XOR — Bitwise Exclusive OR

---

Description

XOR (Bitwise Exclusive OR) writes to Dest a word whose every bit is 1 where Source A and Source B differ, and 0 where they agree. It is an output instruction with no state and no status bits. Use XOR to find which bits changed between two readings of a word, to toggle chosen bits with a mask, or to compare two words for any difference. Do not use XOR for a single bit toggle that must happen once per event; without an ONS ahead of it the bit flips every scan.

---

Operands

Operand	Type	Format	Valid Range	Required	Description
Source A	A whole-number tag, member, element or bit, or a literal	Tag, Tag.Member, Tag[n], Tag.bit, a number	Any resolvable reference	Yes	The first word.
Source B	Same as Source A	Same	Any resolvable reference	Yes	The second word, or the toggle mask.
Dest	A writable whole-number tag, member or element	Tag, Tag.Member, Tag[n]	SINT, INT or DINT; not a literal, not an input pin	Yes	Where the result is written.

A REAL operand is truncated to a whole number first. Dest may be one of the sources.

---

Scan Behavior

Prescan

Nothing. Dest keeps its power-up value.

Rung-condition-in is false

The instruction does nothing. Dest holds its last value.

Rung-condition-in is true

Both sources are read, XORed bit by bit, and the result is written to Dest every scan the rung is true.

Postscan

Nothing.

---

Example

Scenario: A block of digital inputs is read into one word each scan, and the previous scan’s word is kept. XOR of the two shows which inputs changed since last scan, so a single NEQ Changed 0 can trigger a log entry.

Tags:

• Compare_Now — Compare the two words, BOOL
• Inputs_Now — Inputs this scan, INT
• Inputs_Prev — Inputs last scan, INT
• Changed — Bits that differ, INT

Rung 1:

—] [Compare_Now———[XOR Inputs_Now  Inputs_Prev  Changed]———

Scan 1 — Compare_Now = 0. Inputs_Now reads 45 and Inputs_Prev reads 41. The rung is false: Changed holds 0.

Scan 2 — Compare_Now = 1. Changed reads 4: only bit 2 differs.

---

Binary Result

XOR — Inputs_Now XOR Inputs_Prev, INT (16 bits)

                  15 14 13 12  11 10  9  8   7  6  5  4   3  2  1  0
Inputs_Now   45    0  0  0  0   0  0  0  0   0  0  1  0   1  1  0  1
Inputs_Prev  41    0  0  0  0   0  0  0  0   0  0  1  0   1  0  0  1
Changed       4    0  0  0  0   0  0  0  0   0  0  0  0   0  1  0  0
                                                             ^
                                                  1 where they differ

Bits 5, 3 and 0 are set in both words and come out 0. Bit 2 is set in one and clear in the other and comes out 1.

---

See Also

• AND — Bitwise AND
• OR — Bitwise OR
• NOT — Bitwise NOT (XOR with all ones)
• NEQ — Not Equal (act on a non-zero Changed)
• Move/Logical Instructions — Category index

---

Notes

Toggle bits. XOR Word 0x0001 Word flips bit 0 each time it runs. Put an ONS ahead of it or the bit flips on every scan.

Two words equal. XOR gives 0 exactly when the two words are identical, which is what EQU A B tests without a Dest.

Round out the change detector. Follow the XOR with MOV Inputs_Now Inputs_Prev on the same rung so the previous word is refreshed after the comparison.

The block never colours. Watch the rung wire and the Dest value line.