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Mac Plus internal SCSI: noninvasive edition

cheesestraws

Well-known member
Today I successfully installed an internal SCSI disc in a Mac Plus "noninvasively"—that is to say, without any modification of anything original in the Plus, just clip-on things and replacement cables. I thought people might like to see what I did and how. It's been reliable so far today, but of course I make no guarantees; especially, I want to see how it turns out with a bit of temperature cycling. For clarity, nearly all of the work in here is other people's, who will be credited as we go—I just put stuff together.

Here's what the final installation in the Plus looks like:

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First, the power connector. To do this, I made up a replacement cable between the LB and AB. The connector at each end of these is a 11-position Molex KK396, with one connection blanked off. The cable is just wired straight-through, pin 1 to pin 1, etc. The blanked-off connection is pin 2; I like to use hot glue to make an ad-hoc key. It's not great but it's better than nothing.


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At one end of the cable (which will become the analogue board end), crimp two wires into the terminals for 5V, 12V, and the two grounds. One of these should go to the other KK396. The other go to a "Molex" power connector. The result looks like this:

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I attached the molex connector with screw terminals because it was what I had handy, and I don't have the tool to crimp my own. You could probably leave out the 12V—you probably wouldn't want to power a spinning HD off the rather anæmic Plus supply—but I am cursed with completionism.

At this point, you should string the cable between the LB and AB and check that the voltages on your new power connector are in the right places.

Now for SCSI data. We have two issues here: how to tap into the SCSI data on the board without modifying it, and how to turn it into the right SCSI pinout.

For the first I used one of @Stephen's Cloney Clips. Since I am bad at soldering and have no hand-eye coordination, I adopted a couple of tricks which might also help other people. First, I built a test jig on a bit of veroboard to make sure that I'd got all the pogo pins in at the right height. This is just a spare 40-pin junk chip from something I disassembled years ago and some loops of wire; then I can just methodically walk down the pads on the top of the Cloney Clip and the loops of wire with my multimeter in continuity mode and make sure good connection is being made. Two of the pins I installed did require a second attempt, so this was a major annoyance saver.

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Another thing I am bad at is soldering ribbon cables. However, there's a hack here. There is room to solder on right angle pin headers in a kind of ad-hoc surface mount fashion. This is easier, because the row of pins is rigid, and once you've soldered on one end, the rest stay put. For this use case, you want to solder on a row of them on one side of the clip; this row will go on the side of the chip that is towards the back of the board, thus:


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There is then room, if one is careful to put a 20 way ribbon cable from this connector up through the hole in the chassis and up into the main cavity.

To adapt this ribbon cable into a more useful pinout, I used one of @360alaska's adaptors that were designed for soldering onto the chip. Instead of doing it like this, I soldered on a matching set of right angle pin headers to the pads that would go to the chip. Also, instead of soldering on a male socket to the top of the board to take a cable, I soldered a female plug on the bottom of the board to plug directly into a SCSI device—the device in question being a BlueSCSI (of @Bolle's board design).

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So far, it's behaved: I'm a little worried about noise on the SCSI cable given that all the grounds but one have disappeared, but I probably shouldn't be.

I'll keep people updated on how well it works longer-term, but I hope this is interesting to someone, at least.
 

LaPorta

Well-known member
This is the sort of thing I like: mods that don't upset the original structure and function of the machine itself.
 

Mu0n

Well-known member
I keep forgetting to check about the BlueSCSI's status with the Mac Plus - is it still System 7 strictly, or did someone figure out how to add System 6 support? I think I'd try to mimick your setup if that was the case.
 

cheesestraws

Well-known member
did someone figure out how to add System 6 support

I'm running System 6 off it fine. Use a System 6 compatible version of Lido to format it—HD SC Setup, even patched, won't. Lido will complain about something during initialising, but it'll still work. Then install the System the normal way. Don't use the project-provided HFS blanks, they crash early in boot for some reason (I'm guessing drivers). Also disregard the bit in the documentation that says the Lido driver is incompatible with the Plus, that seems to be just wrong (or may only apply to some revisions of the Plus ROM?). I wasn't deeply impressed with the BlueSCSI docs, honestly.
 

Trash80toHP_Mini

NIGHT STALKER
So far, it's behaved: I'm a little worried about noise on the SCSI cable given that all the grounds but one have disappeared, but I probably shouldn't be.
Cable's fairly short, but could be shorter? So not sure crosstalk will be much of a problem for you at Plus SCSI transfer speeds.

Why is that green board used?

Maybe try using a 50 line IDC cable, splitting it, pruning back the alternating 25 ground lines aabout an inch or so while using the remainder of the 25 lines in the proper order on the DIP Clip side. Then you can use a vise to punch a standard 50 pin DIP connector down on the standard ribbon cable and plug that straight into your SCSI device?

If you don't want to do the punchdown, just savage an existing SCSI cable. All but the last 1"-1.5" terminating at the rear will have proper alternate grounding.

edit: if someone has a pic of the cable adapter harness uniserver used to make for the Portable, that'd be a lot better than my explanation. Methodology would be the same.
 

