Sequencer

  6 minute read  

Sequencer steps through a table of values on a timer. Each output channel has its own row of values and they all advance together, holding each column for the time you give it. Use it for a pattern — a sequence of beeps on a speaker, a blink pattern for an indicator light, a fixed series of setpoints.

flowchart LR
    i1(["input — true runs the sequence"]) --> B["Sequencer"]
    B --> o1(["output — one value per channel"])

While input is true the block advances one element every durations[e] seconds and wraps at the end of the table. While it is false the outputs sit at element 0 and the sequence is reset, so it always restarts from the beginning.

Element 0 is the idle value, not the first step. When the sequence starts it goes straight to element 1. A five-element table plays 1, 2, 3, 4, 0, 1, 2 … — element 0 is what you see at rest and once per loop.

Set the durations before you use it. They default to zero, and a zero-duration element advances every task cycle, so an unconfigured sequencer flickers through its table at the task rate.

This block has no enable, no disable and no isEnabled. input false is the idle state.

Signals

Inputs

Path	Unit	Range	Description
input	-	-	True runs the sequence; false parks every channel on element 0 and resets the timer. It is a level, not a trigger — the sequence restarts from the beginning each time it goes true.

Outputs

Path	Unit	Description
output	whatever you put in the table	The active element’s value, one per channel. All channels step together.

Parameters

Path	Unit	Default	Range	Effect
durations	s	all 0	above 0	How long each element is held, one per element, shared by every channel. A zero or negative duration advances in one task cycle, which is what the defaults do.
channel_01, channel_02, …	table unit	all 0	any	One row per output channel, one value per element. Element 0 is the idle value.
ids	-	all −1	any	The IDs this sequencer answers to, used by SequenceSelector to choose between several. Ignored when the block is used on its own.

All are persistent and survive a controller restart. No parameters exist below this block.

The table’s shape — how many elements and how many channels — is fixed when the machine is built. Read the length of durations and of output to discover it.

Setup

1. Read the length of durations to find how many elements the table has, and the length of output for the channel count. Both are fixed.

2. Fill in durations first. Every element needs a positive time, or the sequence will race through it.

3. Fill each channel_NN row with the values that channel should take at each element. Put the idle value in element 0 — that is what the channel shows while the sequence is not running.

4. Leave input false and confirm every channel sits at its element 0 value.

5. Set input true and watch the outputs step. Time one full loop against the sum of your durations.

   Step 5 drives whatever is connected to the outputs. If these values command real motion rather than an indicator, check the whole table before you start it — the sequence loops until the input goes false.

6. Set input false and confirm every channel returns to its element 0 value immediately.

Tuning

There is nothing to tune — the table is the configuration.

1. Keep every duration positive. A zero means “one task cycle”, which is almost never what you want.
2. Remember the sequence starts at element 1. If you want a specific value at the moment it starts, put it there, not in element 0.
3. Put the rest value in element 0. It is what shows at idle and it is also part of the loop, so it acts as a gap at the end of each pass.
4. All channels share one duration row, so they cannot step at different rates. Use a second sequencer for a second rhythm.
5. Nothing here needs re-checking after a task-rate change — the durations are in seconds.

Read each element’s dwell off the width of a step.

Symptom	Cause	Action
The outputs flicker rapidly through the table	The durations are zero — the default	Set a positive duration for every element
The first value of the pattern never appears	Expected: the sequence starts at element 1, and element 0 is the idle value	Put the first step in element 1
The outputs sit at one value and never move	input is false	Set it true
The sequence restarts from the beginning every time	Expected: input is a level, and going false resets it	This is the designed behaviour
One element is much shorter than configured	Its duration is zero or negative	Write a positive value
The channels do not step together	Not possible — they share one timer and one element index	Check what consumes the outputs
The channels need different rhythms	Not possible — one duration row for all of them	Use a second sequencer
The loop takes longer than the sum of the durations	Expected: each element is held for its own listed time, and element 0 is part of the loop	Include element 0’s duration in the total
I cannot tell which element is active	The block does not publish it	Infer it from the output values
The table is too short	Its size is fixed when the machine is built	It needs a configuration change
The sequencer never runs inside a selector	Its ids do not match the selector’s input	Check the ids array

A starting point for a three-element blink pattern on one channel, half a second per step:

durations  = 0.5, 0.5, 0.5
channel_01 = 0.0, 1.0, 0.0

At rest the channel reads 0. Running, it plays 1.0, then 0.0, then 0.0, and loops.

Limits and errors

Limit	Set by	What happens	Reported
durations above 0	Nothing	Not checked. A zero or negative duration advances that element in one task cycle. All zeros — the default — makes the whole table free-run at the task rate	Not reported
Active element	Not published	Which step is running cannot be read from the parameter tree	Not reported
Table shape	Machine configuration	The element count and channel count are fixed when the machine is built. Read the array lengths to discover them	Not reported
output	Nothing	Whatever is in the table. Bound it downstream if the consumer needs a limit	Not reported
Timing	Task rate independent	Durations are in seconds and keep their meaning at any task period	Not applicable
Block state	input false	Going idle resets the timer and the element. There is no separate reset	Not reported

The block raises no errors or warnings and logs nothing. Every failure above shows as a value on a trace, not as a message.

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Verified against motorcortex-control3 3.30.0 (bc348fd).