Block Transfer Error "The Requested Length is Different than Response Length"

Block Transfer Error "The Requested Length is Different than Response Length"

Good day,

I just completed an upgrade from PLC5 to ControlLogix. Everything on the RIO tree pulled up correctly (no yellow triangles with exclamation marks). I then start looking over the message blocks for the BTR/BTW when I notice the following error "The Requested Length is Different than Response Length.

Some background:
1756-L73 controller using a 1756-DHRIO module.
The Block Transfer Read/Write connects to a Prosoft 5601-RIO-MCM gateway, which then connects to a Robicon 454GT drive. This drive was before my time and understanding the full nature of how the registers work is a learning progress.

Now this drive worked fine before the upgrade, which rules out the 5601 gateway and the drive itself.

The message block BTR/BTW reads and writes 64 words. When I look in the configuration file for the 5601, it was setup for only 60 words. I changed the message block BTR/BTW to 60 words. Same error.

I have also verified the R/G/S numbers. Besides that, there really isn't too much for options in that message block, which also makes it very frustrating.

Also to note is I can start/stop and send a reference speed to the drive from the controller, which means there is some communication happening. What I don't see is feedback, errors, running, etc, which is kind of important for control.

Any ways, let me know what you guys figure or things to try. Other message block BTR/BTWs are working fine for the other drives (different models of course, AB 1336), just this one is puzzling.

Thanks for your time!

Check for a Data Size Mismatch. The PLC5 default was 16 Bits, whereas the Logix PLC default is 32 Bits (DINT).

There should also be an Error Code shown, Post that as well.

Stu....

Data Size Mismatch: That was one of the first things I did after using the conversion software.

Error Code: 16#00d3 (see attached).

Thanks,

N25 is a SINT array and not a DINT array? And there are enough elements in the array to write 60 words to?

N25 is an INT with 400 words. In the old logic, N[25] was setup as 16 bit words.

According to AB, that error means;

16#00D3 The scanner requested a length and the block transfer module responded with a different length

Stu....

The error means what it says: The requested length is 60 and the "done length" is not. It's probably not 0 (as seen in the screenshot, which also doesn't have a DN bit) but it should show up there when the .DN bit turns on.

One thing that the PLC-5 did allow you to do is fire simultaneous BTR and BTW instructions at the module; it would just buffer whichever came second, then reply to both.

The ControlLogix and 1756-DHRIO won't do that: The BTW has to be complete (the .DN bit turns on) before a BTR can be executed.

The conversion tool isn't sophisticated enough to know if you've programmed it correctly, so check carefully to be sure your MSG instructions executing BTR and BTW functions are alternating instead of executing together.

Hey thanks for the replies.

I have written the MSG in the same fashion as the other BTW/BTRs that talk to other RIO devices. Only BTW/BTR that communicates to the Prosoft gateway errors.

I contacted Prosoft and they sent me the controllogix ladder logic for the 5601-RIO-MCM. I inserted the rungs (see attached) and added the tags, and sure enough I am able to read and write to the drive again, EXCEPT the same error remains.

At this point I am considering it a nuisance error and will live with it until we upgrade from RIO to EtherNet/IP.

Now I am grinding my gears on modbus... which I suppose I will save for another post...