If you're playing live and want to trigger a riff that doesn't repeat - Arpeggios seem to fit the bill here.

How to a phrase (pattern?) once without looping?

By definition, Pattern Phrases loop during playback. When you place a Pattern Scene event in a “Chain” you can tell it to play and END at any measure of your choosing.

From the Pattern screen, tap “Edit/Job” > “Chain”
Turn Chain ON
Add (+) an Event (the Scene you want to play) — measure 001
Add (+) a second Event (the “End” command — set it at the top of the STOP measure.

By definition, a “Pattern Chain” will play once and stop.
By definition, a “MIDI Song” will play once and stop.

However, an Arpeggio, or set of Arpeggios, might be a solution — provided the phrase you wish to trigger meets the requirements of Arpeggio creation. Arpeggiators can be triggered by a KeyOn event as you design. You can set a Note, or range of Notes, or a Velocity or range of Velocities that will start the Phrase.

Even though a Pattern or a Chain can be started by a KeyOn event you cannot design/refine that further. In other words, any key event will start playback (not what you desire).

The Sequencer can be started and stopped via an FC4/FC5 Type momentary Foot Switch... this opens other possibilities.

Thanks @BadMister. It sounds like using the Chain mechanism is what I'm looking for. In trying it out, I have one question.

When you say,

Add (+) a second Event (the “End” command — set it at the top of the STOP measure.

how do I put in an "End" command? I have a single pattern, 2 bars, recorded as Scene 2. In the Pattern->Edit/Job->Chain page, there are only 2 columns, "Measure" and "Scene" which show Measure 001 and Scene 2. Should I create another Scene Pattern with some kind of "End" command in it? Or should it be added to my existing pattern, and if so, how?

Should I create another Scene Pattern with some kind of "End" command in it?

Exactly. After entering the first step, a “+” appears in step 2. You can enter a Scene # “1” thru “8” or the “End” command... simply use the Data Dial or [INC/YES] button.

You can adjust the Measure, if you desire. The End Measure should be the top of the next measure... if your Scene is 4 Measures and you want it to play once, set the second line of the Chain

005 __ End

...tthe “End” command... simply use the Data Dial or [INC/YES] button.

Aha! Yes, that worked. I never thought to scroll all the way to the end of that list.

So thank you for that. But now I have the problem where the Chain restarts every time I press a key if "Key On Start" is enabled, which is not what I want. I understand that I can use a footswitch to start/stop the sequencer, which may work (looking for instructions on how to set that up) but isn't there some way to trigger it upon switching to a new Scene? Put differently, does changing to a new Scene allow some MIDI or other action to run automatically--and from there I could trigger the Pattern/Sequencer? Seems like this would be a useful function to have.

Also, can you point me to the Yamaha documentation for the Pattern and Sequencer functionality? Seems like there's a lot to learn about this but I can't find it in any manuals.

Also, can you point me to the Yamaha documentation for the Pattern and Sequencer functionality? Seems like there's a lot to learn about this but I can't find it in any manuals.

Since launch, the Pattern Sequencer is an "add on" feature not in the original product offering. As such, the official documentation for the feature is contained within the "Supplementary Manual" which can be downloaded here: https://usa.yamaha.com/products/music_production/synthesizers/modx/downloads.html

Direct link to the USA supplementary manual (as of today): https://usa.yamaha.com/files/download/other_assets/8/1219148/modx_en_sm_c0.pdf

Pages 12-17 have the latest pattern-related features. Pages 27-49 is the bulk of the documentation when the feature was introduced.

Or go to your region's product page if the USA is not your geo.

Certainly there are tutorials, videos, etc to supplement the supplementary manual. I'm going to pull from Montage or MODX alike since they're so related.

https://yamahasynth.com/learn/montage/mastering-montage-modx-pattern-sequencer-features-in-os-v3
https://yamahasynth.com/learn/montage/mastering-montage-pattern-workflow-and-recording-features-in-os-v3-5
https://www.yamahasynth.com/learn/montage/montage-modx-synthtips-loop-recording-with-the-pattern-sequencer
https://yamahasynth.com/learn/MONTAGE%20Series%20Synthesizers/montage-modx-synthtips-quick-beat-making-with-the-pattern-sequencer
https://www.yamahasynth.com/learn/events/tech-talk-live-may-5-pattern-sequencer-basics
https://www.yamahasynth.com/learn/montage/montage-modx-synthtips-1-2
https://www.yamahasynth.com/learn/modx/available-now-modx-os-v2-0 (just an overview for 2.0 when pattern sequencer was introduced)
https://yamahasynth.com/learn/events/tech-talk-live-july-7-os-v3-5-sequencer-live-stream-event
https://www.yamahasynth.com/learn/montage/rehearse-and-erase-functions-in-a-pattern-mastering-montage
https://www.yamahasynth.com/learn/events/tech-talk-spotlight-montage-sequencer
https://www.yamahasynth.com/learn/montage/divide-drum-track-in-os-v3-0-mastering-montage

There are also lots of forum messages with discussions and help from Yamaha. The above may not be complete. Also, the nature of links is that they get changed and invalidated. These links work now 5/2021 - but may not work in the future.

