SCSI for Dummies Thread - How to properly use SCSI

[MrFahrenheit]

I feel that the very basics of SCSI need to be easily found and read in a simple format.

Posting this page so that it’s easily found, and people can more easily get a quick answer on how to correctly and properly use SCSI on the Macintosh.

Rules:

1. The SCSI bus on most Macs is shared, internal and external. It’s all one and the same. The “bus” is any part of the SCSI on the Mac, including the external cables, the port on the back of the Mac, and the internal port (if any) and internal cables. It all makes up the “bus”.
2. No two devices can have or share the same SCSI ID number.
3. Use SCSI ID numbers 0 through 6, in any physical order on the bus. It was general practice for the internal hard disk to be SCSI ID 0.
4. NEVER hot-plug or hot-unplug any SCSI device. They are not a hot-pluggable and carry voltage in the connector. Plugging or unplugging a powered-on device into either an unpowered or powered-on Mac could very likely cause the SCSI chip to blow.
5. SCSI is not plug and play. It’s more like plug and pray (it works without issue).
6. Termination. See next…

Termination:

A SCSI bus is like a water pipe. The SCSI cable carries water (data) between the well (device/disk) and the faucet (computer). Just like a hose or water pipe, it needs to be sealed to prevent leaks. The terminator seals the water (data signal) leaking out either end of the bus/cable. It prevents a data mess!

A terminator is either a block that connects to your SCSI connector, or is a SCSI device/drive that contains always-on termination.

1. Ensure you always have only a maximum of two total terminators on your SCSI bus, combined inside and out.
2. Always have at least one terminator or terminated device present if you have anything at all connected to your SCSI bus.
3. Never place more than one terminator or terminated device on either the internal or external part of your SCSI bus.
4. The terminator always goes only at the very end of your physical device chain. It is always the last thing, because it stops the leaks after it.
5. If you have no internal drive/device, and only external drive/device, you only install one total terminator, because in 90% of Macs the internal SCSI bus will auto-terminate if there is nothing connected there. If you want to be extra cautious, you could place a termination block on the inside of the machine either on a ribbon cable connected to the internal connector or directly on the connector, if you have that kind of terminator.
6. Some devices, such as a BlueSCSI and some models of ZuluSCSI, have an “always on” terminator. Be mindful of these devices and pay attention where you place them on your SCSI bus. These devices are generally meant to be alone and do not expect other devices to exist.

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.

A word of caution related to Iomega Zip drives. Many listings on sites such as eBay indicate the Zip drive is the SCSI version or SCSI-compatible. Most times they actually aren’t, and could ruin the SCSI chip in your Mac (or other computer), if connected. The telltale sign is the model number (SCSI is —Z100S— and the non-SCSI or dangerous one is —Z100P—). In fact, I see numerous listings even right now identifying a parallel port Zip drive as being SCSI.

The dangerous Zip drives also sport a port that shows what looks like a printer icon. The SCSI models show an icon that is the same as used on most Macs (tilted square with a horizontal line on the right). eBay sellers are very unwise in their listing practices and refuse to correct their listings.

 

[joshc]

This is great. Also worth a read is the 'SCSI Devices' chapter of the Macintosh Bible 4th edition, it covers these topics in more depth.

The Macintosh bible : thousands of basic and advanced tips, tricks, and shortcuts logically organized and fully indexed : Naiman, Arthur : Free Download, Borrow, and Streaming : Internet Archive

The Macintosh Bible 4th edition 1992

archive.org

 

[greystash]

Very interesting! Thanks for the explanation @MrFahrenheit, I have never really looked into termination before and this explains a few things I've come across. Very helpful!

 

[chelseayr]

@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)

 

[MrFahrenheit]

@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)

You’re referring to the Power Macintosh 8600/9600.

Hence why it’s likely just safer to describe it as almost always the same internally as externally and to safely assume you only have 7 unique IDs to work with.

Besides, who is seriously going to connect more than 7 SCSI devices to a single Macintosh in 2023? Besides myself, I mean. Lol



It’s hard to see, but there are 7 devices in the picture:

A Yamaha CDRW, an Apple Cd600, two 5.25” magneto optical drives, a 1.3GB Magneto optical drive, a 100MB Zip drive and a 1GB Jaz drive. All SCSI.

