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HD20 Schematics required


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Hi, just picked up a HD20 off eBay that has had its interface/floppy cable cut off. I have a spare 400K external floppy drive, and I was hoping to use its cable for the HD20 (when it arrives). While I wait, I was hoping that someone might have a softcopy of any HD20 schematics, technical manuals that I could peruse while waiting for the delivery.

 

I'm hoping that I can get it operational, but it will make for a nice retro drive style HDD enclosure if it can't be resurrected.

 

Edit: Yes I've searched here and elsewhere, but nary a circuit diagram, nor schematic can I find.

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There are at least half a dozen threads devoted to the lack of information available about the HD20. Endless threads trying to figure out exactly what you are trying to do, compiled over the last 6 years or more. It does not exist within the collective knowledge of this forum. But there is a lot of good information that may help you, including the patent diagram of a similar Rodime drive. What is funny is your casual expectation that such a thing exists in the face of all the threads where the members of this forum have beat their heads against the wall looking for solutions to non-functional HD20s. Laughing at the situation, not at you. But good luck.

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There is a non-working HD-20 on US eBay right now for fairly cheap. If I manage to win the drive (I can't afford much), I will derive some MUCH NEEDED schematics. From there, I will attempt to analyze the logic of the drive if it works well enough to give me some info. If the controller has dumpable ROM, well, then that could certainly help. If I can get enough info, I suppose the next step would be to collaborate on some Mac <-> microcontroller <-> flash drive thing. ;D Nobody around these parts probably knows if 20MB is the max with this interface, but I bet we'll find some wiggle room as details develop.

 

So if you can wait for a bit to see if I get this drive, the first thing I will do is make a connection diagram that shows how one end of the cord connects to the other. From there, all sorts of other experiments. I am an ECE graduate so please have faith that I at least sort of know what I'm doing with these things. :)

 

If anyone has a non-working HD-20 that they can sell me for $30 or less shipped to US (my limit), please let me know.

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There is a helpful pinout of the floppy port here (but not the internal connector of the HD20 alas):

http://support.apple.com/kb/TA48050?viewlocale=en_US

 

And translated to have a nice diagram and data directions:

http://pinouts.ru/Storage/MacExtDrive_pinout.shtml

 

More clues:

http://pinouts.ru/Storage/macfloppy2_pinout.shtml

 

 

Assuming that the HD-20 works using a similar pinout as a floppy drive (which may not be entirely true):

 

DATA:

Rd and Wr seem to be serial lines of unknown baud rate and handshaking.

 

ADDRESSING:

PH0, PH1, PH2 seem to be parallel control lines. There are no obvious direction selecting lines for these, so it seems that they are sent only FROM the Mac TO a drive. It seems that PH3 is strobed to send the 3-bit command present on PH0, PH1, PH2.

 

HdSel may still be a head select line for the HD-20. I am unsure of the number of heads in the drive, but if there are 2, then that's a good sign.

 

 

 

I haven't taken apart a Mac Plus in a while. I'm not sure of the flexibility of the SWIM chip, or whatever it is they use way back then, but it seems possible that data is not sent exclusively over data lines. It could be mixed in and sent as 4-bit parallel with the control lines if that makes it faster.

 

It isn't totally straightforward how this contraption works, or what magic Apple pulled off to make this possible. I would say that this is the most likely scenario:

 

- All lines have identical function as with a floppy drive.

- There is some command sent on the control lines that returns different data through RD when sent to an HD20 than a normal floppy drive. Possibly utilizing the same function that tells the Mac if you have a 400k or 800k disk.

- HD 20 extension (or built-in ROM) sends this command on boot and listens.

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There is a helpful pinout of the floppy port here

 

(snip a bunch of links...)

 

PH0, PH1, PH2 seem to be parallel control lines. There are no obvious direction selecting lines for these, so it seems that they are sent only FROM the Mac TO a drive. It seems that PH3 is strobed to send the 3-bit command present on PH0, PH1, PH2.

 

HdSel may still be a head select line for the HD-20. I am unsure of the number of heads in the drive, but if there are 2, then that's a good sign.

 

I haven't taken apart a Mac Plus in a while. I'm not sure of the flexibility of the SWIM chip, or whatever it is they use way back then, but it seems possible that data is not sent exclusively over data lines. It could be mixed in and sent as 4-bit parallel with the control lines if that makes it faster.

(snip some more stuff...)

