Structured Text

dbh6 · Mar 7, 2013

Hello everyone,

Ok, as i'am becoming a more advanced plc programmer, I have been on more and more service calls lately. Some programs i ran into are written in structured text, and coming from a electrical engineering background, it took me a while to trouble shoot. Anyways i have the sudden interest to understand how to write code in structured text, and i would like to know if anyone has, like a .pdf or a book online or something that i can use to build my knowledge, thanks.

dmargineau · Mar 7, 2013

http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm007_-en-p.pdf

RussB · Mar 7, 2013

Nothing against RS, but they really do not have a very good reputation for following "Standards" which they have not written or own.

A couple more good sources:

http://www.eod.gvsu.edu/~jackh/books/plcs/chapters/plc_st.pdf
Even:
https://en.wikipedia.org/wiki/Structured_text

dbh6 · Mar 7, 2013

thanks guys keep them coming

Ron Beaufort · Mar 7, 2013

first a general word of caution: be sure that you research and understand the difference between the two following assignment instructions:

:= and [:=]

they can give different results when the processor does a "Go-To-Run" operation ... programmers just starting out with Structured Text can be unpleasantly surprised by the differences ...

moving right along ...

if you used the default installation settings for RSLogix5000 then you probably have the following files in the SAMPLES folder of your program directory:

ST_GearChange.ACD

ST_Motion_Example.ACD

these contain some examples of Structured Text programming which you might find helpful ...

dbh6 · Mar 7, 2013

thanks for the heads up Ron, much appreciated

Peter Nachtwey · Mar 7, 2013

There isn't a [:=] instruction in structured text unless this is part of the new unreleased specification.

Anirban Hazra · Mar 8, 2013

Peter Nachtwey said:
There isn't a [:=] instruction in structured text unless this is part of the new unreleased specification.

It is there in Structured Text in RSLogix 5000. It is used for a non-retentive assignment.

Reference:
Publication 1756-RM003k-EN-P - July 2008

Extract(Page 662) from the publication shown below.

Peter Nachtwey · Mar 8, 2013

I am writing a IEC compatible ST compiler. I have the IEC 61131-3 specification for ST. [:=] is not part of the specification.

JesperMP · Mar 8, 2013

Even if RS has added an 'non-retentive expression' to RSLogix 5000 that is not part of the IEC specification, I cannot grasp it.
How can an expression be non-retentive ?
A Tag (i.e. a memory address) can be retentive. That I can understand.
Does it mean that in code you can turn the retentiveness of a Tag on and off ?

Manglemender · Mar 8, 2013

Like Jesper, i too am peplexed by this. What if I had several of these in a program:

myTag := Value_1
// some code
myTag := Value_2
// some more code
myTag := Value_3

Which value or expression would be retained? Presumably the last one processed.

Nick

Anirban Hazra · Mar 8, 2013

Let me try to explain the difference between using a ":=" and a "[:=]" in RSLogix 5000 with a simple code.

Code:

Motor := (Motor OR  On) AND NOT(Off);

IF On

THEN

 Value := 70;

 END_IF;

When the above code is run, then when "On" becomes high "Motor" becomes High and "Value" becomes 70.Then if "On" becomes low
"Motor" remains high and "Value" remains 70.

After that if I make the controller go to "Program Mode", and then bring it back to "Run Mode", "Motor" still remains high and "Value" is still 70.

However if I code like this:

Code:

Motor [:=] (Motor OR  On) AND NOT(Off);

IF On

THEN

 Value [:=] 70;

 END_IF;

Then the same things happen when "On" goes high and then goes low as stated above but this time if then the controller is brought back to "Run Mode" after having switched to "Program Mode" the "Motor" becomes low and "Value" becomes 0,ie. they get reset.

JesperMP · Mar 8, 2013

I think I understand what the function does. But I am perplexed to find that to know if a Tag is retentive or not, you would have to trawl through all the code, and then interpret all the assignments and estimate if a conditional assignment was activated or not. Very confusing and very time consuming.
And what happens if a tag is written to from somewhere else, from an HMI for example ?

Ron Beaufort · Mar 8, 2013

the way Allen-Bradley implements it in RSLogix5000 is this way:

:= is RETENTIVE and basically acts like an Allen-Bradley OTL (Latch) instruction in ladder logic ...

[:=] is NON-retentive and basically acts like an Allen-Bradley OTE (Output Energize) instruction in ladder logic ...

secret handshake: the Allen-Bradley processor handles the retentive and non-retentive assignments/instructions differently on PRE-scan – and in certain SFC (Sequential Function Chart) operations ...

the obvious discussion that this has generated proves my original point ... specifically, many programmers aren't aware of the differences - and sometimes unpleasant surprises are the result ...

party on ...

Manglemender · Mar 8, 2013

Thanks Ron, I think I'll stick with Siemens... much easier.

Nick

Structured Text

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