ControlLogix Bit Field Distributor


Introduction to the ControlLogix Bit Field Distributor (BTD)

The ControlLogix Bit Field Distributor (BTD) Allows us to map bits from one word to another. Particularly, when converting 32 bit data to 16 bit data, the BTD makes this very easy. In this case, we have a 32 bit input module. For the purpose of this document, we’ll assume that our SCADA system can only handle 16 bit data. In this case, we’ll take the first 16 bits, and map the data to the Input16a tag. Likewise, we’ll map the last 16 bits to the Input16b tag. We’ve also used this in RFID readers, where the data is in simple 8 bit format, or even on DeviceNet to remap data from a field device so the bits in the word will match up with the labels on the field device.

Another use for the BTD is to convert data into 16 bit format before sending it to the SLC or PLC-5.

Add the First ControlLogix Bit Field Distributor

To demonstrate, I’ll just add the BTD Instruction to rung 0 of the main routine. I’ll leave this instruction as unconditional. In other words, the instruction will execute every scan.

Notice that our source is Input32, and the destination tag is Input16a. For the first BTD, we’re taking data from bit 0 of Input32, and placing that data into Input16a starting at bit 0. Our length is 16. In other words, we are copying 16 bits.

Add the Second ControlLogix Bit Field Distributor

At this point, we only have 16 bits of data moved. We still need to move the last 16 bits of Input32 into the Input16a tag. To illustrate, look at the example below.

Notice that our source is still Input32. However, this time, our destination is Input16b. We are starting at bit 16 of Input32 this time. We are copying the last 16 bits of Input32 into the Input16a tag.

Test your work

Let’s finalize your edits, and execute the instructions. We’ll see how our data changes.

Notice your data converted exactly as we would expect!

For other information on ControlLogix, please visit the Category Page!

— Ricky Bryce

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