Learning allen bradley - i bought PLC5 and comparing to S7 315

[hewking 1]

hi All

I am trying to learn allen bradley PLC and i bought PLC-5 enhanced PLC/20.

My first impression is very positive how powerful this PLC is for its age. My background is siemens so i am trying to learn allen bradley by youtube or reading manuals and also comparing to siemens s7.

Any chance you give me your feedback please.

1. PLC-5 seems to have 32 concurrent tasks which is similar to having 32 OB1s in siemens which is continuous task. This nearly blew my mind :) Does anybody know how this concurrency is achieved - is there firmware assigning each task a fixed time eg. 20ms and then jumping to another one?
2. Does anybody know why PLC-5 is using octal system which is very unique
3. I heard that allen bradley can simulate things in very sophisticated way using debug files - any chance somebody give me a gist a little bit?
4. Why there are so many ways to use memory - half slot full or double?
5. What is rack really - in siemens its just bunch on modules on a rack :)
6. I also heard that allen bradley when simulating signals its doing it in unique way comparing to simplistic way in which rockwell allows simulation of inputs or outputs during testing - can you help me to get my head around this ?
7. Is rockwell website my best source ? -of course apart from this forum :)
8. Is there any place to find old, educational PLC-5 programs to understand - people dont seem to share it much i believe

Regards

Edited 27 Jan 2023 by hewking

[pturmel 160]

1) They aren't actually concurrent.  All but one have to have periods and priorities to govern execution interruption.

2) Backwards compatibility with prior processors, and only used for the I/O files.  The I/O chassis for PLC-5 was inherited from the PLC-2 and PLC-3 families.

3) Never used this.  But is probably related to the fact that logic can write to the input table if there's no actual I/O occupying the address.

4) Again, old circuit compatibility, with required many compromises and contortions.

5) A PLC-5 Rack is allocated space for 8x 16-bit inputs and 8x 16-bit outputs.  A physical 16-slot rack can be as many as four logical "racks" under this usage/contortion.

6) Probably just an aspect of #3.

7) Yes, grab all the manuals you can find, especially the instruction set reference.

8) Not that I'm aware of.

I must say, the PLC-5 is a terrible choice for learning Allen-Bradley for today's world.  Try to find a used CompactLogix L16 instead.

[Joe E. 259]

I don't know if the AB website is the best resource or not. I started out in the AB world and Siemens world at the same time, working with the SLC 500 and S7-300/400. My PLC-5 experience came later. I never took a class, so my answers are just what I've learned working with them and  using their manuals. There are a few others on here who would be able to give you far more comprehensive answers.

1. I'm not sure about tasks. As far as I know, it has a single task thread where it follows a "standard" PLC sequence: read field inputs into memory, execute code, write output memory to the field devices.
2. The PLC-5s that I used lived in the same physical chassis and footprint as the older PLC-2, which was an octal system. That legacy carried over in some respects.
3. I've never used debug files. I'll have to dig into them
4. Another legacy of the older families, I think. That addressing system lets you pack 16-point I/O modules where the 8-point modules lived previously. Remember that memory used to be a lot harder to come by than it is now, so being efficient with it was a higher priority back then.
5. Rack is more of a logical term than physical with these. I still think in my head about "chassis" instead of rack because of it. I have it broken down for myself like this:
   Group: 16 bits, in 2 octal words (00-07, 10-17)
   Rack: 8 groups (this is a logical rack)
6. Not sure what you're asking here. When simulating code in the 5000 world, you can have the IO tree completely built but each device inhibited and the CPU will chug along just fine without faulting because the devices aren't there. In the 500 world, the configured IO has to be present or the CPU will fault. In the 5 world, the IO memory is all pre-allocated per processor, whether a device is in that location or not. In that sense, it's similar to a 5000 with its modules inhibited. You can read/write the IO while simulating without the IO being there.
7. I've linked a few manuals that I've found useful in my journey
8. Almost all of my work with PLC5s was in running systems, so I don't have any training resources.

https://literature.rockwellautomation.com/idc/groups/literature/documents/qr/1785-qr001_-en-p.pdf
https://literature.rockwellautomation.com/idc/groups/literature/documents/qs/1785-qs003_-en-p.pdf
https://literature.rockwellautomation.com/idc/groups/literature/documents/gr/lg5-gr002_-en-p.pdf
https://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1785-rm003_-en-p.pdf
https://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1785-rm001_-en-p.pdf

I have an installation manual (1785-6.6.1) and an addressing manual (5000-6.4.4) that I can't find on AB's site and attachments don't seem to be working right now.

edit to add:
Pturmel posted just as I did. I would also suggest not starting with a PLC-5, unless you're in a place where you need to support a bunch of them and you have access to someone else paying for the software license. Last quote I got for RSLogix 5 was around $10k about 9 years ago. It's likely gone up since then. My last location had a bunch of them and we actually bought some used PLC-5 CPUs to upgrade PLC-2s because we had the software and a base of spares and they dropped into the chassis. It was a fairly simple migration to get us away from DOS PCs, but is definitely not the ideal route or the path I would take for new builds or for learning.

Edited 27 Jan 2023 by Joe E.

[hewking 1]

@pturmel What a great day already to meet you on that forum Phil :) You are a beacon of knowledge on ignition forum so really happy to see your reply.

Quote

I must say, the PLC-5 is a terrible choice for learning Allen-Bradley for today's world.

It might be too late as i already fell in love with this over 35 years old PLC :) which i bought for cheap on ebay full of cards.

I understand it might be harder learning path then the latest PLCs but there is a lot of good engineering built into this device 

which impressed me already. RsLinx works like magic. Also environment visually good.

Regards

 

Edited 27 Jan 2023 by hewking

[hewking 1]

@Joe E. thanks for a lot of knowledge shared i appreciate it