How to configure a macintosh plus with an external HD 20 SCSI

[Roberto]

A few days ago they gave me an original Apple 20 external HD as in the picture. With the HD I also have the terminator and the scsi cable. In the absence of documentation, I don't know where the terminator (below or above?), and the connector, goes. Then there is a number near the connectors where it is currently set to 5. Can you tell me the exact configuration? If everything goes smoothly, once the computer starts, will it automatically recognize the HD? Thank you

 

[Crutch]

Top vs. bottom doesn’t matter.  Plug the SCSI cable into one, and the terminator into the other.

The SCSI ID shouldn’t matter but be advised the Mac Plus will always boot from the highest SCSI ID, if I recall correctly ... see for example https://macgui.com/usenet/?group=14&id=44109 .  It ignores the “Startup Device” control panel so if you end up connecting multiple SCSI devices, adjust the ID accordingly to give your desired boot disk the highest SCSI ID.  Otherwise it shouldn’t matter, but standard practice would otherwise be to set your HD to SCSI ID 0.

Mac Plusses with early-rev ROMs have trouble booting when a powered-off SCSI device is connected, and may get stuck at a black screen on restart with a SCSI2SD drive connected (and, I think, maybe a regular external SCSI HD, also).  Rev C ROMs won’t have this issue and are readily available.  If you switch on your HD before you power up the Plus, though, it should work fine on a cold boot even with earlier-rev ROMs.

 

[Roberto]

Thanks Crutch, I immediately tried the configuration as you told me but my Macintosh Plus (turning on the HD first and then the Macintosh), does not start, or rather, starts but then resets immediately after to continue so indefinitely . I have tried HD with a Macintosh SE and it works. You told me it should present problems if I turn on the HD after turning on the Macintosh Plus, but oddly enough it gives me problems if I turn on the HD first and then the computer. I tried to turn on the macintosh plus first and then the HD but I don't know how to make it read

 

[Crutch]

Can you boot this Plus from a floppy? What happens if you turn on the Mac Plus with the HD off. Then insert a floppy. Then while it’s booting from the floppy, turn on the HD? Do you see the HD appear in the Finder?

 

[Roberto]

I tried with both the System 3.2 and the System 3.3 following the procedure you described but unfortunately I don't see the HD in the finder.

maybe I need the floppy called HD 20 startup?

 

[Crutch]

No, you shouldn’t need that on a Plus, anyway yours isn’t an HD20 but rather a 20-SC. 
 

It really sounds like a termination issue. Is that a passthru terminator on your scsi cable? I just noticed that.  What happens if you remove it? 

 

[jessenator]

It really sounds like a termination issue. Is that a passthru terminator on your scsi cable? I just noticed that.  What happens if you remove it? 

that does look like a pass-through terminator. I bet if OP took that off and put either it or the other terminator on the CN-50 out on the drive it would work just peachy.

 

[Trash80toHP_Mini]

Offhand, I don't recall Seagate having been an OEM for Apple in that era? That's their ST-225N (for SCSI version) and there's no Apple logo to be seen on the top plate. Might it have been a replacement drive? If that might be the case, check the drive for SIP termination packs, a modder might have set it as end of chain only in that manner?

Did your SE boot from it and if so, with or without external termination? SCSI Voodoo would be the case of SCSI "working well enough" when configured out of spec, as in an incorrect termination config when it really shouldn't be "working well enough" or even at all.

 

[Roberto]

yes exactly Crutch, it is a terminator and from what I have read around it seems that this type of hard disk (20 SC) needs the terminator if there is no other device connected. Anyway I tried to disconnect it but equally my plus does not recognize it. I have to turn on the HD when the plus recognizes the boot floppy, otherwise it goes into a loop and continues to reset, but in any case on my desktop I don't see the HD icon.

I have not understood what you mean jessenator...

Trash80toHP_Mini, I'm sure it's the original disk because I disassembled it from an Apple case with still the seals. I was able to view the HD 20SC from an SE as a second HD (not as a primary) only once; now in any way I start it, my SE sees the HD 20SC as primary and tries to load the OS from it but crashes because the SO of the HD 20SC was installed with the previous Macintosh and I don't know what model was.

I have tried in all configurations (ID selector), with and without terminator.

 

[LaPorta]

Do you have another machine to try the disk on?

 

[Crutch]

Just to be clear, what jessenator and I are saying is, (1) remove the pass-thru terminator at the end of your cable, (2) attach the cable with no pass-thru terminator to one of the SCSI ports on your HD-20SC, and (3) attach just the endpoint terminator to the other SCSI port on your HD-20SC.

That way, you have just one terminator at the end of your SCSI chain.  You are correct you need a terminator, but you shouldn’t have two.

Is that what you tried?

 

[Roberto]

Yes exactly!  what I tried.  considers that HD works, because when I connect it to my Macintosh SE, it immediately starts with booting from the HD20sc.  unfortunately I can no longer dictate the hd20sc so that it starts but as a secondary.  always starts as primary.

note: the adapter you see between the SCSI cable and the HDD connector is necessary, otherwise I cannot insert it!

 

[Roberto]

Hi guys, I just wanted to tell you that I managed to fix the Apple HD 20 SC. Here's what happened:
the HD had a pre-installed System inside (I don't know which version and I don't know on which machine it performed the installation). It was therefore a bootable HD and (oddly), any selection of the SCSI-ID was not taken into account and made me boot always and only from the HD 20 SC. So I put a System 6.0.8 bootable from Floppy on the Macintosh SE to boot from floppy (the Macinotsh gives priority to the floppy to boot, then to external HD and lastly to internal HD. At least for that's me). So, started the System from Floppy I was able to see also the HD 20 SC, and from here I removed the installed system. Seeing no boot disk from the HD 20 SC, all my macintos manage to see it as secondary.
I mainly needed it as HD for my plus. So I connected it to my macintosh plus and started from floppy (System 6.0.8), from here I installed the 5 floppies on the HD 20 SC. The reason why my Plus didn't see the HD 20 SC was because you actually need to turn on the HD 20 SC first and then the Plus; my problem was that the plus was trying to boot the system from the HD 20 SC, and it was definitely not compatible, that's why it kept restarting.
Thank you very much for all for your interest

 

[LaPorta]

Great! Glad it worked. All SCSI peripherals should always be on before the actual computer is.