Strymon Iridium MIDI / EXP Pinout

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Hey everyone,

I have finally pulled the trigger and ordered a Strymon Iridium for some silent home practice/recording.

Even though its delivery might take a week or two I find myself already thinking of how I can build a small midi controller to access more presets.
I have built a such a "digital footswitch" before for use with my Nemesis Delay and it has been working perfectly ever since.

The only problem with doing the same again is the Iridium's TRS-style EXP/MIDI combo jack:
The MIDI Manufacturers Association has finally standardized the pinout for MIDI via TRS not too long ago (MMA RP-054), but according to some rumours Strymon didn't follow those specifications.

The specs of Strymon's new MultiSwitch Plus second that in my opinion. Unlike the original Multiswitch which acts like an expression pedal with different resistors on each footswitch, the Plus model seems to be an actual midi controller.
And here's the problem: The one MIDI cable connecting the MultiSwitch Plus and the Iridium does handle bidirectional (!) MIDI as well as power for the footswitch.

The regular "new" TRS-MIDI is just plain old MIDI with a different connector and still requires one cable for each direction; so there must be something more going on here.

You might (understandably) ask why I even bother and don't just get Strymon's EXP->MIDI cable.
Well, of course that would be the easiest way of doing it, but where's the fun? Isn't that why most of us are here? 
But there are more reasons:
1st) In my opinion the Iridium itself is pretty expensive already and charging 35€ for an adapter cable is almost Apple-tax-price-territory.
2nd) Not needing an additional power supply for my DIY MIDI controller seems more than tempting. (That assumes the Iridium can safely provide enough power!)
3rd) Strymon's latest pedals do all feature this connector. So, everything we manage to find out should also apply to the Riverside and Sunset.

At some point I thought the EXP->MIDI cable must be active and feature some electronics (chunk in the middle of the cable). But allegedly some people use the Iridium in conjunction with the Chase Bliss Faves and according to the Iridium's manual you can hook it up to any MIDI controller with a TRS output.


I really appreciate your help and time!

It would be awesome if we can find something out together!


Greetings!

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Well, I think I have found what I was looking for:

Since it's relatively new to the market there's not that much information on the Iridium's MIDI capabilities.
However, since the Sunset and Riverside feature the same EXP/MIDI combo jack I figured they must work the same way.

Here's what I have been able to find out regarding those two pedals:
Strymon has explicitly stated the MIDI pinout: TIP=Receive, RING=Send/Through, so the SLEEVE must be Ground then.
https://www.strymon.net/faq/how-do-strymon-pedals-receive-midi-messages-sent-via-14-trs-midi-cable/

Over on https://www.thefretboard.co.uk/ someone has asked the same question when Strymon's EXP->MIDI cable wasn't available in the UK at first and another one got a reply from Strymon that included this photo: https://imgur.com/gallery/Md24LCE
https://www.thefretboard.co.uk/discussion/138806/midi-guru-required-midi-to-trs-cable-advice-request-for-strymon-sunset

So, as it turns out Strymon's MIDI cable is not just a simple TRS->DIN adaptor but the aforementioned "chunk in the middle" of the cable houses the actual input circuitry for the MIDI connection, i.e. optocoupler, reverse polarity protection diode and resistors.
Remains the question where they get the 5V for the second side of the 6N138 from???

This ultimately means that the EXP/MIDI jack on Strymon's latest pedals is in fact not a "real" MIDI port but a proprietary multiport that can be adapted to be used with MIDI.
If you take it a step further this also means that you shouldn't connect any old MIDI controller directly as there's none of the "safety circuitry" that any MIDI input should have.
(Some distribution boxes for that are specifically designed for TRS MIDI might have that already build-in.)

At this point things get kind of obvious: If the pedal had all the regular MIDI circuitry build in, the same port probably couldn't be used as expression input.
Therefore, it is understandable that they went this, way in my opinion.


I hope this is information is helpful to someone!


Cheers,

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[ElectricDruid]

There have been several "phantom powered" MIDI devices in the past. It was pretty common for 1980's MIDI filters and so on. They work by using a cap to store the voltage from the actual MIDI signal itself. That voltage then powers the opto.

It's a corner you can cut, and it is against the MIDI spec. Like most cut corners, it can come back to bite you. For example, MIDI is a current loop, and says nothing about voltage. Running your MIDI Input or Output at 3.3V is fine, and these days many things do. Plug your clever little line-powered MIDI gizmo into one of those legal-under-the-MIDI-spec devices and it won't work. Back in the day, that wasn't such an issue because you could pretty much assume 5V for logic/processors and therefore for the MIDI output.