EDIT: 5/23/21 - provided some context including the OP's question. This response is ONLY covering the quoted question regarding documentation. And "the manual" (officially) is the 1st link(s) given to the supplementary manual. The rest are given as FYI resources for further reference (take it or leave it).

Thanks @Jason. You should pin that post for the future.

I have a lot of reading to do.

But now I have the problem where the Chain restarts every time I press a key if "Key On Start" is enabled, which is not what I want. I understand that I can use a footswitch to start/stop the sequencer, which may work (looking for instructions on how to set that up) but isn't there some way to trigger it upon switching to a new Scene?

As I said in my first reply:

“Even though a Pattern or a Chain can be started by a KeyOn event you cannot design/refine that further. In other words, any key event will start playback (not what you desire).”

Definitely, I would say “YamahaSynth.com” and all articles, tutorials, Tech Talks, etc., should be used as a resource. But as a rule of thumb, try to understand the design intention... then you can determine if what you wish to do is possible.

From what I gather, (I do not have a clear understanding of what you want exactly) if you want to have a predetermined Phrase that plays once and stops (that’s an ARP Phrase) and you want it so that you can start it, on cue. You want to start by playing a Multi Part KBD CTRL Performance that includes both real-time and arpeggiated Phrases. (But it starts without the one-time phrase)... this is why you are setting titles to play from Scene 2 (I’m guessing).

If you know the actual design purpose of the Pattern Scenes you would quickly enough see the conflict I warned you about. Touching any key at all will automatically start your Chain. Instead of using “Pattern Scenes”... the following is a possible solution using “Performance Scenes” (without the Pattern Seq).

Performance Scenes are those functions that you can access in realtime... separate from the Sequencer.

It is Arpeggio Phrases that you can design what range of Notes and what range of Velocities will cause them to play.

In other words, it is within the design concept to be playing and controlling a Multi Part Performance (controlling a bass and a right hand lead, for example) without the drum Arp Phrase sounding. If you want to bring in the Drums at a later moment... during the time you are playing without drums you must control the drums by instructing them to be silent.... how? There are several methods...

The Drum Arp Phrase can be set to remain silent until you trigger it by playing within the trigger values you setup... it could be hitting a certain Note at a particular velocity. Only when that trigger requirement is met will the drum Phrase kick in. So you could “hide” the start of the Drum Arp’s until you play a certain key at a certain velocity. Because it is an Arpeggio Phrase it can be set to be played once (Loop = Off)

Alternatives
Also each Performance Scene can be set to recall particular Parts under Keyboard Control, It is possible to have Scene 1 with your right hand lead, Bass Arp and Drum Arp all set to under KBD CTRL. You can prevent the Drums from sounding by telling the Drum Part to play a “MUTE ARP”.

A “Mute Arp” is one that keeps the Arpeggiator active but tells the Part to be silent. It is the equivalent of placing a REST on that musicians chart. The advantage is when you change Scenes, to Scene 2 for example, the drums will play without you retriggering a key... they will simply come in on time!!! On the next downbeat.

This Arp, for example, “Mute 4/4”, placed in Scene 1 on the drum Part will tell the drummer to pay attention to the count but to lay out.
When you move to Scene 2, the drum Arp will automatically come in (no need to trigger a Note) it simply comes in with its pre-assigned Arp Phrase.

A Mute 4/4 Arp in Scene 1 tells the Drums to be silent.
If, as I assume you have been doing - set the Scene 1 Drum Part Arp = Off, while that accomplishes silence from the Drums, but that’s like giving the drummer a break... he ain’t counting, he ain’t paying attention...he’s OFF.

Mute Arp = a musical rest... the drum Arp Part is active and counting the Time, although he is silent, until it comes in — which is the TOP of Scene 2.
Arp Off = a coffee break (meaning you will need to trigger the start of play with a Note On

Make sense?