 

We had a big discussion some time ago about how the floppy drive interface on the Mac 128k->Plus works. (Note that the Plus has an IWM, not an SWIM.) On page two I pointed out this link, which is a detailed account of how to program the IWM, and how the 3.5" disk hardware actually works. Roughly the same information is in "Inside Macintosh". It doesn't have any information about how the HD-20 works, obviously, but reading it will help you understand how the interface hardware works. (Read the appropriate section pointed out in "Inside Macintosh" as well. You can download the original covers-the-Mac 128k/512k volumes in PDF form by searching for it.) It's a lot better than just looking at a pinout and guessing.

 

Here's a few ignorant observations:

 

1: In theory it looks like you could bit-bang a communication stream over the status lines, but it would probably be *very* slow. Reading and writing the registers through the IWM requires substantial setup before each read or write. (We're talking literally about strobing a couple bytes into the controller to read a *single bit* back off the status lines from the drive.) It's much more likely that the HD-20 used the IWM as a UART for the actual data transfer, and *probably* also used it to send control commands, although I'd be less certain about that.

 

2. If you get an HD-20, look for a CPU. I seriously doubt that it lacks a microcontroller of *some kind*. If it does disassembling its ROM and deducing the protocol it speaks from that might just be easier than figuring out a disassembly of the HD-20 init or the appropriate part of the Mac Plus's ROM to do the same. (BTW, has anybody actually done that and managed to make any sense out of it?)

 

3: Here's a flickr picture of the inside of an HD-20. Could that large 40 pin-ish chip be a CPU? Here's another picture. Unfortunately the resolution is just too low. to read the chip markings... But wait! Look, there's labels on the board! Next to the 40 pin chip it says "Z8"... Zilog Z8? There's your microcontroller. There's also an IWM (see below), and a 2764, which is an 8k ROM. There, target acquired. (There's also a 2k static RAM, and the mysterious flat-pack labeled "APPLE". Hopefully that's just a programmed gate array or similar generic piece that's used as glue logic, as opposed to having any intelligence function.)

 

4: Before seeing the circuit board and having it confirmed I seem to remember reading somewhere that the HD-20 actually had a IWM inside of it that was "used to drive a floppy connected to its daisy chain port". Now, what I wonder is if it *may* of actually used that IWM as the other end of the "floppy-serial" link with the Mac. Pure conjecture, but it would make a certain amount of sense. If you're using it as a UART on the Mac side you might as well in the drive as well... of course, it could also be something in that mystery chip that does it. But I like the IWM-UART idea. ;^)

 

Anyway, good luck.

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VERY good information for software reverse engineering, Gorgonops. Thanks to you, we know the architecture and where to dump the ROM from.

 

It may be possible from the hardware side to build a device that intercepts and successfully interprets these IWM signals to/from an ordinary floppy drive. To a certain extent, these signals should be predictable with a bit of research, so we would have some ability to double-check things.

 

After that is mastered for both 400k and 800k, then switch to the HD-20 and speculate about how the signals are different. This stuff is from the mid 80s, it can't be TOO complicated. Heck, I see 7400 series DIPs in those pictures!

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Maybe Rodime is the only thing the 20 Plus and the HD20 have in common...

 

DSCF0140.jpg

 

DSCF0145.jpg

 

DSCF0144.jpg

 

DSCF0143.jpg

 

DSCF0142.jpg

 

DSCF0141.jpg

 

As you can see, I substituted the missing HDD with the one from my faulty LC. It didn't work. Also note, there are two snipped cables, but buggered if I know what they are for. Hell, I can't find much info on the system to start with!

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The chopped wires appear to be for a power light or a fan. It isn't impossible that this happened at the factory. All that can be wrong with it are as follows:

 

  • +5 or +12 Power supply voltages have strayed
  • bad connection between the Centronics-50 ports and the other end of the ribbon cable
  • problem with the drive, i.e. SCSI ID, termination, bad drive
  • bad SCSI cable or external SCSI port on the Mac

 

If this drive works inside of a Mac, test the drive again inside of a Mac, but substitute the power from the Rodime. This will verify the Rodime power supply. If it works, it must be something with the SCSI cable/ribbon stuff, something metal touching the hard drive's board, too long of screws touching something, etc. All the little weird things. This is NOT a complicated problem though. That drive can work with a bit of troubleshooting.

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It may be possible from the hardware side to build a device that intercepts and successfully interprets these IWM signals to/from an ordinary floppy drive. To a certain extent, these signals should be predictable with a bit of research, so we would have some ability to double-check things.

