Shift register method vs step ladder method

[pjones6646]

Can somebody please explain to me the main differences between Shift Register and step Ladder programming also which is the best method to use and why. Thanks to anybody who helps.

PJ

 

[Goody]

Is there a class on Mitsu plc's going on at the moment?

I have to take a guess, as you hav'nt mentioned the brand of PLC.

So I am guessing you mean Mitsubishi's own sequencing method Called STL
Step ladder to other brands simply means ladder programming.

Tell us which brand and then maybe we can help you better.

 

[pjones6646]

sorry, you are right it is mitsubishi. im using the Mistubishi FX0N

 

[kamenges]

Goody-
I think what the instructor is looking for is a broader review of sequencing methods as opposed to how to make them work on a given platform. I'm not sure why they limit themselves to bit shifts, though. I've always liked integer based state machines myself.

Keith

 

[Tim]

pjones6646,
There are three formats of STL programming on a Mitsubishi Plc. I believe this starts with the FX series and newer. See diagram below. I have my preference in one, as others probably do. I wouldn't see why there would be the need for a comparison between one of these and a shift register. I use shift registers in 90% of my programs only because a maintenance tech can monitor the register to see where it is stuck at in the process. Shift registers are normally designed for conveyor processes. Whats odd is the 90% of my programs are "not" conveyor type processes. It's a modified shift register. The machines I program have a starting point and an ending point. Shift registers work great for me in this case because I can turn on one bit and use that bit to follow a sequence. Have you tried to program a shift register and see how it works? There are limitations to it.
Tim
Good Luck! beerchug

 

[seppoalanen]

This is sequence model with shiftregister where 16-bit word bit 0 have set after last step or when word contents is zero.
As you see 7215STEP5 have done as natural-one-shot, it is one step for sending message to InkJet-marker.
Other method is do own separate rung for every step, but monitoring is not so nice.

Program design have shown on web-page:
http://www.kolumbus.fi/sepala/WEB/S7-I7215.htm

 

[Peter Nachtwey]

State Machines

Three different forms of state machines have been mentioned so far. I must admit that I use the same technique as Keith most of the time. All the demos on our website use the integer state machine approach. However, it is not as flexible as the STL or IEC method of using bits to represent states as they allow for parallel branching whereas Keith's ( mine ) or Seppo's does not. I think the second best approach is the Keith's ( my ) technique. The reason is that it is not limited by the number of bits that fit in a word or dword and it is easy to skip steps by adding a constant more than 1 to the state. Another advantage of the integer techinique is that it is hard ( impossible ) to be in more than one state at a time. The bit techniques work well, but one must be careful to be sure that state bits are not accidently left on unless on is trying to implement a parallel branch.

 

[seppoalanen]

If used Peter's and Keit's integer-based technic you can replace step-bits by comparing integervalue=1,2,3,etc. and increment integervalue when RLO is true.
That is good for HMI (PanelViews etc.) where is possible show what process is doing and what condition it is waiting. It is possible also by bit-register, but limited 16 to 32 steps.
Pointing by Integer-value is simple than by bit some cases.
Integer-value can be direct Key-Code for data-table where are more information etc...

 

[pjones6646]

Thank you all again for your help, i have everything i need now (at least for the moment anyway). All take care i can guarantee i will be back if i need more assistance, following the (to put it mildly "Sparsly stocked") college library anyway.

PJ