S7 address registers

[CT782]

Can someone define the address registers for me, as in the ones manipulated with LAR1, LAR2. Are there only 2 in the S7-300? How are they different from the accumulators? Can you use them as extra storage space if you need to load more than the 2 accumulators will hold? are they retentive? I saw in some old posts that they are used with indirect addressing.

Forgive my ignorance, I've never done any programming using these before.

Thanks

Chris

 

[dchartier]

Hello Chris;
The address registers AR1 and AR2 are effectively used by Siemens S7 PLCs for indirect register addressing. They are 32 bit registes that are dedicated to indirect addressing (by the programmer, in STL, or by the CPU), they are not retentive and they cannot be used for anything else. Some of the larger S7-400 CPUs have 4 accumulators; S7-300 only have 2; you cannot use the address register as storage for them, anymore than you could use the stack pointer register or any other internal register dedicated to CPU use.
At the following address (search for ID: 365590, for example) you will find an explanation of Siemens' different types of indirect addressing, including the use of the area registers:

http://www4.ad.siemens.de/-snm-0135030360-1084460314-0000005371-0000000015-1086970874-enm-WW/llisapi.dll

Hope this helps,
Daniel Chartier

 

[CT782]

Thank you, that link was very helpful. I have only been using Siemens PLC's for about 2 years now so I'm still in the learning phase. My background was with GE. I think I had just graduated from Logicmaster to VersaPro 1.something when I switched jobs and wound up in Siemens-land. I have been reading some of you guys posting about indirect addressing, but being so 'green' some of it doesn't make sense, the address registers for instance, the Siemens help file on them was pretty vague, but reading you guys'posts helps me more than you can imagine. I have one customer with machine that has an S7 400 in it and uses indirect addressing almost exclusively for just about everything, it's a pain in the behind to troubleshoot this thing! That's why I'm trying to learn as much as I can about it so I'll understand WHY! you would do such a thing. Reminds me of the way they build cars now.

Anyway thanks again! Hopefully I'll eventually figure this stuff out!

chris