Stupid Siemens S7-300 Questions

[eaw]

Thanks to all who have responded. It has been helpful.
I have already went down a few dead end streets in Siemens using the A-B programming mindset. I'm feeling more comfortable with my plan of attack on this program.

I'm interested in the UDT thread and will try to find it. I used them extensively with ControlLogix.

I haven't started tackling this yet, but this whole Timer idea with some Timers defined (0-255) and the build-your-own IEC style timers? Whats up with that?

 

[RMA]

I'm interested in the UDT thread and will try to find it. I used them extensively with ControlLogix.

If you've already got 15 years experience, this should blow your mind here's the link to S7Guy's UDT Thread, well it's not his Thread, actually, but it ought to be!

I haven't started tackling this yet, but this whole Timer idea with some Timers defined (0-255) and the build-your-own IEC style timers? Whats up with that?

The Siemens standard Timers are a hangover from the old S5 system, in general they're a pain in the neck because they use an odd format for the Time Preset - either BCD based with a maximum count of 999 and maximum Timebase of 10 secs, or Hex based, which means the maximum time is about 2 1/2 hours or so. However, they have one advantage - they count down, which can sometimes be useful. Oh, and some of them can be rententive, how many is CPU dependant. The IEC timers are much simpler, firstly, the number is only limited by CPU memory. The time is simply a DINT (I think, I'm not at work at present, so can't check) and the integer runs from a 10 ms timebase - I'm not sure whether that varies with CPU type, maybe the 400 series can have a 1ms timebase, but I'm not sure. The IEC counters count up and are not retentive, but somebody (KenM?) has published a retentive version here.

Siemens is the first PLC I've had anything to do with, I was previously involved with DCS systems, and I must admit, I found the learning curve quite steep. However, in the five months since I've been here, I've learned more than in the previous four years and slowly I'm getting to quite like the beasts.

 

[RMA]

Don't ask me why in the middle of listening to Act three of the 1968 Erich Leinsdorf RCA recording of "Un Ballo in Maschera" with Leontyne Price and Shirley Verrett (on vinyl, of course!) it suddenly occurs to me that I may be a victim of the "Americans and British, divided by a common language syndrome" - I don't know either!

My comment that S7Guy's example would blow your mind was meant in the sense that with 15 year's experience, you would be able to appreciate what was possible with S7Guy's example. I have not yet reached the point where I can implement it and it is probably not entirely appropriate to my current project, but I assume that in future I am going to be involved in "Maschinenenbau" whether I like it or not and I am certain that I will then come back and see what I can implement from his Post - and the credit will be writ large in NW1 of OB1!!!

 

[JesperMP]

Even I haven't used any Siemens (mostly AB, in past also Omron and Mitsubishi), the use/choose of memory areas seems too complicated. Why it should not be made easier (AB is I think good example)?

The inclusion of Merkers and S5 timers in the S7 system is a quirk, brought by Siemens wish to be able to import programs from S5. This is an understandable decision, considering the enormous amount of S5 code out there.
The trouble with Siemens isnt that there are such quirks, but that Siemens dont make manuals and help texts as intuitive as one could wish for.
That makes it difficult for beginners to grasp the big view of Siemens architecture - quirks and all.

 

[Fred Floggle]

I don't see that M bits are particularly confusing - you could say that exactly the same exists in logix 500 - you have a choice to use Binary staus flags or bit from an N file - the parallel would be to use either M bits or bitwise use of a byte in a db .
The minute you write your first siemens FB , and practice a little with the concept of multi instances , you will see just how powerful this stuff really is .