There's a long discussion of the technicalities on the Synth-DIY list archive, starting with this question:

http://synth-diy.org/pipermail/synth-diy/2015-September/046717.html


Going back to the Strymon EXP/MIDI jack, it sounds like it simply connects directly to a couple of pins on the processor. Those pins could be configured by firmware as either a high output (a 5V supply) and a analog input (ADC input) for use as an expression pedal socket, or could be configured as UART Rx/Tx for serial comms with other devices. For their own devices, that would be enough. For MIDI, you'd need to add the opto-coupler, hence the adaptor cable. Do those seem like reasonable guesses?

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Hi ElectrciDruid!

Yes, I guess that makes sense. It has indeed been bugging me before how many companies kind of ignore the actual MIDI specifications and do their own thing, i.e. voltages.

I think you are right in terms of the Strymon pedals. Since the drawing states "TO UART RX" it would make sense that those connections go right to some pins of the ARM coprocessor that controls the DSP. Also, it makes perfect sense to simply use one of those pins to power the cable's optocoupler. Thereby however, powering anything more than this "active cable" from the EXP/MIDI port seems pretty risky. The SuperSwitch Plus hasn't much going on either, just some LEDs, footswitches and a microcontroller; so, it shouldn't draw much current.

I was hoping for something hardwired to the supply rails, but as this doesn't seem to be the case, I will go with a separate power supply for my DIY MIDI controller.
This will be necessary as I would really like the controller to feature some sort of display.
At least I have found out that I can omit the expensive adaptor cable by building the "MIDI input circuitry" right into the MIDI controller.

Remains the question of what to do with the ring?
Connect it to ground, pull-up resistor to 3,3V/5V/9V, pull-down resistor to ground, leave it floating?

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Again, here's the pic of the adaptor's circuitry:

Can be found here: https://imgur.com/gallery/Md24LCE

I was wondering whether Strymon did at least use regular 5V logic for their MIDI since 3.3V seems to be more common among modern microprocessors.
But the 470R pull-up resistor answers that question.

Has anyone an idea where that connection from "TO UART RX" might be going?

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Has anyone an idea where that connection from "TO UART RX" might be going?

I am referring to the one that goes down vertically. The one to the right should go straight to the ARM-processor from my understanding.

As far as what to do with the ring connection:
Chase Bliss has posted a video on how you can use their faves switch with Strymon pedals featuring the new EXP/MIDI combo jack in which they mention that a TS cable will be fine since those Strymon pedals don't use the ring connection (when only receiving).
In other words: Connecting the ring to the sleeve aka ground should be fine.

To sum everything up so far:
TIP = Incoming MIDI
RING = Outgoing MIDI - if not used GND
SLEEVE = GND
Logic level: 5V

Optocouplers are usually used on MIDI inputs to isolate both communicating devices so one device can't damage the other.
Therefore, it is necessary that the "secondary side" of the optocoupler gets powered by the receiving device itself, otherwise the isolation would be rendered useless as its bypassed.
This however means that neither the Chase Bliss Faves nor the Empress MIDIBox can feature such an optocoupler circuit, which means it cannot be absolutely necessary.
But if it isn't why then do Strymon themselves bother making an active adaptor cable?
And even if it is how do they power it with 5 volts?

I am well aware that this is going way beyond what the average user will care about. If you want a simple way to use MIDI with one of those pedals simply by the official cable or one of the aforementioned switchers and be done with it. But it keeps me thinking how they are doing it.

[ElectricDruid]

Optocouplers are usually used on MIDI inputs to isolate both communicating devices so one device can't damage the other.

True, but not the whole story. The more important reason is to prevent ground loops between different bits of equipment. This is why the MIDI specification requires that the cable screen ground is connected at the MIDI Out socket, but not at the MIDI In. If it were connected at both ends, all their hard work preventing ground loops would be wasted!

Therefore, it is necessary that the "secondary side" of the optocoupler gets powered by the receiving device itself, otherwise the isolation would be rendered useless as its bypassed.
This however means that neither the Chase Bliss Faves nor the Empress MIDIBox can feature such an optocoupler circuit, which means it cannot be absolutely necessary.
But if it isn't why then do Strymon themselves bother making an active adaptor cable?

Sorry, I don't understand what you're driving at here well enough to reply.

