TYPE 0x0651 · PLC FAMILY

SEQUENCER

Step sequencer. Tracks the current step and total number of steps. SATISFIED while more steps remain (current < total). Becomes UNSATISFIED when the sequence is complete or invalid. No Tapscript equivalent exists.

PLC Invertible

Ladder Diagram

Fields

Field	Data Type	Size	Side	Description
current_step	NUMERIC	1-4 B	Conditions	Current step index (NUMERIC[0])
total_steps	NUMERIC	1-4 B	Conditions	Total number of steps in the sequence (NUMERIC[1], must be > 0)

No witness fields required — evaluation uses only the two NUMERIC condition fields.

Wire Format Breakdown

0x0651 0 2 NUMERIC · 3B NUMERIC · 3B Conditions = 14 bytes

0x0651 0 0 Witness = 4 bytes (empty block)

Total 18 bytes

Evaluation Logic

1.Read current_step (NUMERIC[0]) and total_steps (NUMERIC[1]). If fewer than 2 NUMERIC fields → ERROR

2.Validate: total_steps > 0. If total_steps ≤ 0 → UNSATISFIED

3.If current_step < total_steps → SATISFIED (more steps remain)

4.If current_step ≥ total_steps → UNSATISFIED (sequence complete)

Return Values

Condition	Result
Fewer than 2 NUMERIC fields	ERROR
total_steps ≤ 0	UNSATISFIED
current_step ≥ total_steps	UNSATISFIED
current_step < total_steps AND total_steps > 0	SATISFIED

JSON Wire Format

Conditions (step 2 of 5)

{
  "type": "SEQUENCER",
  "inverted": false,
  "fields": [
    { "type": "NUMERIC", "value": 2 },
    { "type": "NUMERIC", "value": 5 }
  ]
}

This creates a 5-step sequence currently at step 2. Three more steps remain before the sequencer becomes UNSATISFIED.

Worked Example

total_steps=5

current_step=0, total=5: 0 < 5 → SATISFIED (step 1 of 5)

current_step=2, total=5: 2 < 5 → SATISFIED (step 3 of 5)

current_step=4, total=5: 4 < 5 → SATISFIED (last step)

current_step=5, total=5: 5 ≥ 5 → UNSATISFIED (sequence complete)

current_step=0, total=0: total ≤ 0 → UNSATISFIED (invalid)

Use Cases

Multi-Step Protocols

Enforce that a multi-phase protocol progresses through each step in order. Each UTXO in the covenant chain carries the current step, ensuring no steps are skipped and the protocol completes fully.

Ordered State Transitions

Model finite state machines on-chain. The sequencer tracks which state the covenant is in, and combined with other blocks, enforces valid transitions between states in a predetermined order.

Workflow Enforcement

Business workflows requiring sequential approval steps (e.g., proposal → review → approval → execution). The sequencer ensures each step is completed before the next can begin.

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