Making sure an SP doesn't change

[Conor]

Hi all, I was wondering if you could help me out. I have an operator entered SP for pH on an Odour scrubber. Every once and a while an SP gets entered, not by the operator, and the system fails. I believe that the problem is in one of the Scada Screen's (Wonderware), which I am trying to trace. Is there anyway that I can make sure that the operator entered SP does not change, i.e. if I get the spurious entered value can I check this and over right. The code for this is in a PLC 5. I am thinking of some kind of checking via an array. I am not sure how to implement though. Thanks, Conor

[Rod_Hackney]

Do a limit comparison on the setpoint you are writing down. Write it to the PLC, if it is within limits (using comparison instructions) copy it to a address used in your control. If if is not, copy over a setpoint you assign in the PLC and then perhaps latch an alarm bit to notify the operator.

[rquirch]

I've seen this happen due to a number of weird reasons :) faulty touschscreen, duplicate address in the wonderware app, address beign overwritten in the PLC by something else, KVM button being pressed when the lid is down etc etc... i've worked aorund it by setting up a "confirmation" pop-up when writing a setpoint. Bring the setpoint up, have the user enter the desired value and press ok Move the entered value to a memory register and not the IO register. Display confirmation pop-up "are you sure you want to enter..." then press ok for a second time. after this confirmed "Ok" move the value in the memory register to the IO and write to PLC. it's a hassle for the OP but it gives you an added layer of security.

[paulengr]

You can lock out data files (entire file) with passwords by using the security functions built directly into a PLC-5. Note that this completely disables write access for the HMI but gives you 100% control from RS-Logix. You can optionally allow only certain data paths to work vs. others such as allowing full access via serial port but not Ethernet. I had a situation not too long ago where I did something similar.

[PLC_man_Stan]

I've also used the confirmation pop up on wonderware, it really is a good practice to adopt, it also gives your operator a second chance at looking at the value they have entered.

[Conor]

Thanks guys. You have given me good pointers to where I shoud go with this.