 

[chelseayr]

Ah I had somehow thought it was a late 68k system but mm I can see exactly why a 9600 would have that. Then again a 9600 is like "who could want to completely stuff that tower up? It would require mortgaging your silicon valley mansion to afford the final price" what with two scsi buses, six pci slots, and twelve high-capacity ram slots hm?? hehe!
(even then seriously tho: i'm just curious which system version couldn't recognize the two buses as separate ones?)

 

[MrFahrenheit]

Ah I had somehow thought it was a late 68k system but mm I can see exactly why a 9600 would have that. Then again a 9600 is like "who could want to completely stuff that tower up? It would require mortgaging your silicon valley mansion to afford the final price" what with two scsi buses, six pci slots, and twelve high-capacity ram slots hm?? hehe!
(even then seriously tho: i'm just curious which system version couldn't recognize the two buses as separate ones?)

Well, if you bought the high end model, it was about $6000 in 1997, give or take.

That’s $11,500 today. Mac Studio anyone?

As for what OS would see it with a single bus, I don’t know. I’ve never heard that one before.

 

[jmacz]

Doesn’t the external port also auto terminate if you have nothing connected to it? ie. if you don’t have anything connected to the external port, you don’t need to put a terminator there?

My mental model (correct me if I am wrong) was that both the internal connector and external port had auto termination if nothing is connected. But as soon as you connect something on the internal connector, you need a terminator at the end of the chain of internal scsi devices. Same on the external, as soon as you connect something on the external port, you need a terminator at the end of the chain of external scsi devices. No devices in the middle should have termination.

 

[Forrest]

There is also a pass-thru SCSI terminator, for those rare instances when the normal SCSI rules don’t seem to work.

 

[joshc]

The Workgroup Server 95 models include the "Workgroup Server PDS Card" which had two SCSI controllers.

 

[ArmorAlley]

I would add to this:

• There are various kinds of SCSI plugs but they essentially fall into 2 categories:
a. Single-Ended (SE) — the original kind and all built-in SCSI in Macintoshes uses this. Essentially it uses 25-pin or 50-pin plugs.
b. Low-Voltage Differential (LVD) — operates at low power at higher speeds. Drives with 68-pin or higher are almost always LVD-drives.
Many such drives also support Single-Ended mode and will have SE/LVD (or LVD/SE) written on the drive. This is important if you want to use a server hard disk from 2003 in your Performa 475. If it is SE/LVD, it will work with an adapter. If it is only LVD, it won't.

Warning 1: SAS is not SCSI from our perspective and SAS drives won't work in any Mac unless, maybe, a SATA card is present]
Warning 2: HVD-SCSI also exists and should be avoided unless you know what you are only using high-voltage differential drives with that card. HVD is rare. The ATTO SiliconExpress 4D is a HVD NuBus card.

• NuBus Macs can take SCSI cards although have become pricey off late. At least 2 of them allow LVD-SCSI (with the 68-pin connector) and these are the FWB JackHammer and the ATTO SiliconExpress IV.

• PCI Macs have a much range of cards available to them. 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. I've never used any of the 2940s but the 160 series cards are APD-19160 or APD29160N, APD-29160. There is a 39160 card but I'm not sure if there is an APD-39160 card.

• The PCI cards are useful in all of the PowerMacs from the Blue & White G3 onwards as they don't have SCSI built-in.

 

[Phipli]

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)

You’re referring to the Power Macintosh 8600/9600.

I think I remember the stock 900 and 950 technically had two busses combined into one in software, as did the 8100. They are effectively one bus in practice.

I'm not aware that this behaviour changed in later OS versions, but haven't specifically gone looking.

I think in terms of a beginner "SCSI getting started", there isn't any reason to muddy the water.

8500, 9500, 8600 and 9600 each have two independent busses. I'd... Ignore that and use weasel words like 'in almost all cases...'

 

[Bolle]

didn't at least one certain 68k system have two scsi buses

You’re referring to the Power Macintosh 8600/9600.

The Quadra 900 and 950 did indeed have two separate SCSI busses with each one having their own 53C96 controller.

 

[Phipli]

My mental model (correct me if I am wrong) was that both the internal connector and external port had auto termination if nothing is connected. But as soon as you connect something on the internal connector, you need a terminator at the end of the chain of internal scsi devices. Same on the external, as soon as you connect something on the external port, you need a terminator at the end of the chain of external scsi devices. No devices in the middle should have termination.

