Re: tlug: Some hardware questions...

[Karl-Max Wagner <karlmax@example.com>]

> ... from someone who knows very little about hardware. Lots of
> questions, please bear with me.

OK. But some of the questions will require a somewhat complex
answer. So it might be that I supposed you to have some
knowledge you actually don't and that some stuff consequently is
over your head :-(. Simply ask then.....

> SCSI
> ----
> 
> What is SCSI exactly and how does it work?

SCSI = Small Computer System Interface. Much redesigned
offspring of SASI ( Shugart Associates System Interface ).

SCSI is a peripheral bus system. It allows the connection of at
most 7 peripheral devices. This is due to the rather primitive
addressing scheme which provides a dedicated address line for
each device. A device is contacted in the following way: The
controller raises line 7 and sends a request to one of the
peripherals. This peripheral acknowledges by raising its line.
By doing so it sends a reply ( with the requested data ) to the
controller. The controller itself may also send back some data.
When the communication is ended both lines are dropped.

The SCSI standard specifies a number of commands and signals 
( in total about 30 of them ). The original SCSI had a bus width
of 16 bits and a top speed of 10 MByte / s.

SCSI II had a few improvements and a higher speed ( 20 MByte / s ).

> Why is it considered better than IDE?

To answer that question we first have to answer the question
what IDE actually is. IDE means Integrated Drive Electronics.
Actually it is the same as the old ST506 standard, just that the
HDD controller is integrated into the disk. The whole drive
behaves like an ordinary peripheral device, e.g. a 16550 serial
controller. The only thing you need is an address decoder to
connect it to a stadard ISA bus. Ever wondered what you find on
those "IDE adapter cards" ? Normally a few latches ( '373 or
brethren ) and one or two GAL's for address decoding. The bigger
chip on there mostly is a 37C65 or similar floppy controller ( with
another GAL for its address decoding ). If you want, you can
easily roll your own. It is pretty simple.

The IDE bus has a maximum clock frequency of 6 MHz or so and is
16 Bit wide. This translates into a maximum data transfer rate
of 12 MBytes / s. Pretty close to SCSI II.

One IDE adapter can drive at most two IDE drives. There is the
peculiarity that they populate the same address space - the
selection is done when establishing contact by simply sending a
command telling the drives which one is going to be active -
Master or Slave.

There are two address spaces for IDE drives, thus allowing a
total of two IDE adapters with four devices in total. 

The problem with IDE is that it is a somewhat haphazard standard
with many things not being clearly specified. Theoretically
there is no reason why SCSI should be better - in fact SCSI adds
a lot of complexity because it translates from one bus system to
another. However, many SCSI controllers have a lot of advanced
functionality built in which the IDE manufacturers then scramble
up to implement, too. Actually there are real IDE adapter
controllers now that implement a lot of the functionality good
SCSI adapters have. However, this is in its infantility still.

With IDE you just can hook up at most 4 devices, with SCSI its
seven. As we have seen in the above, IDE is always playing
catchup with SCSI. So, if you want the most advanced features,
SCSI is the way to go. If that is not so important for you, IDE
is good enough. In the end you will end up with a mixed IDE /
SCSI system because some devices only exist in SCSI ( DAT
streamers, scanners etc. ).

BTW, some SCSI devices implement more than one functionality
( in other words, they are combi devices ) and to tell them
apart, there are the LUNs ( Logical Unit Numbers ). There are at
most 8 LUNs.

> What's the difference between SCSI II, Wide and UltraWide?

One beautiful day they found out that 16 data lines aren't
enough and beefed that up to 32 lines and called the result Wide
SCSI. AFAIK UltraWide is the next step and has 64 lines.
Beautiful side effect: they are - obviously - mutually
incompatible. They forgot to implement a variable bus width.
Megabummer.

> If you can point me to somewhere where this is covered in detail, please
> do.

The official ANSI SCSI specs cover that in more detail than you
probably like......

> Zip Drive
> ---------
> 
> I have an Ultrawide-SCSI Adaptec card. I almost bought an Iomega Zip
> Drive today, before noticing it said it was SCSI II.

Good thinking, yeah !

> Do I need some kind of adpater to slot it in?

You nedd an additional SCSI II card.

> Does a zip drive use an internal or external power source?

Depends. Those designed for building it into your computer take
the usual power plug also used for HDD's and the like. The
external ones come with a wall transformer providing the
required 5 V, 1 A.

> Are drivers necessary? If so, is there one with Redhat Linux 5.1? If

No.

> not, how can a Zip drive work without a driver (or even a hard drive,
> for that matter)?

The ZIP drive behaves like an ordinary SCSI HDD. You treat it
exactly like one.

> Modems
> ------
> 
> I want to buy a 64K modem, but the RedHat compatibility list is not very

Analog or ISDN ? That is the big question.....

> helpful, mainly listing those that don't work, and things like "modems
> that require software drivers for compression, error detection etc",
> "PCI memory mapped modems" are not supported. I have no idea whether a

Oh yes, those infamous Winmodems that abuse your machine for
signal processing. Avoid them like the plague !!!! They will
NEVER been supported under Linux - for obvious reasons.

> given modem suffers from these problems by looking at the box.

When selecting an analog modem, take an external one. Internal
ones have a tendency of being plagued by all sorts of problems -
one of the most common cheesy 16550 emulation.

> I saw an Aiwa CHP 9500 today (or something like that) that looked OK. Is
> this OK? Can anyone tell me any modems that positively work under Linux? 

Practically anything that accepts the standard Hayes AT
protocol. Look into the manual for more info.

About modem manuals: The printed ones don't contain a lot of
info. However, there come diskettes with them things. They do
not only contain drivers, but most often also text files with
the detailed technical manual. So have a look at them before
complaining about lack of a technical manual. Most no name
Taiwan made modems are quite satisfactory ( actually, those
things running under names like Ratoc, Aiwa etc. are actually
Taiwan made ).

If possible, take a modem savvy person with you when shopping.
Don't forget the used ones at different shops like Sofmap.
Sometimes they give them cheaply away because there is no Win 95
driver for them.....

Hopethat helps.

================================================================
"It was hell. They knew it.          Karl-Max Wagner
  But they called it                 karlmax@example.com
    W-I-N-D-O-Z-E"
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