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First post, by aries-mu

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Hey guys,

I did a lot of IDE stuff back in the days, but never put my hands on anything SCSI. For example, something I noticed is people talking about "terminators" (not the robot 🤣), which I had no idea could have been needed.

Is there such a thing like a PDF guide on SCSI, not too many frills, just the basics, but enough to go from concept to a running machine.

Thanks so much!

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Jesus said to them: The beginning, who also speak unto you

Reply 1 of 9, by stamasd

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Here's a quick primer. Depending on the type of bus, you can have either 8 or 16 devices on the bus. The controller itself is a device, so you're left with another 7 or 15 available. Each device has a SCSI ID, usually set by jumpers (the controller's ID can often be set in its own BIOS). There musn't be 2 devices with the same ID. The controller often takes the highest ID (either 7 or 15 depending on the bus). Boot disks will often have ID 0; the CDROM is often ID 6. IDs go from 0 to 7, or from 0 to 15.

Each bus must be terminated at both ends. The terminators are essentially dampeners of signals that get past all devices, and absorb them so they don't get reflected back on the bus. The simplest are passive terminators which are resistor packs that shunt each signal line to the ground. They aren't suitable for high speed buses - practically over 5MB/s you will need to use active terminators instead. Many devices, including controllers, have terminators built-in which can be switched on or off. Usually the controller is at the end of the bus so it will need to have termination turned on in that case (there are configurations where the controller sits in the middle of the bus, and then it must not be terminated). And the device at the other end of the bus must either have its terminator on as well, or else you have to install a dedicated terminator after it on the bus.

That's it in a nutshell.

Some crude diagrams:

Controller (terminated) -----(bus)-----Device1-------Device2-------Device3---Terminator

or

Terminator---Device1-----Device2----Controller (not terminated)-----Device3-----Device4--Terminator

Each device including the controller must have a separate ID, but they don't need to be placed on the bus in any particular order of IDs. There is no equivalent of the "cable select" as for IDE.

Last edited by stamasd on 2018-08-21, 13:37. Edited 2 times in total.

I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O

Reply 2 of 9, by aries-mu

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stamasd wrote:
Here's a quick primer. Depending on the type of bus, you can have either 8 or 16 devices on the bus. The controller itself is a […]
Show full quote

Here's a quick primer. Depending on the type of bus, you can have either 8 or 16 devices on the bus. The controller itself is a device, so you're left with another 7 or 15 available. Each device has a SCSI ID, usually set by jumpers (the controller's ID can often be set in its own BIOS). There musn't be 2 devices with the same ID. The controller often takes the highest ID (either 7 or 15 depending on the bus). Boot disks will have ID 0; the CDROM is often ID 6.

Each bus must be terminated at both ends. The terminators are essentially dampeners of signals that get past all devices, and absorb them so they don't get reflected back on the bus. The simplest are passive terminators which are resistor packs that shunt each signal line to the ground. They aren't suitable for high speed buses - practically over 5MB/s you will need to use active terminators instead. Many devices, including controllers, have terminators built-in which can be switched on or off. Usually the controller is at the end of the bus so it will need to have termination turned on in that case (there are configurations where the controller sits in the middle of the bus, and then it must not be terminated). And the device at the other end of the bus must either have its terminator on as well, or else you have to install a dedicated terminator after it on the bus.

That's it in a nutshell.

Some crude diagrams:

Controller (terminated) -----(bus)-----Device1-------Device2-------Device3---Terminator

or

Terminator---Device1-----Device2----Controller (not terminated)-----Device3-----Device4--Terminator

Each device including the controller must have a separate ID, but they don't need to be placed on the bus in any particular order of IDs. There is no equivalent of the "cable select" as for IDE.

Wow, thank you so much bro!

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you

Reply 3 of 9, by Roman78

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And one other thing about SCSI is, you can use other devices. On IDE or SATA you can use Harddisks or CD/DVD, and on SATA only one device per Cable, on IDE two devices per cable. On SCSI, as told, up to 15 Devices per cable. And beside Hard disks and CD/DVD, also stuff as Tape drives and Scanners, either internal and external.

