MLX 1500 DeviceNet Explicit Message - Amending Class, Instance, Attribute

Hi all, has anyone ever had success with amending (via PLC code) the Class, Instance, Attribute of an explicit message.



Rockwell support supplied this info -

The breakdown of the MG9 table is

Class = MG9:0.TFT

Instance = MG9:0.TFN

Attribute = MG9:0.RBL UPPER BYTE (use bite swap)



The problem is that I have been unable to get some code together to modify the Class, Instance, Attribute within an explicit message.



I can modify the .TFN parameter via a MOV instruction but am unable to modify the others using MOV, COP, or Bitwise AND instructions.



One strange thing is that once you get an expectable config of MOV, COP, AND instructions that pass the verify program check with no errors the program cant be downloaded because you get a "program has errors, cant download" error message



Has anyone ever managed to make this or something like this work?



PS I have had no problem getting a single MSG pole the different nodes (Node = MG9:0.NOD) for a fixed parameter and just need to get the above working to have a good variable message.



Cheers

Generally I only modify the Target Node and the Instance Number, and just use another MSG instruction and MGx:y control file element when I need to read or write a different object.

I get the "Word is not writable at word level" error when I try to validate a program that attempts to set the Class.

What DeviceNet objects are you trying to read or write in your application ?

Thanks Ken, I'm reading from a series of 6 Telemecanique Altivar 71 drives, and reading in general parameters (motor / drive specs & data) to make the HMI a bit more "bells and whistles". It seems strange that you can mod the instance but from the Telemecanique parameters it would appear that the class and instance remain constant while the attribute changes / increments per parameter relevant to a group of common parameters i.e. motor specs.



Cheers.

The usual approach to drive parameters is to use the common "Parameter Class" 0x0F, where the Instance number is the Parameter number. The different Attributes include:

Parameter Value
Link Path Size
Link Path
Descriptor
Datatype
Data Size
Parameter Name String
Units String
Help String
Minimum Value
Maximum Value
Default Valueling
Scaling Multiplier
Scaling Divisor
Scaling Base
...and so on.

This gives a full description of the Parameter and lets the EDS be created just by uploading all the Attributes of each Instance. Virtually all A-B products work this way.

Another approach is to use the Vendor-Specific objects (these begin at Class 0x64) to implement arrays of parameter values. This is how Telemecanique did it, in which each object has one Instance and the variable values are Attributes of that instance.

This means that the scaling and units information either has to be known by the management software or read from the user manual, but it's simpler to implement.

I'm not sure how you're going to be able to write routines to use a single MSG to get multiple parameters out of an Altivar drive.

Does the drive's DeviceNet interface support the Get Attributes All service (Service 0x01) ? That might read the entire set of parameters from each Class (0x64, 0x65, 0x66, etc).