Note:​

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Manual mode overrides software manual mode if both mode bits are set on.​

So obviously resurrecting an old thread of mine...

Can anyone give an example of when one would use the .MO manual mode over .SWM (software manual)?

Is the only difference between the two is that .MO enables use of the Tieback value? Never once have I used .MO for anything.

Using the .MO bit (and setting things up properly... ) will result in a bumpless transfer between manual and automatic control. The .SWM bit on the otherhand is not bumpless.

rdrast said:

Using the .MO bit (and setting things up properly... ) will result in a bumpless transfer between manual and automatic control. The .SWM bit on the otherhand is not bumpless.

Click to expand...

rdast:

Actually, I believe both the Software Manual SWM, and Manual Mode (MO), provide bumpless transfer in the Control Logix PLC.

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf

Page 516

Stu.....

kdcui said:

So obviously resurrecting an old thread of mine...

Can anyone give an example of when one would use the .MO manual mode over .SWM (software manual)?

Is the only difference between the two is that .MO enables use of the Tieback value? Never once have I used .MO for anything.

Click to expand...

kdcui:

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf

I will also point you to Page 516.

Software Manual (SWM) is used to control the Logix PID Output manually through an HMI, or other means, but the Control Element is still controlled from the PID, and the PLC Output. The desired output to the CV is moved into the SO Word. The PID is updated so that when it is put back in auto, there is Bumpless Transfer, with no disturbance to the Controlled Process.

Manual Mode (MO) is used with an external Manual Control Station where the Operator manipulates the Control Element directly, not the PLC PID, and the PLC Output. This is monitored by the PLC, and the PID is updated so when the Operator goes back to Auto, there is Bumpless Transfer.

I have not seen the Manual Mode used for quite a while, and I have always thought it was there because the Operators didn't trust that new fangled thing to do the job right.

Stu......

Thanks for the info Stu. I actually read all that in the documentation, but was still unclear as to why I would need to use MO over SWM, so I figured I'd ask for some examples.

One example of where you would use it is if you had two loops controlling the same variable. For example if you wanted to control flow but not exceed pressure you might have a pressure loop and flow loop. Normally the flow loop would operate and the pressure loop would track the position by using the MO and Tieback so its output was at the same position as the output generated by the flow loop. When you reached the allowable pressure you put the pressure loop in auto and it controls without a bump. There is a bit more to do to co-ordinate the loops but MO basically tells the loop it is not in control. The tieback tells the loop where the control variable (valve or pump speed usually) is so it can take over from that spot. It is also meant to mimic old style Manual Loaders to allow the controller to take over from a manually set postion.
You could also use it to force the Primary loop to track the setpoint of the secondary loop in a Cascade Control scenario.
All of this is easier to handle in the PIDE but not everyone has access to or the knowledge to use the Function Block Programming in RSLogix5000.

Personally, I've never seen MO used. I always use SWM. They seem to work pretty much the same way. SWM and SO together or MO and Tieback together. SO is the same units as the PID output (0-100%), where TIEBACK in scaled using MAXTIE and MINTIE. I can't think of a single thing that I can do with the MO and TIEBACK pair that I can't do with the SWM and SO pair. There may be some additional code required for one vs the other but the same function can be achieved. That said, I see no reason to use both, so I'll stick with SWM.

Think of it as the A-B PID instruction's appendix. No one really knows why it's there, so they pretty much ignore it.

If anyone has an example where the MO and TIEBACK are neccessary, I'd love to see the code. I'd even be interested in seeing an example where it was just a cleaner design.