 

The operation of the IWM when it comes to talking to floppy drives is well-described, so I don't see it being worth building a signal analyzer solely for that. There are already devices like the "SVD" which can be connected to an Apple Disk ][ controller and emulate a drive, and the data stream portion of the interface is the same for the Disk ][ and the Macintosh drives. (Where they differ is the use of the "control lines"... a Disk ][ uses the wires to send raw stepper-motor pulses to a completely "dumb" peripheral, while the Mac drive has those status registers and accepts "high-level" commands like "step" and "change step direction". Meanwhile, the Mac 400k drives added a "PWM" signal to control the speed of the motor. Ironically the 800k drives didn't use it.) It's reasonably safe to say there's enough documentation out there that if someone wanted really wanted to build a "Macintosh Disk Drive" emulator they could probably just go straight to prototype hardware without bothering with a protocol analyzer. (Particularly since in terms of hardware they'd basically amount to the same thing anyway.)

 

After that is mastered for both 400k and 800k, then switch to the HD-20 and speculate about how the signals are different.

 

Contrarywise, a signal analyzer *would* be useful if you found a *working* HD-20 to play with. Clearly the easiest way to get one would be to build a *floppy drive emulator* with a vastly oversized Flash memory (like an SD card) and get it working. Once you've nailed that down you add a "passthrough connector" that can duplicate all the signals sent through the device and log everything it sees in a RAM/flash buffer. Hook it up between a suitable Mac and an HD-20, put the drive through all its paces (formatting, reading, writing, whatever), and then get to work analyzing your logs. Once you think you've got some idea what you're doing then a few firmware changes to your floppy emulator should be able to turn it into an HD-20. Easy Peasy?

 

(No, not easy, it's a blooming heck of a lot of work. But it's the only way it'll get done if you restrict yourself to "public documentation" of the interface and clean-room reverse engineering techniques. Disassembling the software driver on the Mac side and the firmware of the HD-20 might let you skip a few steps.)

 

This stuff is from the mid 80s, it can't be TOO complicated. Heck, I see 7400 series DIPs in those pictures!

 

The real question is what that big square chip is. It has "Apple" written on it, which is worrisome. It could be something generic, but my gut feeling is that it's an ASIC, probably a gate array device. If that's the case a schematic derived from the circuit board isn't going to be that informative... you'll see a bunch of signals going in and out of that thing, but without knowing what it *does* you'll still be in the dark. On the bright side all/most of its logic *may* be dedicated to driving the hard disk drive. The patent document covering the disk drive itself is interesting reading. The interface seems to function a lot like a simplified version of ESDI, in that it still requires a "host controller" to serialize the data stream, but the actual RLL encoding and data separation happen on the drive itself. In addition, the drive also has its own Z8 microcontroller which allows it to accept high-level-ish commands for head positioning. Thus the "host controller" may well fit into a single modestly-sized-for-1986-ASIC... and the more of that ASIC that's dedicated solely to being the hard disk controller the better.

 

(I'm also guessing that the RAM chip is used as a sector buffer for reading and writing to the drive since the data rate of the IWM isn't likely to be sufficient to provide the data stream directly.)

 

So... if all that supposition around the "mystery chip" is correct then it's possible the Mac interface may be composed entirely of the Z8, its program ROM, and the IWM. (I'm still guessing here that the IWM in the HD-20 is used as a UART to talk to the Mac. I'm really in love with that idea... it's just so wrong.) If that were all the case, and I'll grant that's a lot of guessing, duplicating the device as an updated version of the original hardware might be possible. They still sell enhanced Z8-family chips so if you were willing to pull an IWM from a dead Mac you might even be able to reuse the portions of the original code which spoke to the host Mac, only leaving you to rewrite the portion that spoke to the disk drive to use whatever your new storage medium is instead.

 

(Admittedly you probably wouldn't want to build a device that required pirating ICs from dead Macs but, eh, figured I'd mention the idea. A fast microcontroller like a Propeller could probably emulate an IWM easily enough.)

 

Anyway. We'll see just how for naught this discussion ends up being. Lots of ideas get thrown around, it's rare someone actually makes them happen.

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I see that the IWM floppy drive interface is well-documented, that is extremely helpful. The reason that I want to start by analyzing the floppy drive is for that very reason; I can have the solution to work toward, and I think that's what you're talking about with the floppy emulator. There are a lot of small, possibly unknown details that make it difficult to blindly dive in and analyze logic, lots of timings that we may not be 100% certain about, so if I can get that to check out first with an ordinary floppy drive, I will learn what to expect and know that my testing rig is working properly. It also gives me that hands-on experience that I require to properly understand something like this.

