Assigning Variables to Bits of a Word

[dholden]

I have a variable (P4100) that I need to read the bits of and assign to variables. I was thinking it would work like this:

 

//Digital Inputs

PTR Input01->P4100.0.0;

PTR Input02->P4100.0.1;

PTR Input03->P4100.0.2;

PTR Input04->P4100.0.3;

 

However clearly this is not the correct approach, how would one do this correctly.

 

By the way the Inputs are read from a ModbusTCP variable into the local P-Variables like so:

 

cmd"ModbusRegisterRead 0,0,4,P4100"

 

Which is why i am not simply addressing the corresponding I/O card Addresses.

[curtwilson]

PMAC P-variables are floating-point variables, so the locations of individual bit values "float" (are not fixed), and you cannot assign pointer variables to individual bits.

 

First, you will want to define fixed-point M-variables to user buffer registers. For example:

 

M4100->u.user:4096; // Sys.udata[1024]

M4101->u.user:4100; // Sys.udata[1025]

M4102->u.user:4104; // Sys.udata[1026]

M4103->u.user:4108; // Sys.udata[1027]

 

Then you can use:

 

cmd"ModbusRegisterRead 0,0,4,M4100"

 

Now, you can define other M-variables to individual bits of these fixed-point registers.

 

ptr Input01->u.user:4096.0.1; // Bit 0 of Sys.udata[1024]

ptr Input02->u.user:4096.1.1; // Bit 1 of Sys.udata[1024]

ptr Input03->u.user:4096.2.1; // Bit 2 of Sys.udata[1024]

ptr Input04->u.user:4096.3.1; // Bit 3 of Sys.udata[1024]

 

Note that the starting bit number comes before the bit width in these definitions.

[dholden]

A couple of questions,

First, why did you choose u.user:4096 as your starting point?

 

Second how would you do the same thing (get a bit from an value) for:

Sys.ModbusServerBuffer[0]

[curtwilson]

"Why did you choose u.user:4096 as your starting point?"

 

No particular reason. I just did not want to imply that you had to start at offset 0, and many people use lower locations for other purposes.

 

Registers in the user buffer can have M-variable pointers assigned easily to individual bits. This provided the closest working alternative to what you provided.