SCSI for Dummies Thread - How to properly use SCSI

[chelseayr]

sorry @ArmorAlley was rereading now and noticed your post, i'll just note that although not many 'getting started' people would run into them i'll nevertheless suggest that if someone new to buying into a mac hobby sees a scsi drive that has one big single rear interface (eg https://disktransfer.co.uk/IMAGES-fs/scsi-sca-80-pin-hdd-for-data-transfer.jpg ), do not buy it .. what you saw basically is 'sca 80' or a form of more modern lvd/hvd ultrascsi
[I only wanted to mention this because apparently a sca hdd can be had in very low capacity so it would be too easy to think that a 4gb drive you found in a local ad would go with your new first vintage mac but..umm..yeah..]

 

[Realitystorm]

Coincidental timing, I started working on a SCSI basics post a few days ago for my site. I've noticed more and more new members on the FB classic mac groups who don't know how to setup a SCSI chain. Good job here, I knew about the FX and Macintosh Plus weirdness, but some of the other information here was new to me.

 

[Phipli]

Should probably mention that a lot of SCSI CD drives either don't work any more, or struggle to read CDRs. Because if you didn't know, you'd think there was something wrong with the setup.

 

[MrFahrenheit]

The post was intended to be a “this covers 99% of basic situations” not an in-depth and comprehensive guide on all possibilities and exceptions. For that, you’ll have to send $19.99 for my book

In any case, if y’all want to discuss all of the different complexities that’s fine, but hopefully someone can still find page 1 to read the important basic info.

In the age of Twitter and 140 characters, most people aren’t even reading more than 5 sentences without “being tired out”.

Perhaps what the community should do is work on a thread where a PDF book is ultimately created that educates and informs the new user to all of the intricate details required to use and maintain a vintage Mac.

Including bridge machines, how to download software, repair, recapping, the SCSI details mentioned here (plus more, I’m sure), virtual SCSI devices (BlueSCSI/ZuluSCSI, etc) usage, etc.

If everyone wants to contribute someone could write sections and have it made into a PDF for download. Then when problems come up people could point the user to the book, and say “click here, read page 52, second paragraph”.

I was simply trying to inform people about how SCSI works “for dummies”. People in 2023 often think SCSI is like USB, just plug a device in. Or hot plug it. And then they have trouble. Or people don’t understand the ID situation or where you place termination (or don’t know that you need a terminator at all). Those were the people this post was targeting.

Not the guy who owns 34 Macs including a WGS 9150 with 4 SCSI cards. Likely they already know.

 

[Skate323k137]

In regards to BlueSCSI, ZuluSCSI, SCSI2SD, and other such devices, pay close attention to power requirements of these devices. Some draw their power from the SCSI bus itself, and that’s fine, provided they are by themselves in the bus. If you add more than one or have multiple different kinds, you will have to externally power these devices to avoid problems.

IMO this is some of the most important advice here from a stability and power supply standpoint. Some people assume power draw is always less with these disk replacements and to some extent that's true, however, drive motors sometimes operated on 12v or something not driving the logic ICs. You can actually end up needing a fair amount of 5V instead.

Anything you install internally you should just get a power connector for it as well. Give it power and a ribbon cable just like a normal HDD. Even if it works on termination power seemingly fine, a couple bucks for a cable for a nice power path from the main PSU to the device is more than worth it. This will ensure stable power for the device, and less troubles if you happen to also use an external DB25 BlueSCSI or something at some point that needs termination power (sans USB power, which isn't a bad idea TBH).

 

[ArmorAlley]

sorry @ArmorAlley was rereading now and noticed your post, i'll just note that although not many 'getting started' people would run into them...

I was thinking of myself when I got back into macs in 2009. I had some knowledge but the more modern stuff alluded me. This was as much a message to me then as it is to people new to the hobby. SCA-drives are admittedly rarer and pricier than back in 2009.

 

[bigmessowires]

Should probably mention that a lot of SCSI CD drives either don't work any more, or struggle to read CDRs.

What's the failure point in these old SCSI CD drives?

Is auto-termination actually a specific function that's implemented somewhere, or just what happens normally from an electrical standpoint when nothing is connected? Termination is something that's needed when you're driving a signal on a transmission line - it's a resistor at the end of the line that absorbs signals, preventing signals from reflecting at the end of the line and coming back a short time later to interfere with new signals at the source. But if there are no SCSI cables connected, then there's no transmission line, or only a very short transmission line from the SCSI chip to the connector on the motherboard, so no termination is needed in that case?

 

[Phipli]

What's the failure point in these old SCSI CD drives?

I think the diodes get weak.

Is auto-termination actually a specific function that's implemented somewhere, or just what happens normally from an electrical standpoint when nothing is connected?

Apple brag about in the C610 dev note, so I think it was a swish new active thing. there is more likely to be an internal cable with no device connected than external. Perhaps that is why? Regardless, I know some machines don't like having an unterminated internal bus. The computer can even detect it.

 

[sstaylor]

It seems like at least some models of the IIfx wanted an internal terminator if there was no internal drive attached. Can't think of any others offhand.

 

[MrFahrenheit]

It seems like at least some models of the IIfx wanted an internal terminator if there was no internal drive attached. Can't think of any others offhand.

The original LC also. I have an LC that shipped without an internal hard disk, and it had an Apple terminator PCB plugged into the SCSI Port.

