Control Logix Code for Profibus PA devices

[Wayne L.]

I am trying to write some code using the AB CLX platform.
I have set up a small Profibus network with 3 devices attached.
My SST-PFB-CLX Scanner Module is located in Slot 2 of my rack.
Each device sends data in the form of a IEEE-754 Floating Point value using 5 bytes.
The first four bytes are used for data the 5th byte is used for device status.
The device status is not required.
Each device displays a value of 0 - 100% but the value that comes into the PLC is Eg. -16234 & 23908.
How do I get these to read in Engineering Units?
Can someone help!!

 

[ghettofreeryder]

Those values are in the form of 2 16 bit words. A float is a 32 bit value. On an AB PLC, you can convert these values to a float by copying them to a float.... eg if those 2 16 bit words were in INT[0]and INT[1], you would write COP INT[0] REAL[0] 1, where the one is the number of destination adresses you want to copy data too, meaning 200 words of int data is 100 values of real

 

[dchartier]

Hello Wayne;

Sorry I missed your first post.

Basically your problem is that bytes/bits from the PA coupler need to be swapped so the Controllogix processor can make sense of them.

We use the BTD instruction 4 times to reconstruct a REAL register containing the PA value.
In the illustration below, from one of our projects, we first read from the SST card the first 16 bits and the last 16 bits of the PA transmission. We use a subroutine to apply the BTD instruction, so don't worry about the variable names. Here is how it executes:

Hope this helps,
Daniel Chartier