Amiga 500 MIDI Interface Options

[appiah4]

Hey there EAB, I've been gone for a while but now I'm kind of sucked back in.. I've been engrossing myself in retro x86 stuff for half a decade and ended up acquiring quite a bit of MIDI gear, in the form of both external synthesizers as well as internal wavetable daughterboards. Now I want to make use of some of these with my Amiga, but I have no MIDI interface. So I was wondering.. are there any modern options to add MIDI capabilities to an Amiga? Maybe an opensource Github project with an off the shelf BOM and a gerber I can get printed at JLCPCB? I am way more experienced with soldering than I used to be, having built my own Snark Barker and PicoGUS in the last few years, so a DIY project is definitely something I would enjoy. Cheers!

[Karlos]

MIDI is basically just serial. MIDI interfaces are optoisolated to protect the devices at each end but beyond that sent the same rs232 signals. I'm sure there are schematics.

http://davbucci.chez-alice.fr/index....nt/midiint.inc

[appiah4]

Originally Posted by Karlos

MIDI is basically just serial. Most midi interfaces from the day were optoisolated to protect the serial port but beyond that sent the same signals. I'm sure there are schematics.

http://davbucci.chez-alice.fr/index....nt/midiint.inc

Well, I have found this: https://github.com/amigamess/AmigaMidi

[offog]

The Datel MIDI Master boxes sometimes come up at sensible prices (<£10) on eBay, but it'd be very easy to build a clone - I've attached photos of the PCB. It's the usual MIDI arrangement with a 74LS chip driving the outputs through 220R resistors, and an optocoupler for the input.

[appiah4]

Thank you but reverse engineering that PCB in Eagle or KiCAD is beyond me. I'll probably end up building the AmigaMidi kit, which is more or less the same thing with only single MIDI In and Out, which is good enough for my needs!

[Krashan]

Originally Posted by appiah4

are there any modern options to add MIDI capabilities to an Amiga? Maybe an opensource Github project with an off the shelf BOM and a gerber I can get printed at JLCPCB?!

KA12 MIDI interface

It is a proven project, I have made over 600 pieces of it. :-)

[Jope]

Originally Posted by Karlos

Most midi interfaces from the day were optoisolated to protect the serial port but beyond that sent the same signals.

The opto isolator is part of the MIDI spec. In my opinion it should never be left out, you might (will) end up with a ground loop and have a bad time.

https://learn.sparkfun.com/tutorials...implementation

[Karlos]

Originally Posted by Jope

The opto isolator is part of the MIDI spec. In my opinion it should never be left out, you might (will) end up with a ground loop and have a bad time.

https://learn.sparkfun.com/tutorials...implementation

There was no suggestion it should be left out.

[Calabazam]

Originally Posted by Krashan

KA12 MIDI interface

It is a proven project, I have made over 600 pieces of it. :-)

Yes it does work fine, i use it on my A1200.

[appiah4]

Originally Posted by Krashan

KA12 MIDI interface

It is a proven project, I have made over 600 pieces of it. :-)

Thank you, I will look into it. There are no renders on the github page, and I can't view the Gerbers at work. Going by the BOM it is a single MIDI In/Out board with a DB25 cable?

[Jope]

Originally Posted by Karlos

There was no suggestion it should be left out.

Your use of the word most might carry the implication that the opto is optional to some readers, so this compelled me to add some information to the thread. Nothing personal.

[Karlos]

Originally Posted by Jope

Your use of the word most might carry the implication that the opto is optional to some readers, so this compelled me to add some information to the thread. Nothing personal.

You are technically correct. Which is the best kind of correct. I shall amend accordingly.

I've also used nullmodem cables to send MIDI data between Amigas.

[Krashan]

Originally Posted by appiah4

There are no renders on the github page, and I can't view the Gerbers at work. Going by the BOM it is a single MIDI In/Out board with a DB25 cable?

The PCB fits inside standard DB25 shell.

It has two shielded two-wire cables ended with DIN5 plugs.

[Jope]

Originally Posted by Karlos

You are technically correct. Which is the best kind of correct. I shall amend accordingly.

:salute:

I've also used nullmodem cables to send MIDI data between Amigas.

Yeh, between computers you probably won't get as many problems with a null modem. Synths, drum machines, mixers etc are much more prone to ground loops and such.

[pandy71]

Originally Posted by Jope

Yeh, between computers you probably won't get as many problems with a null modem. Synths, drum machines, mixers etc are much more prone to ground loops and such.

To prevent this you need to have fully isolated MIDI interface and to be honest i never saw such solution either for Amiga or other computer.

[Krashan]

Originally Posted by pandy71

To prevent this you need to have fully isolated MIDI interface and to be honest i never saw such solution either for Amiga or other computer.

Every MIDI device has (should have, according to the standard) optoisolation on receiving side. So when you connect such a device to Amiga MIDI interface, it is fully optoisolated. One optocoupler is inside the interface, the second one in the connected device.

[pandy71]

Originally Posted by Krashan

Every MIDI device has (should have, according to the standard) optoisolation on receiving side. So when you connect such a device to Amiga MIDI interface, it is fully optoisolated. One optocoupler is inside the interface, the second one in the connected device.

Yep but this not prevent problems similar to grounding loop to occurs - you still may face issues especially in case where you dealing with larger cabling, power cables and capacitive coupling.

[Krashan]

Originally Posted by pandy71

Yep but this not prevent problems similar to grounding loop to occurs - you still may face issues especially in case where you dealing with larger cabling, power cables and capacitive coupling.

Having two cascaded optocouplers instead of one won't help a bit in this regard, but creates another interesting problem: how to power the cable between them. Energy created in illuminated phototransistor is not sufficient to lit the LED in the second optocoupler.

A MIDI cable is constructed in a way that transmitter lits a LED in an optocoupler inside receiver. Then two wires of a MIDI cable constitute a differential pair. EM field generated by it is very small, external EM disturbances cancel out. A good cable also has a shield around the pair, connected to ground of transmitter (but not connected to ground of receiver, or, according to MIDI standard, connected via 100 pF capacitor, to ground high frequency radio interferences). Then even if a long MIDI cable goes along AC high power cables, there is no coupling.

[pandy71]

Originally Posted by Krashan

Having two cascaded optocouplers instead of one won't help a bit in this regard, but creates another interesting problem: how to power the cable between them. Energy created in illuminated phototransistor is not sufficient to lit the LED in the second optocoupler.

A MIDI cable is constructed in a way that transmitter lits a LED in an optocoupler inside receiver. Then two wires of a MIDI cable constitute a differential pair. EM field generated by it is very small, external EM disturbances cancel out. A good cable also has a shield around the pair, connected to ground of transmitter (but not connected to ground of receiver, or, according to MIDI standard, connected via 100 pF capacitor, to ground high frequency radio interferences). Then even if a long MIDI cable goes along AC high power cables, there is no coupling.

You can provide power with isolated DC\DC converter. Cable capacitive coupling is common if you are using power cables - this unavoidable so whenever power island is beyond your control (i.e. you are powering various devices from various sources) then full isolation is more than just recommended.
Also nowadays you have plenty devices powered by SMPS as such most of them is galvanicaly coupled with power network (trough Y class capacitors) anyway.

MIDI is same concept as current loop so industrial practices can be applied to MIDI as well. In home where everything is powered from same phase - same power bar this is not critical and overall risk is way lower but still - personally i would keep Amiga safe at all cost.

[Krashan]

Originally Posted by pandy71

personally i would keep Amiga safe at all cost

There are optic fiber MIDI extenders. If $1000 per pair fits your definition of "all cost". ;-)