My experience with the original LC is that if you completely remove the internal drive and cable, external SCSI becomes unusable or unreliable unless you insert a terminator into the internal bus (or terminated drive, working or not, same thing).

 

[aladds]

Perhaps what the community should do is work on a thread where a PDF book is ultimately created that educates and informs the new user to all of the intricate details required to use and maintain a vintage Mac.

I think a complete range of "Outside Macintosh" PDF books could be put together covering various topics and would be an excellent resource for everyone. There's a lot of knowledge on this forum and now's a really good time to get it recorded somewhere that isn't only accessible via forum search!

 

[trag]

@MrFahrenheit didn't at least one certain 68k system have two scsi buses that otherwise with the "incorrect" use of the lower version of supported system 7 would be seen as a single bus instead causing problem if one drive on each bus had the same id? (I think it was 7.6 as minimum that correctly saw both buses as separate ones, or was that around the 7.5 era, I'm not sure hence just simply saying it was lower version for sake of this question)

That was the Q950, not the PM9500/9600.

The Q950 pre-dated SCSI Manager 4.3, which supported multiple unique SCSI busses on the same Macintosh.

Later Macs, such as the PM9150 have SCSI manager built into the ROM, but the Q950 did not and was the first Mac with two SCSI controller chips.

After boot up, once SCSI Manager 4.3 loads as an extension (?) then the two busses will be seen as distinct. But this can still cause SCSI ID conflicts at boot time -- probably mainly if there are devices on both of the busses which share the boot device's SCSI ID.

With all the modification of ROMs going on now days, it would be interesting to create a Q950 ROM with SCSI Manager 4.3 built in.

The JackHammer card, BTW, loads a version of SCSI Manager 4.3 from its boot ROM, so I think if you have a JackHammer installed in the Q950 that may avoid the issue.

 

[trag]

The Adaptec 2940-series, 19160 (or 19160N) and 29160 are popular cards and unless they are flashed with the Mac-ROM, only cards with the APD-prefix (Apple PowerDomain) should be used.

Adaptec later created a number of cards with the "2940UW" identifier. Only the ones based on the Adaptec 7880 chip are convertible to Mac use.

There was a 2940UW 'Dual' or 'Duo' or some such which wasn't.

The 2940UW is pretty easy to convert because there's a set of DIP holes right on the board. Remove the PLCC flash, install a blank DIP flash of the right type, flash the card with the PowerDomain software and boom. IIRC.

I wrote an article about this conversion for Mike Breedan's XLR8yourmac.com back at the dawn of time. Mike has long since removed such old content.

Also, I was fairly new to electronics at the time, so there's a ton of probably useless soldering detail in the article.

BTW, Adaptec used to have some fantastically useful SCSI instruction documents on their FTP site. Which was taken down a couple of decades ago, but great stuff with really nice diagrams. Also, there was a very extensive and well researched SCSI FAQ on one or more of the news groups. Probably something like comp.sys.SCSI or maybe mumble.electronics.SCSI

It would be well worth while to find that old FAQ.

 

[trag]

There might be some useful information here: https://68kmla.org/bb/index.php?threads/scsi-voodoo-howto.25878/

And there's a link in that thread to the comp.periphs.scsi FAQ on SCSI that I mentioned above.

 

[GerhardNL]

Very helpful info - thank you so much!

 

[MrFahrenheit]

For anyone saying “SCSI is hard!”, I present to you my just hooked up SCSI chain, simply following “the rules”. Booted and works first try without issue.

 

[s_pupp]

You are the champion, my friend!

 

[-Micky]

The 2940UW is pretty easy to convert because there's a set of DIP holes right on the board. Remove the PLCC flash, install a blank DIP flash of the right type, flash the card with the PowerDomain software and boom. IIRC.

Not necessary. You can flash it with the flash utility from Adaptec.

Adaptec - Adaptec Driver: PowerDomain 2940UW firmware, version 4.1 - Binhex Download Detail

And voila you have a PowerDomain 2940UW.


Micky

 

[trag]

Not necessary. You can flash it with the flash utility from Adaptec.

Adaptec - Adaptec Driver: PowerDomain 2940UW firmware, version 4.1 - Binhex Download Detail

And voila you have a PowerDomain 2940UW.


Micky

Software and flash chip are larger on the PowerDomain than on the PC version of the card. The PC firmware won't fit.

It's been more than 20 years, so memory and all that. I remember there was a version that seemed to work in the smaller flash chip, and I can't remember if it actually did work, or if there were some underlying issues if one did that.

I can't decide whether to curse or praise Mike Breedan. On the one hand, he ran XLR8yourmac.com for years and provided great information, news, and useful compatibility database and forums. Then after everyone had poured their hard earned knowledge in, he took them down. But the details would be there, if the resources were still up...

 

[volvo242gt]

Software and flash chip are larger on the PowerDomain than on the PC version of the card. The PC firmware won't fit.

It's been more than 20 years, so memory and all that. I remember there was a version that seemed to work in the smaller flash chip, and I can't remember if it actually did work, or if there were some underlying issues if one did that.

I can't decide whether to curse or praise Mike Breedan. On the one hand, he ran XLR8yourmac.com for years and provided great information, news, and useful compatibility database and forums. Then after everyone had poured their hard earned knowledge in, he took them down. But the details would be there, if the resources were still up...

Found it, I think...

Accelerate Your Mac! - Converting a PC adaptec SCSI card to mac

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