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adderthorn

macrumors member
Original poster
Nov 22, 2020
44
38
I'm having an issue connecting my SCSI HDD to my Mac Plus. Some time ago it used to work as a boot disk. Now, not only will it not boot, but I can't get it to be recognized by the mac plus. I've attempted to open Apple HD SC Setup and it reports that it was "Unable to locate a suitable drive connected to the SCSI port."

I know the drive is working because I've connected it to my PowerBook 1400 running Mac OS 8.6 and am able to mount it. Although, curiously, it will not mount automatically and I need to open Drive Setup to mount...maybe that means something? Any thoughts as to what I can try?
 

MacTech68

macrumors 68020
Mar 16, 2008
2,393
209
Australia, Perth
I assume you have a method of booting the plus, and would suggest using "SCSI Probe 4.3" control panel to see if it reports on the SCSI BUS. If it does, it's either driver on HD, or ROM. If SCSI Probe shows a device, but the name looks scrambled, then termination should be checked.

What SCSI ID is the drive set to ?


SCSIProbe v4.3 :
https://forums.macrumors.com/attachments/scsiprobe_4-3-sit-zip.475470/
 
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adderthorn

macrumors member
Original poster
Nov 22, 2020
44
38
Thanks for the replies! It seems the MacPlus is not seeing it as terminated based on SCSI probe? Odd since I am using a HDD box that has a terminator attached. The ID should be set to 0, based on the fact there are no jumpers installed. This is the model drive I have and this was the only documentation about it I could find:

 

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MacTech68

macrumors 68020
Mar 16, 2008
2,393
209
Australia, Perth
Here comes what was termed "SCSI Voodoo" - Turn off the Mac & external drive. Then REMOVE the external terminator and try again.

Yes, this MIGHT work.

LATE EDIT:

I have a feeling that this is another drive model that was modified by IBM for Apple and had the terminator enable jumper removed when manufactured. This would probably mean they have termination enabled at the factory with no easy means of disabling it.

So, if this drive is the "Apple Badged" variety, it's possible it is already terminated on the drive itself.

Slightly better diagrams from IBM:
It would appear from the best photos I can find that the "J" jumper set is not present on the Apple Badged drives.
 
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adderthorn

macrumors member
Original poster
Nov 22, 2020
44
38
Here comes what was termed "SCSI Voodoo" - Turn off the Mac & external drive. Then REMOVE the external terminator and try again.

Yes, this MIGHT work.

LATE EDIT:

I have a feeling that this is another drive model that was modified by IBM for Apple and had the terminator enable jumper removed when manufactured. This would probably mean they have termination enabled at the factory with no easy means of disabling it.

So, if this drive is the "Apple Badged" variety, it's possible it is already terminated on the drive itself.

Slightly better diagrams from IBM:
It would appear from the best photos I can find that the "J" jumper set is not present on the Apple Badged drives.
Thanks for the idea. While removing the terminator did not work, it is, alas, something I would not have thought of doing myself.

Thanks for the diagram too, that is much more helpful than the one I found. Indeed the drive is Apple badged (has both Apple and IBM logos on the labels). You are correct that there are no "J" jumpers, or if there are, they are covered by labels. The only jumpers are at the front of the drive and have 14 total pins (two rows of 7). I will see if I can find a jumper and put it across pins 1 & 2 and force the drive to address 0.
 

adderthorn

macrumors member
Original poster
Nov 22, 2020
44
38
Adding some more puzzling information. I played with the jumpers and still cannot get the Plus to recognize the drive. I installed the newer version of the SCSI probe on my PowerBook and checked and the drive shows up at ID 0 and profiler shows it as well. Could there be something wrong the Plus that it cannot see SCSI devices anymore?
 

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MacTech68

macrumors 68020
Mar 16, 2008
2,393
209
Australia, Perth
It certainly could be. It could also be the SCSI cable or the contacts of the cable/ports.

Without "known good" equipment, it makes diagnosis much more difficult. Contact cleaners can help, but are not guaranteed to fix all poor contacts. You could try using a multimeter to make certain you have good, low ohm continuity from the Mac Plus motherboard SCSI port to the pins on the hard drive.

There are very few components associated with the SCSI port, you can see these here:

820-0174-A Mac Plus Schem SCSI 5380 Detail crop.png


820-0174-A Mac Plus Layout SCSI 5380 Detail crop.jpg



You'll notice that the Diode CR1 is not present. If your board is the same, this means that termination power MUST be supplied by the drive itself. This is entirely possible and some drives do and also supply Termination Power back to the SCSI BUS. However, I can't tell if the IBM WDS-Lxx series does or not.

Passive termination requires 5 volts to "tie up to" via resistors, it also "ties down" to ground via resistors. So, without the +5v 'Term Power', it can't work.

As far as I'm aware, this model of drive was only ever fitted by Apple as an internal drive. This would have required termination. Since there is no easy way of selecting termination on this drive, it MUST be terminated. The question is, where did it get termination power (from BUS or from drive) ?

The only caution would be that these IBM drives seem to deploy a special IC for termination. I can find NO data as to whether these are passive or active devices, or even semiconductor ICs.

The only thing I could suggest is to measure the TERM Power pin on the socket you're plugging the terminator into and see if you get +5 volts.

Pin 38 on a centronics connector should be Term Power. On the back of your drive should be a female receptacle (lower image in diagram):

SCSI 50 Centronics.jpg


Next would be to determine what kind of terminator you have? Pic of the rear of your external drive case & terminator may assist.
 

adderthorn

macrumors member
Original poster
Nov 22, 2020
44
38
Thanks for the information! Sorry I've been late to reply, it's been busy for me. I've attached a lot of pictures of my drive and enclosure as well as SCSI probe and profiler from my newer machine running 8.6.

My MacPlus does not have the diode (my PCB looks as shown), although again, the drive used to work in this very Plus, so I'm not sure it's that. I'll try measuring the output for term power.
 

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MacTech68

macrumors 68020
Mar 16, 2008
2,393
209
Australia, Perth
OK, try these:

Check the fuse circled here - should measure zero ohms.

hdd_bottom crop fuse annot.jpg



And, move the existing jumper to the position arrowed below:

jumper_back crop annot.jpg


jumper_front crop annot.jpg
 
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