STEP7 GRAPH and HiGRAPH

[JesperMP]

Doen anybody know what really is the difference between these two option packages for STEP7 ?
Siemens sales pitch is very sketchy on what you really get.
Why would I choose one over the other ?

 

[kennyb]

Sorry JMP,
I've never used either of those packages and never really had the necessity. When I asked a couple of Swiss programmers (heavily into S7) they just shrug their shoulders and say it's nothing special, don't worry about it, you don't need it?! Isn't it just another method of representation like LAD/STL/FBD?

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It's hard to make a program foolproof because fools are so ingenious.

 

[RGB]

Jesper:

S7-Graph lets you build sequencers; HiGraph is for state-diagram programming.

 

[Ken M]

Jesper

S7-Graph is Siemens' name for their implementation of the IEC61131-3 SFC (Sequential Function Charts) language. (Except I notice PLCOpen and the IEC have stopped referring to it as a 'language', and say it should be regarded as a "method for structuring control programs".)
It's a very visual tool designed ideally for situations where you have a process with a specific procedural path - do this, wait for the response, now move on to that, check the feedback, now do the next thing ... and so on. It encourages you to lay out the idealised control flow for everything in the machine/plant. The step/transition method also makes fault-finding really easy. The process is stuck at a step? No problem: the next transition hasn't come true. That's it. No more wading through yards (sorry, metres) of code to find out.

Hi-Graph on the other hand is a Siemens' own-brand programming language with no counterpart in the IEC61131 standard. I think it was developed in conjunction with some universities in Germany plus some input from big German customers of Siemens like VW and BMW. It is also very visual, but instead of the strict SFC approach of one step / one transition, Hi-Graph is more of a state machine approach. Define your process or operation as a series of related states, and then define the conditions which would cause you to leave State 1 and go to State 2. And State 3. And State 4. etc. Now do the same for State 2, and so on. Its evry good at dealing with processes where there is no consistently predictable path through them, but you do need to prevent 'accidental' activation of states. If you don't explicitly code a transition between states it won't ever happen.

Both S7-Graph (and all other SFC compilers I've seen) and Hi-Graph carry a memory overhead, but if your process justifies them, the advantages are worth it and memory is cheaper than downtime.

Regards

Ken.

 

[jvdcande]

Both S7-Graph (which is equal to SFC or Grafcet) and S7-HiGraph (which is equal to State Diagram) are graphical programming systems based on states and transitions. When you know the system, it's not hard to translate from one of these systems to more 'conventional' programming methods, such as statement list or ladder.

Back in the early 80's I was working with a PLC for which only statement list was available on a dedicated programming device. We wrote programs pf up to 8192 statements (memory limit). We designed the program on the drawing board using State Diagram and then hand-translated it to statement list. We were used to it in those days and had no problems at all. Nowadays I use SFC more often, but up until now I've never ever used S7-Graph, nor S5-Graph. I always draw it on paper and translate it manually. This gives me the big advantage that I can use SFC on PLC's were this programming method is not included at all. I even use the method to design complex relay controls, pneumatic controls or electronic controls.

A method to translate state diagrams is described on this Mechatronics page, for SFC or Grafcet you can view this Mechatronics page .

Kind regards,

 

[JesperMP]

Thanks all for the excellent info

Ken, if are you saying that GRAPH (SFC) only has one-step/one-transition, then how can I create a state diagram where the program flow can branch out in several directions ? It sounds more like a sequencer (OK, thats what RGB say as well).

I also create my state diagrams on paper before I code it manually.
Actually I wouldnt like to be without pen and paper for the first stage of breaking down the problem. But I was hoping that one of these packages could ease the implementation and maybe bring additional benefits.

 

[seppoalanen]

Maybe its power is with the WinCC. WinCC can read the graph code and updates sequence-view when PLC program have changed.?