Laser 128 questions (was Re: hyperc, etc.)

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <3f4vtc$k8j@nexus.uiowa.edu>,
Larry Kingery <kingery@xerxes.cs.uiowa.edu> wrote:
> 
> A couple of Laser 128/ex2 questions:
> 1) Has anyone hooked a hard drive to one of these things?  Any special 
> requirements?

The Laser has a slot, doesn't it?  You should be able to put a SCSI
card in there and use a SCSI hard drive.  You should also be able to
connect one of the SmartPort hard drives (from Sequential Systems) to
the disk port, assuming the Laser has a full implementation of
SmartPort.

> 2) What about a 3.5" external?  Can I plug my 5.25" external
> non-daisy-chain floppy into the final position in the chain?

I'm not familiar with details of the Laser's operation, but you can
certainly do this on the Apple IIc.  The 5.25" drive MUST be connected
after the 3.5" drive(s).

> Also, if anyone reading can shoot me a synopsis of the interrupt
> handling scheme employed here, I'd appreciate it.  I understand
> daisy-chaining as a method of prioritizing input interrupts, but the
> concept of differentiating individual drives on output has got me.

Eh?  Disk drives do not use interrupts in any way.  The IIc has an
"external interrupt" signal on the drive port, but it has never been
used, as far as I know (and certainly not for floppy disk drives).

The daisy-chaining scheme works in different ways, depending on which
sort of drive you are talking about.

If there are no 3.5" drives involved, and you have an external 5.25"
drive connected (for a total of two drives), then all signals are sent
by the computer to both drives, but there are two separate select
lines: one for the internal drive and one for the external drive.
Only one of these select lines will be active, so the deselected drive
ignores all of the other signals.

On a IIgs or IIc+, you can have two external 5.25" drives.  In this
case, both select lines are sent to the first drive, which looks at
the "drive 1" select line, and swaps the signals over in its
daisy-chain connector.  The second drive then sees what it thinks is
the "drive 1" select line, but is actually the "drive 2" select line.

With a UniDisk 3.5 drive in the chain, there are a few differences.
The UniDisk is an intelligent SmartPort device.  It uses a
communications protocol (SmartPort) to exchange packets of data with
the computer.  Every command sent by the computer has an address,
which specifies which drive is to be selected.  The UniDisk 3.5 drives
are assigned numbers starting with 1 for the drive closest to the
computer.  (The assignment method involves disconnecting the
daisy-chain signals at the back of the drive until the drive's address
has been assigned by the computer.  The first drive has its address
assigned, then it ignores further identification commands, and the
second drive has its address assigned, etc.)

Once the addresses are established, all UniDisk 3.5s will receive data
sent by the computer, but only the addressed drive will act on it.

If a 5.25" drive is connected to the back of the UniDisk 3.5s, it
ignores the communications to and from the 3.5" drives.  If the
computer wants to control the 5.25" drive, it uses the same method as
it would if the 3.5" drives weren't there, and they pass through all of
the signals without acting on them.

> Finally, I need more drams.  I know that I need <=120ns.  Does it do
> me any good to go any faster? 

No.  Faster RAMs will cost more, but provide no benefits.

I'm surprised you need 120 ns.  Even the IIgs (which accesses memory
at 2.8 MHz) only needs 150 ns RAM (120 is usually used).  The Apple
IIc and IIe can get away with 250 ns RAM.

In fact, they might be able to get away with 350 ns - many II+'s I've
seen use 350 ns RAM chips.

The RAM has to be fast enough to handle two accesses each clock cycle,
i.e. one access every 500 ns (one by the video circuitry, one by the
CPU).  There is some setup time required, so the RAM has to be somewhat
faster than 500 ns.

Even in an accelerated Apple II, memory accesses still take place at 1
MHz (or 2.8 MHz in a IIgs).  The accelerator caches memory contents so
that it can avoid re-reading memory if a location is accessed again.

(There is one exception: the TransWarp IIe has 256k of fast RAM on the
card, so it runs at high speed except while accessing I/O locations or
multi-bank auxiliary RAM cards.)

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand