HD20 Schematics required

[Mac128]

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.

 

[Dennis Nedry]

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.

 

[Mac128]

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.

 

[JDW]

Did someone say "800k floppy drive cable"?

 

[Dennis Nedry]

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.

 

[Dennis Nedry]

Well, my foot is planted in my mouth.

I just found an image of the back of a UniDisk 3.5 drive and it does appear identical, except it is longer and platinum. :I Does anybody know of a drive with with this cable that came in beige?

 

[Mac128]

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.

Just slid my 400K drive's cable through the HD 20 case, and aside from not having a rounded bottom (leaving a tiny gap), fits perfectly. The 3.5" 800K Unidisk came out at the exact same time as the HD20 and uses the exact same strain-relief rubbber pass-through cable, except it's off-white rather than beige, and there's slighlty more internal wiring. Only the 5.25" Unidisk will not physical pass through as it's rounded on both the top and bottom. This is presumably to keep someone from using that cable on a Macintosh which is electrically incompatible. But no worries, this is an old Mac forum where memories are assumed to suffer from bit-rot. ;-)

And I already mentioned the drive that came with the identical Unidisk cable in beige - the common Macintosh 800K external disk drive, released only 3 months later for use with the Mac Plus. But as I said, the 400K drive cable will also fit, though not identical, if you are concerned about color-matching. And the dual platform Platinum-colored Apple 3.5" drive cable is also identical, if color is not an issue.

One interesting note, all Mac drive cables are either actually a taupe color, or Platinum. Cross Platform cables are Platinum and off-white, matching the plastic cases. While Apple II family only cables are true beige and match the plastic cases. Later 5.25" Platinum drives came with matching Platinum cables as well, but continued the unique strain-relief case passthrough that prevents it being used on a Mac without alteration.

 

[Dennis Nedry]

Thanks for understanding my bit rot Mac128. Have you by any chance used your HD20 off of a disk drive cable? I just get paranoid about that stuff, like maybe Apple switched 2 wires so the traces on the board didn't have to cross each other or something. This drive is SO old and so strange, I feel like there's at least a chance and that worries me.

 

[Mac128]

I don't disagree, especially with the HD 20 - better safe than sorry. But no, never used my external floppy cable instead of the HD 20 cable. I could probably use my HD 20 cable on a floppy disk drive, though I doubt that's proof positive it would work the other way a round.

 

[Gorgonops]

If you have both cables available a few minutes with a continuity tester (or a gutted flashlight and a few scraps of wire) should be sufficient to determine whether the pinouts are the same. Not exactly rocket science.

 

[Dennis Nedry]

That's my plan when my HD 20 comes unless someone beats me to it. I have an external 3.5" drive or two to compare with.

 

[Mac128]

Dennis, couldn't be happier that you are taking on this project!

While you're at it, will you see if you can figure out what code Apple used to disable the HD20 under MFS? It can be formatted MFS, but requires by default that the HFS file system be loaded, or present in the case of the 128K ROM or laters. This means it cannot be used on a stock 128K, for which there is otherwise no reason it should not be able to be used, other than if fully loaded would be slow as molasses under MFS as the Finder struggles to keep track of 20MB of data. But it would be a great device to transfer files back and forth between a more accessible compact and the 128K. Even if only a small capacity of it is used, it would eliminate the two floppy drive limitation of the 128K, and vastly increase the storage space of a 128K, the current max of which is 1600K, without swapping floppy disks. I would also be happy to find a way to load HFS on a 128K, just to see how it actually copes with the file system using the added burden of the INIT code to the system heap.

viewtopic.php?f=7&t=1492&hilit=HD20&start=25#p21460

 

[Dennis Nedry]

Replicating a "black box" and actually understanding it well enough to make a major functional change, like using a different filesystem, are two very different things. I believe that I can replicate the original functionality, or at least document it well enough to be extremely helpful. I don't know enough details at this point to tell exactly what modifications are feasible; a lot of things will probably end up remaining unknown and just repeated from observations. It seems to me that if there is a 20MB volume limit with this interface, and a limit of 2 HD20s, at least some attempts to provide an emulated second HD20 should be worth investigating.

I have a Willem ROM programmer that supposedly can dump the 2764 ROM chip inside of the HD 20. Given that it's only 8 kilobytes, disassembly should at least be somewhat feasible and should shed some light on the Rodime and floppy interfaces. Once dumped, I shall certainly share the 8k file for others to poke around in. Theoretically, we can determine where the Z8 starts execution and disassemble from that point. From there we can determine what register is what, which ports correspond with what hardware, and form up all sorts of cool theories.

If we crack the external disk drive interface, we can build a new board that converts those signals directly to flash. If we crack the Rodime interface, we can build a new board that converts THAT to flash. Both have advantages and disadvantages.

