Data Pointer

[alemberskiy]

I'm new to PLC, please excuse my amateur question:

In J1939 Configuration Parameters, "Transmit PGN" tab, first parameter is 'Data Pointer'. Description is as follows:

 

"This is the start of the vector that contains the data that will be transmitted from the PLC."

 

How exactly does this work? How do i change the data i transmit?

 

Thank you.

[Simon]

A vector is a group of consecutive registers (usually MI values in Unitronics).   In J1939 the data area is usually 8 bytes long, so the vector is going to be 4 consecutive MI values (eg MI10, MI11, MI12 and MI13.  The vector is addressed by using the first element (MI10) and the length (4).

 

By changing the data in this vector, you change the information that is being sent when you trigger the PGN send operation.

 

Since the SPNs can be any length, you may have some work to do when converting the 8 bytes of PGN to and from the individual SPN values it contains.

[alemberskiy]

Thank you very much, that's what i thought, but then i looked at the example someone posted, and there was a reference to MI and then very next one was used for something else. I think it was not a very good example, that's all.

[Simon]

From that example you have highlighted one of the common errors that gets made with Vectors.  Make sure you know where your vectors are and don't accidentally allocate them to something else.

 

Visilogic doesn't automaticlly mark vectors as "in use" so I usually type a description for each operand in the vector.  You can highlight a range and the system will name all of them together.  This will then mark them as "in use".

[alemberskiy]

Will do.

Thank you very much for your help.