Tag values - converting 16-bit integer to two 8-bit integers

Ignition
1

I’m wondering if there is an easy way to convert a 16-bit integer tag value into two 8-bit tag values (one for the high byte, one for the low byte).

Here’s how i’ve been doing it:

  1. I have an integer value coming into an OPC tag.
  2. I make an expression tag that converts the integer into a binary string using the toBinary function. The toBinary function seems to strip leading zeros, so I also have some code to pad the binary string with zero’s to make it 16 bits long.
  3. I make 2 more expression tags. One tag reads the first 8 characters/bits of the binary string, the other reads the second 8 characters/bits. Now I have the high and low bytes of my original integer stored in 2 separate tags, as binary strings.
  4. I make 2 more expression tags. I use the fromBinary function to convert the two 8-bit binary strings into integers.

Anyone know of a more straight forward way to accomplish this?

Note: I know I can consolidate some of the expression tags, it’s just easier to explain my question this way.
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2

Probably not even close to the best way, but run this script on value change:

import struct
a = 4634
s = struct.pack('>H', a)

first, second = struct.unpack('>BB', s)
print first
print second

I’m curious to know the need? Packing bits was a real need back in the PLC5 days, but today?

3

You guys are overthinking this.

High Byte: ({IntegerTag} >> 8) & 0xFF
Low Byte: {IntegerTag} & 0xFF

8 Likes
4

Wow haha… Thanks.

5

Is it possible to do that conversion in the original tag definition or do you have to pull it into a tag and then create an expression tag to do the conversion. It would be a lot cooler to do it all in one tag

6

You need a separate tag. The Modbus driver doesn’t have byte-level addressing because registers are 16-bit.