And even if it is how do they power it with 5 volts?

I already explained that bit. The power is provided by the MIDI *signal* itself. The line provides a 5mA loop, so you can store that power and use it to run the device. No other connection is necessary. Caveats already mentioned apply.

[quote
But it keeps me thinking how they are doing it.
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Absolutely this! It's always good to work out how these people are pulling off these "clever tricks" to see if (a) they are actually that clever and (b) if we can do the same thing ourselves.

[Error!]

Sorry, I don't understand what you're driving at here well enough to reply.

-> The transistor side of the optocoupler needs 5 volts for operation. If you took those 5 volts from the transmitting MIDI device there would be a direct electrical connection between the two devices and thereby a) you would outplay all the isolation (ground loops etc) and b) 5V without an associated ground potential isn't a usable difference in potential aka voltage.
In order to provide isolation, the transistor side of the the optocoupler needs to be powered by the receiving MIDI device.

But I can't imagine that every TRS-MIDI guitar pedal (e.g. Strymon, Chase Bliss, Empress, Meris) supplies +5V as phantom power on their receiving MIDI contact.
So, I guess, all the MIDI-connection-boxes (Empress, Chase Bliss) rely on isolated pedalboard power supplies to prevent ground loops and connect to the MIDI devices without any extra isolation via optocouplers.

The only thing that I can't wrap my head around is where the +5V in Strymon's adaptor is taken from (top right corner of the schematic posted above).
Sure, you could use phantom power on the signal line but I understand "TO UART RX" as "straight to a pin of the processor".
And with the RING and SLEEVE of the jack being essentially ground (when only receiving MIDI) how else would they supply +5V to the adaptor circuit?

[ElectricDruid]

I think you've basically answered your own question. Your logic went as follows:

1) If you take power from the transmitting device, you lose the opto-isolation.
2) In order to maintain isolation, you'd have to take power from the receiving device, but that's impossible
3) Therefore they must do (1)

I think (1) is true. If you start ignoring the MIDI spec to do a line-powered device, you can ignore the bit where it says "must avoid ground loops" too. In the case of the old MIDI filters, it wasn't such an issue since they were then *sending* MIDI to another MIDI input that *was* isolated. So the filter effectively just became a connected part of the transmitting device.

In Strymon's case, I'm changing my mind. Initially I thought it was a classic line-powered MIDI device (e.g. transmitter powered). But thinking about it some more, I think they've done (2), e.g. receiver powered. They've got three pins, and one of them can be configured to send power when required (by making it a logic output and setting it high). Like that, their cable turns incoming MIDI current loop signals into a UART serial data stream and is powered by the receiving device. As far as the transmitting device is concerned, it's a completely legal MIDI input.

When you connect these Strymon devices using TRS cables to talk to each other, I think you're just connecting UART Rx/Tx pins between devices. With that there is no isolation, but I guess they either don't care or they've done something else clever internally that we don't know about.

[nzCdog]

Hello, sorry for the dredge but I wanted to post here as I have recently built this circuit... and it works!

Power for the 6N138 comes from the Ring of the TRS connector (receiving device). I use this cable to control my Strymon Iridium from my Boss MS-3.  Thanks to the earlier posters, this thread helped get the cable working, and I also found this thread on the midibox forum helpful.

Vero layout below... Cheers  

[RichC]

i have built this circuit but couldnt get it work for some reason. Anyway i ended up buying the Strymon EXT Midi cable, but now im having another problem!

I have a Strymon cloudbust pedal that im using the cable with, but the problem is it will only work when connected to a MIDI OUT port, not a MIDI THRU! This not much use and the only MIDI OUT i have (from my HX Stomp) is connected to a MIDI THRU box which then sends MIDI to all my other effects.

Any ideas what would be wrong here and how i can get it work?

[FiveseveN]

connected to a MIDI THRU box

Hello and welcome!
What are the specifics of this "box"? Have you tested it with other "through" outputs from other devices or just the box?

[RichC]

connected to a MIDI THRU box

Hello and welcome!
What are the specifics of this "box"? Have you tested it with other "through" outputs from other devices or just the box?

It's a Mindburner MIDI THRU box.. my other effects units that i connected to this are working fine.
I have also tried using the MIDI out/thru port of one of my other effect units with thru enabled and that doesnt work eitehr - it only seems to work when it's connected to the MIDI Out port on my HX Stomp.

I diddnt think there was any difference electrically between a MIDI out and a THRU port, so im not sure why it works with one and not the other...