Auto internal termination with nothing connected was introduced with the C610 and wombats (mentioned in the Dev note). Terminated an empty external SCSI was never needed.

 

[volvo242gt]

The Workgroup Server 95 models include the "Workgroup Server PDS Card" which had two SCSI controllers.

Even the logic board has two SCSI controllers, as both @Phipli and @Bolle mention above. My non-WGS 95 Quadra 950 shows two SCSI busses in SCSIProbe under System 7.5...

 

[Bolle]

The Workgroup Server 95 models include the "Workgroup Server PDS Card" which had two SCSI controllers.

And the WGS95 card has two busses on it yet again, so you end up with a total of 4 SCSI busses with one of those in a Q900/950.

 

[cheesestraws]

May I suggest we do not derail this thread with specifics about specific machines and edge cases? @MrFahrenheit's original post is really useful precisely because it applies to basically everything—if you take one of the machines that technically have two buses, generally, if you treat it as per the instructions in the first post, everything will still work fine.

I love a good technicality, but I think we should try to keep this thread as a "let's not confuse people who are new to this" zone. You don't need to know whether you've got SCSI Manager 4.3 installed to successfully stick an external hard drive on the back of your Mac.

 

[chelseayr]

anyhow just to continue with the original topics i'll add these reples of mine;
1. although I know most people would never run into them, still - what of the 'black terminator' (as I heard it was called) specific to the IIfx?
2. the "bastardized" scsi on performa systems was still a full scsi-1 bus, but with the crippled grounds it didn't work well with longer cable distances and I think (was this an apple or unofficial cite?) only three peripherals maximum could be chained for that reason
3. generally the scsi host is terminated (whether this is onboard or card) but in some cases the terminator may be optional or not even there in the name of that the host is in the middle of instead of the end of a scsi chain, hence you have either host(t)-hdd1-hdd2-hdd3-hdd4(t) versus hdd1(t)-hdd2-host-hdd3-hdd4(t) setups .. its an interesting less common usecase tho
4. I know usage of terminator has already been mentioned but should the guide also explain active versus passive ones too?
5. I know at least one magazine's q&a page went into this (I'm quite certain it could had been in either macuser or macaddict issue) but ideally you should turn your scsi peripheral on first and make sure it has initialized itself to life before turning the system on but even then although this is really not advised, it is sometimes possible to turn the peripheral on after-the-fact and use a third party software to make the system scan for the bus again
6. scsi id has already been described but do we want to go into lun's or not? considering that although multi-lun drives were not very commonplace there is for example a 3-disc changer drive which may physically appear as one single scsi drive to the computer but to the user it shows up as three cd drives otherwise in a manner speaking

 

[chelseayr]

@cheesestraws just saw your post now and yeah sorry about a lot of quadra chat overnight

either way I looked up about the quadra 950 and indeed I did find exactly what I had thought was there in the name of this quote which seem to indeed suggest that you would need to run at least 7.5 if you wanted to utilize a lot of scsi devices on a single system for some reason;
Although there are two separate SCSI buses, System 7.0-7.1 "folds" them together so the operating system sees a single virtual SCSI bus. Thus, under System 7.0-7.1 (and only under those systems) you must make sure that all devices on both chains have unique IDs.

 

[joshc]

what of the 'black terminator' (as I heard it was called) specific to the IIfx?

This is covered elsewhere already.

Make your own Macintosh IIfx Black SCSI Terminator 590-0705-A

The Macintosh IIfx has a special black SCSI terminator compared to the standard passive terminators. Unfortunately, the black terminators list for upwards of $200 on eBay. Furthermore, there is confusion and myths surrounding whether they are necessary and how they actually differ electronically...

68kmla.org

SCSI Terminators: Determining Passive vs Active

Like most of you, I use a solid-state memory card to replace dead SCSI drives. When repairing or testing old Macs, I usually place the MacSD, ZuluSCSI, or SCSI2SD drive externally. Often, these old computers do not come with a working internal drive. Depending on the Mac model, I sometimes need...

68kmla.org

I agree with @cheesestraws, best to keep this thread as a general SCSI guide rather than trying to explain every single specific scenario/nuance.