Best way to impliment a SCADA map

godfrey

Member
Join Date
Apr 2002
Location
Charlotte, NC
Posts
412
My boss and I are in disagreement as in how to put a SCADA map into our PLC programms. We provide PLC based control cabinets with door mounted OITs for a variety of processes. There is usually a separate integrator who will provide a SCADA computer to which our PLC will be linked.
The SCADA must read status bits and process variables and also write bits and setpoints to our PLC. Here is where we disagree:
I want to let the SCADA write to the same registers that my OIT writes to. A SLC 500 example would be: N7:0 is the flowrate setpoint. There is a numeric data entry button on the OIT addressed as N7:0 for this. If SCADA writes to N7:0 or if the OIT button writes to N7:0 it works.
My boss says, create a separate SCADA map file like N47. Let SCADA write to the N47 file only. Then I program the N47 addresses to write to N7:0 and other like addresses. WHY??? Because if the integrator's map is off, he will be writing to a different address and screw things up. My boss wants a disable bit in front of every line of our code in the SCADA map so that I can prove that errors are due the map being off.
Is there any merit to this? How do you do it?
 
we do a lot of installations where information is passed
from one system to another and i absolutelly agree with
your boss...

First:
ask yourself what are the merits of writing directly
to setpoins? you saved 20 integer words and one or two
rungs!?


Second (just an example):
imagine more complex world where you have let's say five
touchscreens, three computers and 2 other PLCs all
writting to *your* PLC and even worse - they all
write to the same N7 file!!!
imagine that vital paramters have changed or get
corrupted periodically. how do you propose to find
the culprit? what if customer suddenly changes mind
and asks you do log WHO did parameter change or
to assign access levels? How do you ensure that
setpoint values are in range? By hardcoding
them individually in each panelview?
 
Last edited:
Sorry godfrey, I agree with your boss too... :nodi:

I question why the integrator wouldn't be able to "screw things up" with your method as well?... :confused:

beerchug

-Eric
 
How will your Boss's idea really work? I order to maintain two sets of setpoint files in your program you will need to implement some way of determining which value is the current setpoint. You will also need to copy current setpoints into the SCADA file so the outside HMI's can see the current setpoints before they change them.

I tried to do something like this about ten years back for a different reason, it turned into a real mess.

Maybe I am missing the point?
 
I do my own SCADA as well as all the electrical control system design, build the panels, write the software and commission the job.

I partly agree and disagree with your bosses idea. Typically for adjusting timers or any other numbers the SCADA writes directly to the channel where the setpoint is stored by way of +/- buttons (thumbwheels) on the screen. These buttons turn on and off bits in the PLC. The bit then increments and decrements words on a rising edge trigger. This way the number can only change by 1 each button press. The number is shown in the centre of the thumbwheel for each digit and one can see it change each time the button is pressed, fast network to the PLCs.

The other method I use is to set up a table for selection of generator set order, for example. The current setting is shown and the selected setting is shown. When the operator is happy with the selection, a "set" button is pressed. This then moves the current settings to old settings and the new settings to current settings. A comparison between current settings and old settings then takes place to determine if the order has changed. If there is a change in order, a sequence of events is activated to start the required generators for the new settings and then shut down the generators that are not required.

If you require some screen shots or sample code, please respond here and I will post some.
 
To get down to some of the details you are asking about...

When doing data transfer using the standard files in a PLC-5 or SLC-500, I suggest you use separate files for the following

Bits From HMI (example B100)
Bits To HMI (example B101)
Integers From HMI (example N102)
Integers To HMI (example N103)
Floats From HMI (example F104)
Floats To HMI (example F105)

This will help you to optimize data reads and writes from any of the direct type drivers out there. A new transaction will be created each time you change file type or do a read or write. This file organization simply recognizes what creates a new transaction. I would further suggest using this concept whether an external HMI is accessing the data or not. Your program and OIT (PV or PanelMate or other) should be using this layou out.

Bandwidth may not be an issue if using Ethernet. Using DH+ or DH-485 will benefit from this approach as they are bandwidth limited. For example DH+ can handle about 12 transactions per second at 57.6K.

To answer your question about having a separate block of data addresses for others to access is something that can go both ways. If you are involved in a project where the contractual relationship does not allow you to work closely with the party doing the SCADA/HMI, then I agree with your boss. If you have a close relationship contractually with the party doing the SCADA/HMI, then I lean toward your point of view. You boss is anticipating finger pointing and working toward keeping the fingers pointing at you.
 

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