RSVIEW32 Derived tag issue

[Buckaroo 0]

I don't see many questions here regarding RSVIEW32 but my hope is someone can point me in the right direction. I have a project that has been up and running since before I came on board. Recently I have been tasked with modifing it. Here is the backround: There are multiple vessels that can contain a variety of products. So in the project all the vessel tags are set like so VESSEL_1_prod where this indicates the product in the vessel as inputed from a PanelView and read as a string. Also tag of VESSEL_1_temp where it is reading the temperature of the vessel. The temperature is maintained at a certain setpoint based on the product. What I am trying to do then is create an alarm by using a derived tag where I read the tag VESSEL_1_prod and if it is A then I want the alarm value to be xxx if it is any other _prod I want the alarm value to be yyy. So my derived tag name is VESSEL_alarm_value and the expression I am trying is IF VESSEL_1_prod == "A" then xxx else yyy. I stop and then start my project and check in the tag monitor where I have placed the VESSEL_alarm_value tag and it always shows the "ELSE" value no matter what I put in there that is what comes up. I am not sure if this relates to the _prod tag being a string type or not. I do not have much experience utilizing strings or this RSVIEW32 software. I will then be using this derived tag in an event but I need to get this part working first. Any help would be greatly appereciated. Thanks Tom

[ScottC 8]

Are you starting the derived tags file that contains VESSEL_alarm_value? There needs to be a command DerivedOn <filename> somewhere that executes when the project starts.

[Buckaroo 0]

Yes. The derived tag I created was added to a derived tag file that is started with a startup macro. I guess a question I have is can you use the expression area of a derived tag to evaluate a tag value for a certain string and then give the derived tag an analog value based on the string? That is what I am trying to do but can't seem to get it to work. Thanks Tom

[ScottC 8]

Is VESSEL_alarm_value a new tag? If so, I think the Tag Server has to be restarted before it will go on-scan. I guess the easiest way to do that is to close the project and restart it. The expression you have should work.

[Buckaroo 0]

Yes that is a new tag I have created and yes I have restarted the project. One thing I did was changed my expression to check the value of a different tag which was an analog type to test and see if I could get it to work. So the expression if VESSEL_01_temp >= 159 then 74 else 45 and I have the derived tag of VESSEL_alrm_value. Restarted my project and watched in the tag monitor and it was working where the tag VESSEL_alrm_value would change between 74 and 45 based on the temp tag. That worked great. Now as I said in first post I want to use tag VESSEL_01_prod which is the product name as a string. This is not working for some reason and may be due to how I have it set up. I do not get any compilation errors when I go to accept the derived tag. I am going to go back and do some testing to see if I can get this to work. Thanks Tom

[Buckaroo 0]

Ok I have it figured out and working now. The problem was I did not take into account the string length. In the tag database the tag is set to read a PLC address with type as string and length of 8. The reason my expression was not working is because I had it set like this: If VESSEL_01_prod =="stuff" then 100 else 50. It would not work because what is inside the quotes needs to be a length of 8 so changing it to "stuff " where there are 3 spaces after the last f and before the closing quotes. After that worked great. On a side note I tried Rockwells LIVE CHAT option for help. Worked pretty good. Anybody else tried this from there website? Any other thoughts or opioions on it? Thanks Tom

[Eddie Willers 2]

Strings have to be surrounded with quotation marks if you are using them in an expression. This Derived Tag expression worked for me, in RSView32 7.40: if ( Vessel_1_Product == "Soup") then 99 else 44

[Eddie Willers 2]

Aha ! String Length, the secret bugaboo of most String issues. I was using an Internal tag, so I was cheating a bit. Thanks for posting the followup !