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Power Mac 6100/66 DOS Compatible – MIDI

I couldn't resist this over-sized-pizza-box when I saw it on Facebook Marketplace. I never really check that area of the web, as the interaction mechanism is too loose... half the time you never even get a response from sellers and it doesn't feel real-time enough to trust if anything is actually available. Anyway, I threw a curve-ball offer and the seller responded (6 hours later)! The unit was picked up in short time, well actually... I had to wait for Victoria's 5-day lockdown to end. Yes, that was a while ago! It's been sitting in the junk room waiting for a video adapter cable thanks to it's HDI-45 video plug.

It's a beautiful unit. A much larger pizza-box than the LCs and it's even DOS Compatible!

Period-Correct Screen

By the time the video adapter cable came from America, I was ready to test the unit. I'd actually recently picked up a Trinitron monitor from the tip shop and it also needed testing! After a good sanitisation all round, everything was plugged together. To connect the Mac to the Monitor, I needed the HDI-45 to DB-15 adapter and then another DB-15 to DE-15 to get to VGA standards. With everything connected I got nothing... just a black screen... but I did get a startup bong! Myoldmac has a great reference on the video adapter settings and I had to use this as my adapter seems to have lost its packaging and the settings sticker on the actual unit. After a miriad of tests, the correct answer was to have 'separate sync' configured.

Ok, with a picture showing, I could now configure resolutions... only up to 832x624? What gives? Oh, This machine has 640kb of non-upgradeable VRAM limiting my choices. There's a good write-up here describing its system limitations. Turns out the AV Model has a separate card with 2MB allowing more resolutions on an external monitor. That wont work for me though... I have a DOS card in there!

MIDI By Roland

Before we get to DOS, let's just get my external SC-88 going. It's got a serial connection and I've always wanted to test it. Firstly, grab a game that'll support it and get that installed. Actually, there're too many options to choose from there. For fun, I'll load up DOOM II as, well, it has the easiest gameplay and a great soundtrack!

Grab a standard serial cable, make sure you choose a port that's free (i.e. not in-use by AppleTalk as you won't get a single message and it'll just not-work) and plug it all together. Make sure the SC-88 is set to "Mac" and that it has been power cycled since that switch was changed. Then power up the Macintosh. Don't expect anything to be hot-pluggable! But even then, nothing would be visible in the Quicktime Settings.

At this point in time, you can look over here for hints and then here for cable wiring and then here for the Roland manual and keep guessing why that Quicktime panel doesn't work... but don't bother.

No amount of cable-foolery got the basic Quicktime Installation to enable the General Midi checken-boxen. Instead, I software-foolery'd around until a version of OMS both extracted and installed. Note, if you've just installed MacOS, do not use the version of StuffIt from the Internet Utilities folder. It's archaic and will fail to extract things. Make sure you get a recent version installed first!

I need to discuss the middle shot above. It's telling you that, if AppleTalk is enabled, one of your Serial ports might be in use and the CPU will also be under pressure. Fortunately, our AppleTalk config is over Ethernet, so no Serial ports are in use! Is this software version for Macs with zero Ethernet capabilities? Eek... Anyway... auto-detect away!

Haha.. it found 16 Roland SC-88s... If I was a rich man? Must download some Beatles. After this, many duplicates were cleaned up until there was only one. From there, I scrolled down to check the other port...

The Yamaha MU80 didn't fair as well... it itself threw an ILLEGAL COMMAND error and the software decided it was a shitty Modem.

So.. just force it in the settings...

And then try again...

Music! Doom II sounded amazing.

DOS Compatible

PC Setup was already installed on the hideously-themed OS. I tried to boot it up, but the screen just went black! Turns out that unlike the previous Power Mac 7220, this one needed an external cable to route the video through. Googlin' around, there were a few options/schematics, but unfortunately this specific Power Mac had the HDI-45 Video port and therefore needed a hydra cable containing a different pinout. Thanks to the internet though, someone has already done the build and the pinout was available (and more over here). Here it is again for safe-keeping:

Each of the three plugs coming off the main plug need 13 wires each! I don't think I've ever had a standard 'round' cable in my arsenal that had more than 8 wires. Maybe I should trash a printer cable? Maybe I'll just heat-strink single-strands? If I cut the heat-strink up enough, it can 'telescope' down when I'm soldering and then I can slide the pieces up, bit-by-bit? Looking a bit like bamboo? It won't be too flexible though!

