Need help understanding this program ML1200

Hi folks,
I am wondering if you guys can help me understand what the intended operation was for some logic that relates to the operation of the analog output (only 1 in use) going to a small VFD for speed reference.
This is a small system that controls some “special effects” using a ML1200. It was provided years ago with no documentation whatsoever, and the company/guy was impossible to track down. A year or so ago, the HMI died, and an employee wiped the program, so I did what I could to re-create the HMI, and in the process tried to document the .RSS file the best I could. (so some or many of my symbols/descriptions could be totally wrong). In Jan 2018, the analog I/O card (1762-IF2 OF2) died and I replaced it.

Last week the system was not working and the following things had/were happening:
DC power supply failed/failing flickering indicator LED
Pressure switch bad (A.D. PSD 25 series which I have seen fail-to-short circuit), thought this was the issue and replaced both items.
Upon restarting, 5A fuse to the small teco VFD input power was blowing. We pointed at either the pump/motor or VFD, replaced VFD (cheaper and easier).
Original VFD was discontinued, so recommended replacement model was used. I landed the analog output on the wrong terminals and believe that I likely fried the IF2 OF2 card. It would not work. Replaced it, the card works now.
During this replacement, as before in Jan, just replaced wire for wire on the analog module. But this time, realized that there seemed to be no power supply in the circuit for the input/pressure sensor. I don’t see how it could have ever worked that way, and certainly wasn’t working, so finally I decided to wire it as it “should” be (IMO) and now that works. Pressure reads. But how was it working before? Now we are getting to the major confusion points I’m asking for help with.
Tried to run the system again and the pump does not ramp up, and the logic seems to be causing this by only put a minimum value in O:0.1
When the logic was “bypassed” and I could send values to O:1.1 and run the pump. Ran the pump full speed, blew the fuse again. So the first VFD was probably fine. And motor/pump is probably bad (?)
So, this is now quite a lot of hours spent on a fog/effects machine, so Friday afternoon I JMP LBL’d the logic for this output and just made it “work” (yes, sounds like a bad idea to me too). (this is the FOG PLC TESTING.RSS)
Watching the amps while speeding up the pump, I found it drawing about nameplate amps at 30hz, so I’m sending that value (15000) to O:0.1 to the VFD.

The logic “seems to me” to do the following:
Takes raw analog input value from a pressure sensor (I:1.0) and “filters” through a MVM using a mask of 32760 (N9:12)
The MVM outputs to N9:11
On rungs 34/35 (N7:11) is used in LES and GRT instruction to MOV a minimum of 6552 and max of 32760 directly to the analog output O:1.0
(I don’t see how the value could ever get above 9 using the mask of 32760, and anytime I saw the program online, 6552 was indeed being MOV’d to O:0.1)
On rung 36, if N9:11 was not LES or GRT in previous Then SUB 6552, then SCP 0-1450 (0-1450 is the range of the Pressure sensor)
Then (still rung 36) N9:11 is scaled 0-32760, and the output of this SCP goes directly to the analog output O:1.0

I can’t fathom how the thing was working with the analog input not working. Maybe the VFD was setup to receive the start stop, but it’s output was not referenced externally (?). I kind of think I see what the “guy” was doing since he re-scaled the really small # coming from the mask, but why do that at all?

Thanks for any insight you may have,
Byron

We can start with the MVM. The value of 32760 will allow any bits to pass except for bits 0, 1, 2, and 15, so any analog value between 8 and 32760 can be moved. The 0s block the data, the 1s allow it to pass. The instruction help description actually has a nice visualization of this if you are able to look at it.

The first SCP takes the raw value, between 0 and 26208 (the raw values minus the minimum of 6552) and converts it to PSI. The second SCP takes the PSI value and reconverts it to a value between 0 and 32670 to output to the drive. It seems to me that the scaled values should be between 6552 and 32670 to keep the value between the min and max range in the rungs with the LES and GRT instructions. Perhaps the scaled min has been incorrectly modified at some point. The analog input may work again with this modification. The only reason I can see for using the two SCPs in this manner is to watch what is going on while online. The pressure and output to the drive could be monitored at the same time. Otherwise, he could have just moved the value of N9:11 into O:1.0 as long as the LES and the GRT were not true.

In a nutshell, these rungs are supposed to keep the analog output value between 6552 and 32670. I don't understand why the pressure readout would control the drive's frequency in this manner, but I have definitely seen stranger things.

Thanks MikeyN,
I was reversing the function of the 1's and 0's in the Mask! Makes more sense now.

"I don't understand why the pressure readout would control the drive's frequency in this manner, but I have definitely seen stranger things."

Yes, I am still having difficulty with understanding this operation and what it actually does.

Would I be correct in describing it as:
The pump starts, and the minimum value of 6552 is sent to the output to get the pump spinning.
Once the pressure value rises enough, the pressure value is then used to control the output to the VFD.
As the pressure increases, the output increases, until maximum 32760.

As the pressure rises, the pump speeds up? As a method to ramp up the pump?

I don't think you are wrong in your description. That is not the way I would have done it, but I don't know what kind of equipment either you or the creator has(d) to work with. If the drive didn't have ramp capabilities, that sounds like a very creative way to get it done.