Converting Logix Ladder Logic to Step 7

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Thread Starter

Tim

I am a newbie with PLCs in general. I have just created a program using A-B Logix software and now I am informed that I need to convert it to Siemens Step 7 because the PLC I have been given is a Siemens S7-200. I have a number of latch instructions in the A-B software and I can not seem to find any instruction that is compatible in the Step 7 software. The rest of the program I have managed to convert but it just will not work without the latches included. Could someone please help me? Is there any such thing as a latch in the Step 7 software?

Thank you.
 
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Trevor Ousey \(lists\)

Tim,

The -(S) instruction is the same as a (OTL) and the -(R) is the same as a (OUT).

Regards, Trevor
 
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Michael Griffin

Set (S) = latch. Reset (R) = unlatch. This is common terminology with many brands of PLC.
 
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Look in the tree under "Bit Logic". You should see two blocks, "SR" and "RS". They are "Set Reset" and "Reset Set". They might do what you need.
 
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DAVE FERGUSON

Aaaaaaaaaaaaaaaaah, but with all latches and unlatches, sets and resets, the key is safety... What does this particular processor do when the
processor loses and restores power?

AB PLC5, for instance, retains them so you need to write a start-up routine.

Be careful.

Dave
 
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Michael Griffin

In reply to DAVE FERGUSON: Siemens S7-200 memory is retentive as well. Memory is held for about a week with a supercap, or you can install a battery. Program memory is in EEPROM. Data can be saved to EEPROM under program control. A small range of flags (M) can be configured to be automatically saved to EEPROM on loss of power (the supercap presumably provides the power for this). If you wish to have a special start-up routine, there are special memory bits (SM) for this.

You raise a valid point though that one for one translation of PLC programs do not always work because the instruction sets are not equivalent. In this case, items that merit attention are the SM (special memory) bits, and timers. The SM bits provide special features. Timers have types and time bases which are determined by the timer number. You have to plan in advance which timers you intend to use for which functions.

Generally though, the S7-200 is quite easy to program. It is very "conventional", and has little or no resemblance to the S5 or S7-300/400

A minor point to mention is that it would be better if you had phrased your comment as being concerned about "machine malfunction" rather than safety. Safety of course, is addressed by means other than PLC memory flags.
 
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DAVE FERGUSON

I agree and understood this when commenting.

My bigger point had to do with the fact that people all the time are using latches and unlatches in programs without REAL knowledge of them. I try to avoid them or use software seal ins instead.

There are many cases where on a power up or after a failure... something takes off that wasn't planned for due to a latch retaining memory on, but then again this is poor programming.

And in my book, any unwanted movement of a machine that is unexpected can be a safety issue, but I do understand your point.

As another thread on programming says "Do you want to play chopsticks or Chopin?" or something to that effect.

Dave
 
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