Cracking the Rodime interface allows us to continue using the original controller board and its daisy chain port, the most authentic solution, and maybe the easiest. Cracking the disk drive interface allows ANYBODY with a ROM burner and a soldering iron to build an external disk drive port hard drive with a few affordable parts, but replaces the original HD20 interface board. More hacking is required to provide the daisy chain port, and in all honesty, that part is likely to be skipped in all of the excitement unless it's fairly straightforward. I suspect that the complete replacement version is low-power enough to be powered off of the floppy port.

We have to disassemble the HD20 a bit before we really know which direction to go with this. I imagine that I will have the drive in a week or so. My favorite microcontroller board is loaned out at the moment, so it may take a bit to analyze signals. Stay tuned!

 

[jongleur]

OK, my HD20 with the butchered data cable arrived, and all I've been able to do is take a few photos which are over on RetroMacCast (http://retromaccast.ning.com/photo/hd20-close-up-of-cut-cable?context=user). Haven't been able to get out the spare 400k drive to "borrow" the cable as we are in the process of adopting the neighbour's 3 cats and are getting the downstairs ready for their arrival. Inside the HD20 is dusty and will need a good brush and blast before I do any work with it.

 

[Dennis Nedry]

That doesn't look like the original cable anyway. It looks like a 400k drive cable. Compare to some of the other pictures posted here. Very interesting.

I'd pull the internal connector before powering it on in case the chopped cable has created short circuits. But you should be safe to turn it on and have a listen once that's removed.

 

[Mac128]

Its hard to say exactly what the limits are with the HD20. Its entirely possible the HD20 could support a larger drive, but Apple didn't offer it based on System limitations, or merely price point. Look at their first SCSI drive, it only offered 20MB for many months, yet the case is capable of supporting any size drive you can fit into it, and supported by the system. So the HD20 may well support a greater than 20MB limit.

Also, the HD20 only officially supports one additional daisy-chained drive, but others have reported no problems with three and up to six daisy chained. Hard to say why such limitations were invoked by Apple, maybe to force the consumer to buy more than one Macintosh? But, as you say the onboard ROM dump will divulge much.

The HD20 is also designed to hold a 5.25" full height drive. The original Macintosh ROM contains a driver for the Apple Widget drive per Inside Macintosh, which drive was a full height 5.25" drive. So it's entirely possible the original HD20 drive was intended to be Apple's own Widget, until that too was discontinued along with the Lisa in early 1985. This may in part explain why the HD20 was announced during the 2nd quarter 1985, but not actually delivered until the 4th quarter, as they retooled it for the Rodime.

And yup, that's the 400K drive cable alright ... so if any damage was done, then there's no reason not to try a replacement 400K cable.

 

[Dennis Nedry]

Also, the HD20 only officially supports one additional daisy-chained drive, but others have report no problems with three and up to six daisy chained.

That's very good to know!

 

[jongleur]

Post by Mac128 » 29 Nov 2010, 08:16And yup, that's the 400K drive cable alright ... so if any damage was done, then there's no reason not to try a replacement 400K cable.

Post by Dennis Nedry » 29 Nov 2010, 08:15That doesn't look like the original cable anyway. It looks like a 400k drive cable. Compare to some of the other pictures posted here. Very interesting.

I'd pull the internal connector before powering it on in case the chopped cable has created short circuits. But you should be safe to turn it on and have a listen once that's removed.

Curiouser and curiouser, thanks for the quick ID on the remnants of the cable gentlemen. When I get the chance I'll clean the dust out, remove the butchered cable and give it a power-up test, then try the 400k's cable.

I'll let you know how it goes.

 

[JDW]

Well, well, well...

http://cgi.ebay.com/Rare-Macintosh-Hard-Disk-20-drive-software-case-/290506026430?pt=PCC_Drives_Storage_Internal&hash=item43a3822dbe#ht_3225wt_1141

Interesting how fast these "rare" HD20 drives pop up on EBAY, one after another. And this one has its cable intact too, as do most I've seen on EBAY. Which makes me ponder the definition of "rare" a bit... Ahah! So that is the definition of a "rare HD20"!!! An HD20 with its cable hacked off! A cable-less HD20, my friends, is indeed "rare." But the one on EBAY at the moment, is not.

 

[jsarchibald]

Yeah, it goes without saying that the US is a treasure trove of Apple stuff. But here in Australia, we can't just snap our fingers or wait for a few weeks for something to come up. I have been looking for the past 6 months, and one has never popped up. There has been no 128Ks and only one 512K. A TAM, forget about it. So, you may have it easy but we are not in the same boat as you - hence rare.