Stephen

Well-known member
For the first I used one of @Stephen's Cloney Clips.
I’m happy you were able to make progress - frankly, the Cloney Clip has many barriers to entry and failure points. With the 3D printing, pogo pins, and finicky cabling its an accomplishment. There’s a simpler iteration I’m exploring but it’s more “one time use”.
 

cheesestraws

Well-known member
I'm happy you were able to make progress - frankly, the Cloney Clip has many barriers to entry and failure points. With the 3D printing, pogo pins, and finicky cabling its an accomplishment

Ahh, well, the trick with the 3D printing for me was 'pay someone more competent but still local to do it for me', which worked a charm, funnily enough ;-). Pogo pins the right size were slightly tricky to find over here, but I've got what's probably a lifetime's supply now...

I think I've actually got a reasonably reliable technique for building them now, so I'm going to try to build up a few more to check my working, but it's looking good. The test rig helps a lot. I suspect it's one of those things that's always going to be a bit fiddly, because it's got mechanical parts and needs to mechanically fit things, and is relatively small.

Thankyou for making it. I think it opens up a lot of opportunities. My next plan for one is to see if I can use it as a logic analyser test clip...

Got linkage? I've seen one something like I understand yours to be somewhere, but don't remember where.

 

cheesestraws

Well-known member
Why is that green board used?

Convenience :). Yes, probably the "right" way to do it would be as you described, but as aforementioned, I hate working with ribbon cables, because I'm bad at physical reality and they flap around like fishes. And by using the adapter, I didn't have to worry about pinouts at all. I'm a big fan of using work that other people have already done and made available.

Honestly, yes, it's fine for noise, I think: I ran a load of disk benchmarks last night and nothing bad happened and speeds were consistent. I'm just being neurotic :).
 

Trash80toHP_Mini

NIGHT STALKER
AH! @Stephen nice work! While you're rethinking your project take a look at this one:

KillyKlip-and-Such-01.JPG

It's not for clipping onto DIP legs, but you get the idea.

My notion would be to extend the "pin spacers" to fit between the legs of any given DIP chip. Springiness of the metal would make for really good contact. Dunno how your pogo pins work, but finding a suitable metal strip to cut would be a lot better than scavenging and modifying random pins as in the link.

Your printed clip-cap would make it perfect for any DIP chip. I'd keep it two part, maybe with posts on the ends of the upper clip surface for easy removal? Dunno if there's room, but maybe make a customized PCB for this great Plus SCSI hack.

Hope this isn't too far out on tangent?

Discussing this would be perfect in the 3D Printed Objects thread. I've got a box with 250 AMP 88315-1 10 contact ribbon cable jumper pins I can post pic of over there that might be perfect for your project.
 
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Crutch

Well-known member
This is really great @cheesestraws ! A couple years ago I was really interested in getting internal SCSI on my Plus. (I got less interested when the excellent SCSI2SD 5.5 became available, which I now just clip onto the back of my Plus like a tiny backpack using a right-angle adapter, but this is still really cool.) I will have to try this eventually.

Same question on the power though …. Isn’t there a simple mod where you can add a diode to the Plus mobo and thereby get term power on the SCSI bus? (I have never actually tried this.) Wouldn’t that prevent needing to hack the power cable together?
 

cheesestraws

Well-known member
Sorry, I somehow totally missed all these posts!

Side question, is that the BlueSCSI? If so, can it be bus powered like the SCSI2SD units?

Yes, it's a BlueSCSI, using @Bolle's respin of the board design which I use because it has more standard mounting holes. Nothing would stop a stock BlueSCSI being used in this configuration, it's just what I have in stock here. It can be powered from the bus, but read on as to why it isn't :).

Portable and Mac Plus termination power is weird. The SCSI2SD has standard Molex, the BlueSCSI you’d need a Molex To Berg adapter to make up for it.

The Plus doesn't have a Berg connector in it either—power for the floppy comes down the ribbon cable. So you'd have to either sort out termination power or use an external power connector like I did.

Isn’t there a simple mod where you can add a diode to the Plus mobo and thereby get term power on the SCSI bus? (I have never actually tried this.) Wouldn’t that prevent needing to hack the power cable together?

Yes, there is, but the termination power goes straight to the external port through that diode and doesn't turn up on a pin of the SCSI chip, where I was tapping stuff off. If you want it internally you either have to add the diode then tap off power at the far end of it (which would have involved soldering a wire onto the board, which was against the rules for this mod :) ) or you have to add an extra diode. @360alaska's adapter board does actually have landing pads for power to provide termination power, but to use those would have either required board soldering (as said) or having a diode floating in mid-air (messy). I did draft out a modified version of that board that had the diode on it, but since I already had the crimps and connector housings in the house, it seemed simpler just to build the harness rather than wait for a modified circuit board to arrive.

But, as I said before, the "magic" here, inasmuch as there is any, is in @Stephen's and @360alaska's work; my contribution here is just a series of pragmatic decisions that have been shown to work together. But those decisions are far from the only way to do this, and is likely not to be the best way. I'm very open to being told there's a trick I have missed, and perhaps that modified board with a diode on it would have been neater.
 
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