Are you sure your DB15 to MIDI cable contains the necessary circuitry to get a proper MIDI signal or just route the signal direcly from the DB15? At page 27 there's an example how it should look like.

I really doubt my cable contains any electronics beyond the pure copper wires. And yet, it works fine with the mt-32 which for me was ruling out it was a cable issue. I may try to get another to give this a go, any suggestions on where to pick it? Last time I ordered from aliexpress 🙁

here?
https://www.serdashop.com/JoystickMIDICable

Thanks @keropi, just ordered one. I'm still unsure this is the problem but it's worth a try, and anyway I wanted to have a spare cable 😀

What games/software are you using?

Also, how's the audio output of the SC-55 connected? If you have routed it through the sound card's line-in, make sure it's not muted.

I tried several games, including doom2 and tetris classic. Doom2 is especially nice to setup, allowing you to select the midi port address.

The output from the sc-55 is connected to my external mixer. This mixer is connected my amplifier, but it also has a headphone output I was using for the test. I also tried the headphone output in the sc-55 unit to isolate the cause of the problem, and still no sound.

The front panel of the unit looked pretty still, really as if it was not getting any midi input. However, while playing Doom2 I could plug the lead to the MT-32 and listen to the music, unplug it and connect it to the sc-55 and it would remain silent. Plug it again to the mt-32 and again music.

So weird, it may something related to the cable after all. I'll have to wait and see if the new cable works. A funny thing I noticed is that the MT-32 would not work if I routed the midi signal from the input of the sc-55 to the mt-32 through the midi-through port. It's really as if the sc-55 was firewalling any midi input coming from my 486 🙁

Did you already try resetting the SC-55 to factory values? Does it play the built-in demo song?

Yup, I did init it by switching it on having both instrument left and right keys pressed. The demo songs play well. Hell, I can play midi files from my laptop using the usb<->midi cable. It's the 486 that won't be able to play music through it via the db15<->midi cable, but it would play the mt-32 fine. Crazy!

keropi wrote:

here?
https://www.serdashop.com/JoystickMIDICable

Thank you for letting me know about this! I was going to buy some on eBay, but for one, you cannot be sure, that it will work properly with MIDI and also, strangely, they're not that cheap on eBay.

Ok, I got my setup working using the following hack: I connected the db15-midi cable directly to my mt-32, and used the midi-through of the mt-32 to feed the sc-55. Worked like a charm! The cable is obviously missing some electronics but the mt-32 copes with that pretty diligently.

On a related note, Doom 2 soundtrack sounds just beautiful on the sc-55!

Thanks to everybody that suggested solutions. I'm now waiting for the new cable which I hope will be able to drive the sc-55 properly (will confirm when I get it). I may end up sticking to this solution anyway.

Definitely sounds like a bad midi cable. It could be the gp-to-midi cable too. I had one go out on me and it took a good while to figure it out.

Alright, got my cable today ordered from https://www.serdashop.com/JoystickMIDICable and I can confirm it works perfectly fine when connected directly to my sc-55. So mistery solved, it was the cable! I am still amazed of the ability of the mt-32 unit to deal with such an imperfect cable and even deliver a compatible midi signal via the midi-through port to my sc-55. Neat.

Thanks @keropi for sharing the link to the compatible cable.

Cheers

Excellent news, glad all worked out in the end!

I've encountered the same problem with an ESS1868 card. Switching sound card to a Yamaha YMF-718 card fixed it in my case.

Diagnosing this issue further.

When I looked at the MIDI signal on the oscilloscope, you get a nice strong 0 to 5V square wave on the output when the ESS1868 is not connected to anything.

When I connect the MIDI cable to my SC-55MkII, I check the signal going into the optocoupler input, I only see about a 1 volt differential between 1 and 0 state, so the square wave is only swinging between 4V and 5V. Looking at the other side of the optocoupler, you do not see a full signal either (should definitely be switching between 0 and 5V after the optocoupler)

This means that the ESS1868 is weak at driving the signal to 0V on a low impedance output. On some devices, like the OP's MT-32, or my own MT32-PI device, there is still enough oomph to switch the optocoupler properly and get a signal into the synth, on other devices, not so much.

When I test with the Yamaha, I see a more substantial swing of 2-3V on the input side of the optocoupler. Not perfect, but enough to get a clean signal on the other side of the optocoupler.

The OPs solution worked great because the MT-32's optocoupler is sensitive enough to get the MIDI signal from the ESS1868, then the full square wave signal is run through a couple of TTL like inverters to ensure strong signal strength before being outputted on the MIDI through output.

An active cable (with built in redrive circuit) would also definitely helped.

I guess I'm a bit late to this thread, but for anyone wondering what was going on with why it wasn't working.