SQC — Sequencer Compare

---

Description

SQC (Sequencer Compare) steps through a table of expected input patterns and reports whether the live inputs match. Each time its rung goes from false to true it advances the position, ANDs both the table element and the Source with the mask, and sets the control tag’s FD bit when the two agree. Use SQC to check that a machine’s sensors read what they should at each step of a cycle, and to raise a fault when they do not. Do not expect FD to track the inputs continuously; it is evaluated on the advance only, and holds until the next one.

---

Operands

Operand	Type	Format	Valid Range	Required	Description
Control	CONTROL tag	Tag or Tag[n]	One SQC per control tag	Yes	Holds LEN, POS and the status bits.
File	INT or DINT array tag	Tag	At least Length + 1 elements	Yes	The table of expected patterns; element 0 is the parked position.
Mask	Whole number or tag	0x000F, Tag	Any	Yes	Only bits set to 1 in the mask are compared.
Source	INT or DINT tag, member or element	Tag	Any	Yes	The live input word.
Length	Whole number	4	1 to array size − 1	Yes	Number of steps.
Position	Whole number	0	0 to Length	No	Starting position at power-up.

---

Scan Behavior

Prescan

LEN is loaded from Length and POS from Position. FD is clear.

Rung-condition-in is false

EN is cleared. Nothing advances; FD holds its last result.

Rung-condition-in is true

On the scan the rung turns true, POS advances by one, wrapping from Length to 1, and FD is set if File[POS] AND Mask equals Source AND Mask, cleared otherwise. DN is set while POS equals Length.

Postscan

Nothing.

Condition	EN	DN	FD	POS
Power-up	0	0	0	Position (usually 0)
Rung false	0	holds	holds	holds
False → true	1	POS = LEN	masked match	+1, wrapping LEN → 1
Held true	1	holds	holds	holds
RES on the control	0	0	0	0

---

Member Bits

Bit	Mnemonic	Data type	Set by	Cleared by	Operation description
Enable	.EN	BOOL	The rung being true	The rung being false, or RES	Mirrors the rung condition.
Enable unload	.EU	BOOL	Never	RES	Reserved.
Done	.DN	BOOL	POS reaching LEN	The next advance, or RES	The sequencer is on its last step.
Empty	.EM	BOOL	Never	RES	Not used by SQC.
Error	.ER	BOOL	Never	RES	Reserved. Bad operands are refused at validation.
Unload	.UL	BOOL	Never	RES	Not used by SQC.
Inhibit	.IN	BOOL	Never	RES	Reserved.
Found	.FD	BOOL	A masked match on the advance	A masked mismatch on the next advance, or RES	The inputs matched the table at the current step.
Length	.LEN	INT	Length at power-up	Never (RES preserves it)	Number of steps.
Position	.POS	INT	Each advance	RES (to 0)	The current step.

---

Example

Scenario: The indexing table from the SQO example has a sensor at each station. At each step exactly one sensor should be made, and the expected pattern table says which.

Tags:

• Seq_Chk — Check this step, BOOL
• Expected — Expected input table, INT[5], loaded 0, 1, 2, 4, 8
• Inputs_Word — Live sensor word, INT
• SQC_Ctl — Control, CONTROL, Length 4

Rung 1:

—] [Seq_Chk———[SQC SQC_Ctl  Expected  0x000F  Inputs_Word  4]———

Scan 1 — Seq_Chk = 0. Inputs_Word reads 1, station 1’s sensor made. Position 0, FD dark. The SQO rung above and the SQL rung below are their own pages’ examples.

Scan 2 — Seq_Chk = 1. Position advances to 1; Expected[1] is 1 and the masked input is 1: FD lights.

Had the sensor word read 3, FD would stay dark: a second sensor made is a mismatch. A rung XIC SQC_Ctl.EN XIO SQC_Ctl.FD → OTE Step_Fault raises the fault.

---

Control Tag Errors and Troubleshooting

Error definitions

.ER cause	Condition that sets it	.POS / status value	Meaning
None	.ER is never set by LadderIDE	—	A File that is not an INT/DINT array, or a Length of array size or more, is refused at validation.

Troubleshooting hints

Symptom: .FD is set although the inputs are wrong now. Likely cause: FD is evaluated on the advance only and holds until the next one. Fix: Advance the sequencer when the check should happen, or use a CMP for a continuous test.

Symptom: .FD never sets. Likely cause: The mask hides the bits that differ, or the table and the inputs use different bit positions. Fix: Compare Expected[POS] AND Mask with Source AND Mask in the Tag Browser at the failing step.

---

See Also

• SQO — Sequencer Output (drive the outputs the same table describes)
• SQL — Sequencer Load (record the expected table from a good cycle)
• CMP — Compare by expression (a masked compare without a table)
• Shift/Sequencer Instructions — Category index

---

Notes

Pair with SQO. Drive an SQO and an SQC from the same rung, on separate control tags, and the outputs and the expected inputs step together.

The block colours neutral. Watch the rung wire, the Position row and the DN/FD indicators.