Reply 4 of 9, by aries-mu

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Roman78 wrote:

And one other thing about SCSI is, you can use other devices. On IDE or SATA you can use Harddisks or CD/DVD, and on SATA only one device per Cable, on IDE two devices per cable. On SCSI, as told, up to 15 Devices per cable. And beside Hard disks and CD/DVD, also stuff as Tape drives and Scanners, either internal and external.

Thanks!

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you

Reply 5 of 9, by PC Hoarder Patrol

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There are quite a few guides out there, targeted at various audiences / levels but this one at scsifaq.org covers most of the basics (although a bit dated now and with some dead links)

http://www.scsifaq.org/scsifaq.html

Reply 6 of 9, by Zup

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A few things to remember...

- Terminators must be set on BOTH sides of the bus (before the controller and after the last device). It doesnt' matter if it is an internal or external bus.
- Keep on mind that some controllers/devices have may use terminators (i.e.: a controller usually have its own terminator to terminate its side of the bus, a disk may be configured to be a terminator). Those internal terminators can be enabled with jumpers.
- In some controllers, the terminators are resistor packs. Removing them means you have an un-terminated bus.
- There are two kinds of SCSI buses: Single-Ended and Differential. All devices on the same bus must be set to operate on the same kind of signals.

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Reply 7 of 9, by Jo22

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On SCSI, devices are numbered by their LUN numbers, often starting with LUN0.
https://en.wikipedia.org/wiki/Logical_unit_number

SCSI was also pronounced "SKUZ-ee" / "scuzzy".
ATAPI was based on the SCSI command set.
SCSI was derived from SASI. Serial SCSI is called "SAS" and can optionally work with SATA devices.
https://en.wikipedia.org/wiki/Serial_Attached_SCSI

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Reply 8 of 9, by stamasd

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I should have mentioned in my SCSI primer above that you won't be able to boot from SCSI devices unless the adapter you use has its own BIOS extension. The main PC BIOS doesn't know how to boot from SCSI devices. There are some low-end SCSI cards, mainly meant to be used with external devices such as scanners, printers etc. which don't have their own BIOS and aren't bootable. You can attach disk drives/cdrom units to those and they will be seen by the PC but they won't be bootable; they will also be very slow.

I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O

Reply 9 of 9, by aries-mu

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PC Hoarder Patrol wrote:

There are quite a few guides out there, targeted at various audiences / levels but this one at scsifaq.org covers most of the basics (although a bit dated now and with some dead links)

http://www.scsifaq.org/scsifaq.html

Great, thanks!

Zup wrote:
A few things to remember... […]
Show full quote

A few things to remember...

- Terminators must be set on BOTH sides of the bus (before the controller and after the last device). It doesnt' matter if it is an internal or external bus.
- Keep on mind that some controllers/devices have may use terminators (i.e.: a controller usually have its own terminator to terminate its side of the bus, a disk may be configured to be a terminator). Those internal terminators can be enabled with jumpers.
- In some controllers, the terminators are resistor packs. Removing them means you have an un-terminated bus.
- There are two kinds of SCSI buses: Single-Ended and Differential. All devices on the same bus must be set to operate on the same kind of signals.

Oh thanks for letting me know!

Jo22 wrote:
On SCSI, devices are numbered by their LUN numbers, often starting with LUN0. https://en.wikipedia.org/wiki/Logical_unit_number […]
Show full quote

On SCSI, devices are numbered by their LUN numbers, often starting with LUN0.
https://en.wikipedia.org/wiki/Logical_unit_number

SCSI was also pronounced "SKUZ-ee" / "scuzzy".
ATAPI was based on the SCSI command set.
SCSI was derived from SASI. Serial SCSI is called "SAS" and can optionally work with SATA devices.
https://en.wikipedia.org/wiki/Serial_Attached_SCSI

Thanks so much!

stamasd wrote:

I should have mentioned in my SCSI primer above that you won't be able to boot from SCSI devices unless the adapter you use has its own BIOS extension. The main PC BIOS doesn't know how to boot from SCSI devices. There are some low-end SCSI cards, mainly meant to be used with external devices such as scanners, printers etc. which don't have their own BIOS and aren't bootable. You can attach disk drives/cdrom units to those and they will be seen by the PC but they won't be bootable; they will also be very slow.

Very good to know, thanks!

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you