SWPB — Swap Byte

A DINT is 4 bytes and an INT is 2. Not every device stores them in the same order. A drive, a scale or a gateway that sends a number or text in Modbus registers often puts the bytes the other way round, so a value that should be 16#1234 arrives as 16#3412, and text arrives with each pair of letters swapped: 'ABCRDO1E' where the device sent 'BARCODE1'. SWPB puts the bytes back in order.

SWPB is a statement of its own that writes its last operand. It gives back no value, so Reading := SWPB(Raw, HIGHLOW); is refused: write SWPB(Raw, HIGHLOW, Reading);.

SWPB(Raw_Register, HIGHLOW, Reading);

OperandsCopy Anchor Link

The order mode is a word written straight into the statement, never a tag. Structured Text spells the third one HIGHLOW, with no slash; a ladder box spells it HIGH/LOW.

What each order mode doesCopy Anchor Link

Picture a DINT as four bytes A B C D, with A the highest:

An INT has only two bytes, so every order mode does the same thing to it: swaps them. 16#1234 becomes 16#3412. An INT source may go into a DINT destination. The swapped INT keeps its sign there, so 16#12AB becomes 16#AB12, which as an INT is -21742, and the DINT holds -21742. A DINT source cannot go into an INT: half its bytes would have nowhere to go.

Example — swap every register in an arrayCopy Anchor Link

A device sends text two letters to an INT, high byte first. Swapping each INT in place puts the letters in the order the controller reads them:

FOR i := 0 TO 5 DO
    SWPB(Registers[i], HIGHLOW, Registers[i]);
END_FOR;

The source and the destination may be the same tag, as here.

Common MistakesCopy Anchor Link

• Writing HIGH/LOW in Structured Text. That is the ladder spelling; here it is HIGHLOW.
• Putting the order mode in a tag. It is a word typed into the statement.
• Choosing the wrong order mode. Compare the value you get with the one the device's manual shows: two letters swapped in each pair is HIGHLOW, two halves swapped is WORD.

RelatedCopy Anchor Link

• Logical & Bit Instructions (Ladder) — SWPB as a ladder box, beside AND, OR, XOR and NOT
• COP — copy registers into a STRING once their bytes are in order