I started chopping up wires to use for the individual pins... until it occurred to me that pins were shared across all three secondary plugs. How does one share a wire succesfully? I'd actually need to construct 1-to-2/3 wires that split out into the differing plugs. At the final plug end, they can bridge all the pins they need to, but they need to get there first, without causing wire bridges 'outside' the initial plug housing.

So, with this in mind, I twisted two or three wires together where the ground feed needed to be distributed. I even used resistor legs to shift the major solder blob away from the plug solder pins.

Somewhere around the middle photo above I went slightly insane... and slept. Soldering and routing that many wires was a nightmare!

IT WAS EVEN WORSE ONCE A SINGLE END PLUG WAS ON!

But finally, a hydra-sorta-styled cable was assembled. All wiring was double-checked and then it was plugged together:

No way. It worked. I re-loaded the DOS image hard disk file from the original installation of Mac OS and found a copy of Win95B with an app that seemed to be a collection of "The Age" articles. Who-the-fxxx would want to have a source of that trash?

Unfortunately, it seems that all the data was on CD! So I shut it down.

Shiiiit.. haven't seen that in a LOOOOOONG while. I then tried to shut down the actual Macintosh... after Apple-Return switching back to MacOS...

What dat? Does this thing have an actual physical clicky power switch? i.e. non-atx-style soft power off? I pushed it and it did indeed click! I never noticed when turning it on!

MIDI via the DOS Joystick Port?

I was really hoping to hook up my Roland SC-88 to the joystick port, but no go. A quick google of pinouts shows that MIDI TX/RX signals are on pins 12 and 15 and these are tied to +5V and GND, which means it's actually REALLY DANGEROUS to hook up any MIDI device to this joystick-only port. Actually, I blab a lot on here, so let's just re-make that point...

Looking at the cable pinout table further above, it shows that we also don't have any spare pins on the 26-pin plug from the DOS card to squeeze MIDI through, so no wonder they prioritised video over the game port. Interestingly though, couldn't they have possibly spared two of those ground wires to pass MIDI data through?

It then occurred to me that we might be able to route the MIDI data through the Macintosh' Serial Port. To do this, I first connected my SC-88 (which has a very convenient Macintosh Serial Port on it!) to the Printer port and configured PC Setup to route COM1 to this.

From here, I downloaded and installed softmpu. This driver loads into RAM and intercepts all calls to the IO of the MPU on the Sound Card. The DOS Card fully supports the MPU, it's just that the output pins are literally un-wired! So, with the virtual tap, we can send the MIDI data out to COM1 instead, which is then virtually connected to the Macintosh Printer Port which is then hard-wired to the SC-88!

But it didn't work... softmpu just froze up the DOS side whenever executed.

Can we wire up the MIDI Port?

I then did a bit of Googlin' about those missing traces from the CT2501 Vibra 16 chipset. It seems that many have discussed it before, but each time it's ended in silence. Macintosh Garden's Comment Thread even tells me that it would've never worked and half of me wished I'd read that at the start as I probably would've not even bothered to wire up the port!

In 2019, BaldGoldEagle brought up the exact topic again on Vogons and got a good amount of detail, even after a direct bit of abuse from someone who didn't quite listen at the start (yeah, I'm talkin'bout you derSammler). The basic answer was to find a PC soundcard that has the same chipset (CT2260, CT2810, PB3110, CT3110, CT3930) and trace the pins to the port to work out what we'd need to build. The thread ended with a user offering to do so, but then the usual silence... I've just replied to see if anything came about!

A little more googlin' brought up a beautiful post dishing out on all of the DOS cards as they were built. Initially, the Houdini cards were mentioned as supporting MIDI! That was quickly corrected in the replies and the same result was reached: crickets.

I'll post again about MIDI once I've worked on it all a little more. (This now works! More information here!)