Block Transfer Read/Write & Messages, PLC5 to ControlLogix Conversion

Hello, I am currently converting a project from PLC5 to ControlLogix. I have migrated the program from Logix5 to Logix5000 and any Block Transfer Read, Block Transfer Write, or Message is converted to a message in 5000.

I have a few questions:

1. In the old system there was only 1 rack with a PLC5, the other 4 simply had I/O and communicated that back to the PLC5 over DH. These BTR and BTW instructions were only used for the analog data somehow, correct? If yes, can you elaborate on this a bit please.

2. As I said earlier, there were also message instructions. My understanding is that message instructions are only needed when communicating between multiple PLCs. So if there was only one PLC5, why were message instructions being used?

3. The upgraded system will only have 1 rack with a PLC (1756-L73) and will be communicating with the other 4 racks over ethernet. Since ethernet is being used, are these BTR, BTW, and message instructions even necessary anymore? Do they need to be replaced with a MOV or COP instruction?

If more information is needed please let me know. I'm new to the industry and this is the first project I was put on so I'm definitely a noob here.

Edit: to be clear, we are using the AB PLC5 to ControlLogix 1756 Conversion Kit. All old racks and cards will be upgraded to ControlLogix racks and cards.

See Comments below.

K_Lich10hr said:

Hello, I am currently converting a project from PLC5 to ControlLogix. I have migrated the program from Logix5 to Logix5000 and any Block Transfer Read, Block Transfer Write, or Message is converted to a message in 5000.

I have a few questions:

1. In the old system there was only 1 rack with a PLC5, the other 4 simply had I/O and communicated that back to the PLC5 over DH. These BTR and BTW instructions were only used for the analog data somehow, correct? If yes, can you elaborate on this a bit please.

Click to expand...

Don't confuse Data Highway Plus (DH+) and Remote I/O (RIO). DH+ is a Peer to Peer Communication between PLCs and other smart Devices. RIO is for Communicating with I/O Cards in Racks and Devices that are setup with AB RIO (i.e. PanelView) etc. They both use Blue Hose, but serve different purposes.



Digital I/O is natively scanned by RIO, but Specialty Cards like Analog, Counter Cards etc. require Block Transfers (BTW/BTR). This is due to the history of the 1771 I/O.

K_Lich10hr said:

2. As I said earlier, there were also message instructions. My understanding is that message instructions are only needed when communicating between multiple PLCs. So if there was only one PLC5, why were message instructions being used?

Click to expand...

You are generally correct about the Message Instruction (MSG) in the PLC 5. It is hard to know what the MSG is communicating to without seeing the PLC Program and knowing the Plant layout.

K_Lich10hr said:

3. The upgraded system will only have 1 rack with a PLC (1756-L73) and will be communicating with the other 4 racks over ethernet. Since ethernet is being used, are these BTR, BTW, and message instructions even necessary anymore? Do they need to be replaced with a MOV or COP instruction?

Click to expand...

You would not use BTW/BTR Instruction with Ethernet, they are used for RIO Communications.

K_Lich10hr said:

If more information is needed please let me know. I'm new to the industry and this is the first project I was put on so I'm definitely a noob here.

Edit: to be clear, we are using the AB PLC5 to ControlLogix 1756 Conversion Kit. All old racks and cards will be upgraded to ControlLogix racks and cards.

Click to expand...

Ok, it is good thing you mentioned this. If all the 1771 I/O is being replaced with equivalent 1756 I/O Cards over Ethernet, then the I/O Setup will be just like any normal ControlLogix Project with Ethernet I/O. That is an entirely different subject.


Time to start reading Manuals.


https://www.rockwellautomation.com/en_NA/literature-library/overview.page


Hope this helps.


Stu....

sthompson that is an excellent concise reply

EICS said:

sthompson that is an excellent concise reply

Click to expand...

Well, thank you sir.


I do try to make my answers that way, but I am not always successful.


Stu....

K_Lich10hr said:

Hello, I am currently converting a project from PLC5 to ControlLogix......

Click to expand...

I do not know your application, but using the conversion tool can be "damaging" to the outcome.

1. Free-running (or self-resetting) messages will need to be timer driven, the code in a Logix machine runs much faster, and you can easily eat up the bandwidth of the communications. You might be using 100Mb/s ethernet, but please remember the poor old backplane that runs at 5Mb/s.

2. PID loops will almost certainly need to be re-tuned in the new controllers. Although they use the same base equations, they do not perform the same, in my experience.

3. Possibly the biggest hurdle is the different rounding algorithm used in Logix5000. PLC5 and SLC used the traditional xxx.5 = round UP, whereas Logix5000 controllers use the "round-to-even" method.... 2.5 rounds to 2, 3.5 rounds to 4, etc. This may have significance if your PLC5 code relied on the round up method. And this doesn't just apply to math instructions, ANY destination tag that is integer, when faced with the result of a floating point calculation, will perform rounding, even a MOV will do it.

And just so as you know, DIV, 11, 2 = 5, but DIV, 11, 2.0 = 6

The former uses all integer data-types (even though I've quoted them as literals), so the controller performs integer math on the operands, so all results are truncated. The latter forces the controller to "elevate" all the operands to REALs, does proper REAL math on them, and then rounds the result to fit into a destination integer tag.

Operand and Destination tag data-types really do matter in Logix5000....