 

I do not have a logic analyzer by the way. I will probably be doing this with a microcontroller and record timing data to be picked apart with a computer. i.e. for each line, how long it's a zero, how long it's a one, how long it's a zero. The computer can then reconstruct the waveforms from this information.

 

This discussion will result in AT LEAST some schematics and useful information. It may take time for me to analyze the logic as I am between jobs and moving soon, but once things settle, we should have some signals dumped out for mapping.

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Hi, just picked up a HD20 off eBay that has had its interface/floppy cable cut off. ...I've searched here and elsewhere...

Ditto Mac 128's chuckle:

viewtopic.php?p=140834#p140834

 

jongleur, you did fulfill your word in that other thread though. You bid and won!

 

But what I want to know is, who in the heck would hack off a hard drive cable like this, especially on something as glorious as an Apple HD20? The previous owner must have been either a madman, or quite pissed at Apple.

 

Here are photos (inside and out) of my HD20, with cable intact:

http://www.flickr.com/search/?q=HD20+-HD20SC+-error&m=tags&ss=0&ct=0&mt=all&w=66071596%40N00&adv=1

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But what I want to know is, who in the heck would hack off a hard drive cable like this, especially on something as glorious as an Apple HD20? The previous owner must have been either a madman, or quite pissed at Apple.

 

I suspect that it went out in a kerb side cleanup, and someone came along and cut off the cable. They do this to then sell for scrap copper, but who knows, and what was left was rescued. I paid a AU$70 for mine, but now I need to either replace the data cable, or find another unit on eBay.

 

Currently it is sitting under my 512k - looks good, but I'd love to get an operational HD20 eventually.

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I got a load of computers from someone via Craiglist a few years ago where he had even more he was just dumping.

 

Many years prior, he had been heavily into drugs, as had his roommates. One day, one of his roommates went through all his computers (he had them more as a pack rat, having run a small computer business in the early '80s, than keeping as a collection,) and removed all the RAM, CPUs, and cabling, to take to a recycler for cash. Yup, cut off hard-wired cables and everything.

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but now I need to either replace the data cable, or find another unit on eBay.

If that's all you need ... Considering the HD20 plugs into the DB19 floppy drive port and offers a floppy drive passthrough, as well as designed to emulate a giant floppy disk, I would be shocked if a cable from a 400K or 800K external drive wouldn't work just fine. In fact the 20 pin connector on the HD 20 logic board is identical to the one on the external floppy disk and compact Mac logic boards. You should even be able to use a 20 pin ribbon cable from the Mac to the HD20 side-by-side, cases off, if you don't have an external handy. That would at least tell you if it works. You'll need at least an SE with two floppy connectors and/or an internal HD, or a Plus with an external hard drive to boot from, since you will be dedicating the internal floppy connector for the HD 20.

 

However, excepting Anonymous Freak's example, I would expect the notoriously finicky Rodime drive within the HD 20 ceased functioning long before the cable was cut off. Here's hoping I'm wrong and AF's scenario applies. Good luck.

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I don't recall ever seeing an identical cable in a floppy drive, i.e., one that would fit through the opening in the back of the HD20, so it seems that this cable was at least molded specific to the HD20. I suspect they're electrically the same, they do appear to use the same connectors, so you can probably use a disk drive cable, but given that you can just wait a bit for me or someone else to give you a definitive answer on it, then we can eliminate any chance of damage to your drive.

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I don't recall ever seeing an identical cable in a floppy drive, i.e., one that would fit through the opening in the back of the HD20

Then you've never seen a Macintosh 400K or 800K external floppy drive. Both of those cables, as well as all Apple 3.5" drives, are all interchangeable, with the exact same 20 pin connectors, length of interior cable and grounding connections. They fit into the HD20 like a glove. In fact it may be the exact same cable use on the beige Macintosh 800K external drive, which I don't have handy at the moment.

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I admit that my experience is rusty. I had an HD 20 that I took apart many years ago, that's what I'm going off from. I have an old Apple 400k drive here with a cable that certainly would NOT fit through the case of the HD20, and I seem to remember that my UniDisk 3.5 cable didn't fit. There are certainly other 3.5" drives that I have not considered. I'm sorry if I assumed a knowledgeable position on the subject. I agree and never disputed that all of the connectors will fit together, and it seems unlike Apple to throw in one with different connections from one end to the other. I would make every effort to be certain with such a valuable drive though, should it be in working condition minus a chopped cable.

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