PLC's and MODBus

[rmaynard]

The company iv been working for have recently moved into using equipment that utilizes PLC's and remote management, while im up to speed on networking and remote access the PLC amd MODBus side of it is new to me. As im installing the equipment id like a greater understanding of PLC programming and specifically MODBus over TCP. What im after is any info anyone has on MODBus and MODBus registers, Books or PDF's would be great and also anywhere i might be able to get hardware to practice on.

Cheers
Rob

 

[Knobhead]

ModBus or ModBus Plus?

Which ever you have, you have to assign a global address and a block of contiguous registers for each node in the configuration. You can assign individual register blocks (16) but you will be making a rod for your own back if you do.
When we first used ModBus I had to move used registers so that I had blocks of contiguous registers free for each address. Problem was although Modicon would move registers and all calls to that register easily, it didn’t move the descriptors so I had a lot of editing to do.

One thing to watch, although two PLC’s are from the same manufacturer (supposedly). We found the Modicon 984’s had a reversed bit pattern to Modicon TSX. Magelis panels could be configured either way, which was fun when I had to transfer registers from Magelis to TSX via the 984’s.

PS just to add to the confusion we also used ModBus’s predecessor PeerCop as well. Trying to remember what went where was a nightmare. 5 x 984’s, 3 x TSX’s and 4 x Magelis panels.
This was the first major plant in the UK to be totally controlled from the outset by PLC’s, originally Modicon 384’s. So we had hangovers from the early 60’s. 4 node, 4 line networks was the worse bit.
The 984’s replaced the original 384’s, the TSX’s and Magelis were added later. Hence the muck up over contiguous registers, people would just look for a free register and use it. 1556 registers to go at and someone (a total pratt) in the past had plonked something in the middle of a block.

I was brought up with Texas Instruments PLC’s, they were carp. Forever locking out on the watchdog timer. The watchdog operated a relay, we sorted that out. I can’t remember who donated their terminal driver. It was jammed in the relay for years.
So the Modicon 384’s didn’t surprise me when I moved to another company and i was chucked in the deep end. What did surprise me was the 386’s use of DX function blocks, they were a god send.

 

[rmaynard]

Thanks that's a lot of info, the system uses modbus tcp, iv never really got into PLC,s and what not but it'd interesting

 

[WaterSparky]

If your company buys alot of gear off of one manufacturer it maybe an idea to see if you can get some training days with them as they can be very knowledgeable, we do this with Schneider and have a few a year! Good way to learn.

 

[Knobhead]

Schneider also provide some of the worse training. They don't listen to what's required and go in to autopilot mode.

A shopfloor guy wants to know how to fault find and do a quick work around a fault, end of!
Certainly not how to set up and configure the I/O nor did they want the topography.

It wouldn’t have been so bad if I hadn’t had to show the “instructor” how to set up the configuration.

A three day course, only the last ½ day was what we wanted. The rest was total mush to the guys at the sharp end, nothing they needed to know. They had all switched off by then!

It was an expensive total waste of time.

Everyone on the course had worked on PLC’s since being apprentices. They knew about addresses and the topography, it wasn’t what they wanted!

 

[WaterSparky]

When was your last dealing with them tony?
And who was the trainer?

I can't comment of the price, as afaik I think we get our training free. But usually a course is setup to what we ask for and then we get one drawn up and then we attend with our training rigs, nothing like the real world but a good base to work from.

I have to admit you are right and you do usually get some guff at the beginning, also problem is like you say they go into autopilot mode at points what seems like child's play and basics to them is sometimes not so, well for me anyways. Ive had to reel them back and re-ask the questions at times.

Sounds like your training was poorly planned maybe.

 

[Knobhead]

Our company paid for the training. We gave a list of what we wanted which the company passed on. It was ignored.

 

[shanky887614]

Schneider also provide some of the worse training. They don't listen to what's required and go in to autopilot mode.

A shopfloor guy wants to know how to fault find and do a quick work around a fault, end of!
Certainly not how to set up and configure the I/O nor did they want the topography.

It wouldn’t have been so bad if I hadn’t had to show the “instructor” how to set up the configuration.

A three day course, only the last ½ day was what we wanted. The rest was total mush to the guys at the sharp end, nothing they needed to know. They had all switched off by then!

It was an expensive total waste of time.

Everyone on the course had worked on PLC’s since being apprentices. They knew about addresses and the topography, it wasn’t what they wanted!

Depends who you speak to tony.

I know 3 of the engineers I would follow without question and they listen to what you are asking.

With a couple of them we just dismiss what they ask/say


There currently planning on setting up testboards, ahu's etc at there office to train on as well

 

[Robotstar5]

Schneider also provide some of the worse training. They don't listen to what's required and go in to autopilot mode.

Have to agree, the last one I did ended up as a sales pitch for the (then) new Portal software, the only saving grace was a good spread put on for lunch!

 

[Knobhead]

Depends who you speak to tony.

I know 3 of the engineers I would follow without question and they listen to what you are asking.

With a couple of them we just dismiss what they ask/say


There currently planning on setting up testboards, ahu's etc at there office to train on as well

When I took delivery of our first TSX the programming guide didn’t arrive. It never did for that matter.

Configuring it was pretty straight forward, it was PL7 I was struggling with. To get started I just needed a few basics. I phoned our local Telemecanique technical rep, chatted with him for about ½Hr and set to. He called in to see me about a week later and it was sat on my desk running the program I’d originally written for Modicon. He was one of the good ones.

At the same time I nailed him over another project.
I loved his reply, “I can’t say it will work because you’re pushing things over the limit. It should be OK. Let me know if it works.

RoB2 will confirm a similar conversation as he was working on the same thing for a different company at the same time. We didn’t know each other back then but we had both came up with the same solution. Neither of us told them the outcome, we must be a pair of rotten sods. It did work, my job would have been on the line if it hadn’t.
We were really pushing standard contactors by putting 1.7KV on them. I didn’t tell the rep I’d already tested them to 5KV.

I suppose like any company they are sales driven, but when I want training on one product I don’t want to hear about the new totally unrelated toy.

PS Eurotherm were as bad, another two days of my life wasted listening to sales drivel.