Old PLC to New PLC communication

I have a 1756-L55/A PLC with a 1756-DNB DeviceNET module, a 1756-DHRIO Scanner and an EtherNET module. Using RSLogix 8.02

What is one of the best ways to communicate with a new modern Allen Bradley PLC?

We want to upgrade a machine. But, move things over to the new PLC in phases. And, keep the old one online until everything is moved over.

I would have to research if you could do Produce/Consume Tags over Ethernet with a version 8 controller; back then it might have been over ControlNet and the backplane only.

But if not, you can definitely send and receive tag read and write messages via the Ethernet module.

Getting your hands on v8 software is probably the hardest part !

By chance could you upgrade the fimware of the L55 to v16?

If not, as Ken mentioned you should be able to send messages back and forth.

I second the idea to update the firmware to the highest level that CPU can handle. I was in a situation similar to yours about 10 years ago. An extruder in our company was programmed using Logix v2.xx. There was only one computer that had that version installed and everyone was afraid to do anything to that extruder. I used a spare CPU and converted the program to the highest rev available for that CPU. Once the conversion was complete, I swapped CPUs. I kept the original CPU for a month and when no conversion problems were found, I updated that firmware. My suggestion is to start there.

Good point drforsythe, you can get another processor ready to do a swap and keep the old one "on deck". The processor could have upgraded firmware and newer comms setup with minimal risks. Produced/Consumed tags are pretty sweet, just make sure to create an array bigger than you think you'll ever need since you can't create them online.

A quick check from where I buy my used stuff shows an L55 can be had for sub $200 delivered and that includes a year warranty.

If you wanted to go the route of buying a used processor, I think a L61 is the same price if not cheaper than a L55 on the used market.

So, you think that produced/consumed tags would be easier than messages?

Messages you can do all online
Messages will be less risk if you want to do changes online.

Produce and consume is a mixed bag of online and download changes. Therefore its more risk since you have to download to setup the produce and consume.
Produce and consume tags you need to put the processors in the IO config tree.
Just look at rslinx and build the same structure in your tree.
Create your tags: I like to name my tag starting with the following format:
Produced_To_PLCname
Consumed_From_PLCname
This way you can easily identify a produce and consume tag.
3rd step is save what you have online upload tags.
Now go offline
Go to your Produced tag, there are type that needs changed, and other RPI settings, and how many consumers.
Go to your Consumed tag, do basically same thing but you need to select the tag from other processor and other feature settings.
And then download these changes.

Jim3846 said:

Messages you can do all online
Messages will be less risk if you want to do changes online.

Produce and consume is a mixed bag of online and download changes. Therefore its more risk since you have to download to setup the produce and consume.
Produce and consume tags you need to put the processors in the IO config tree.
Just look at rslinx and build the same structure in your tree.
Create your tags: I like to name my tag starting with the following format:
Produced_To_PLCname
Consumed_From_PLCname
This way you can easily identify a produce and consume tag.
3rd step is save what you have online upload tags.
Now go offline
Go to your Produced tag, there are type that needs changed, and other RPI settings, and how many consumers.
Go to your Consumed tag, do basically same thing but you need to select the tag from other processor and other feature settings.
And then download these changes.

Click to expand...

I like to keep identical tag names in both processors - it provides a visual "link" which can aid in fault-finding. In fact I create the tag in one controller (either), copy/paste it to the other so I definitely know I'm getting an identical structure (which it has to be). I then modify the copied tag to be either produced or consumed, depending on where I made the first one. It took longer to type that than it takes to do ...


You can use the filters on the tag database to identify Produced and Consumed, but how often do you need to ? Once it's in and working it's a forget about it job, just like your I/O module tags. Consumed tags scream at you anyway with the "Base Tag" column telling you where the base tag is being consumed from...


I like to make UDT's for produced/consumed data, for several reasons.....

1. I can include the Connection_Status member, which has just 2 BOOL flags (a) The "connection" is valid, and (b) the remote controller (the producer) is in run mode. Just those 2 bits tell me the consumed data is 100% valid.

2. I can add "spare" data locations for when I need that little bit more.... So long as I keep my UDT size below 500 bytes there is little overhead for making it bigger than I actually need at the time.


3. I only need create the UDT once, then a quick copy/paste into the other project and the job is done, with guaranteed success, even if I misspell a member name ....