Re: HD20

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

you <you@somehost.somedomain> wrote:

> >No one has cracked the lid and 
> >looked at one?
> I am sitting in my shop with an open HD20 in fromt of me.  Inside we find a
> Power Supply a fan, a disk, and a board.  The disk is a Rodime Model 552.
> Connector seems to be a 26 pin affair similar to a Apple II SCSI card
> connector A DB25 (DE? ;) SCSI  ribbon will fit here.

It may be using all 26 pins, which means you couldn't use a DB-25.  (And
yet, it is a "B" in this case. ;)

> which leads to a matching connector on the baord.  again a 26 pin connector.

Hm.  If that is the only connection to the drive mechanism (apart from
power) then I'm not sure what type of interface it is.

The original ST506 interface (as used in the ProFile, for example) uses
two ribbon cables.  One is 34 pins, and is basically the same as the
floppy drive interface used in the PC.  The other is about 20 pins, and
carries parallel data to/from the drive.

I haven't seen a SASI interface, but I expect it is 50 pins, like SCSI.

It could be a SASI or SCSI interface which eliminates most of the ground
lines (like the Apple II and Mac DB-25 SCSI connector), but I've never
heard of a drive mechanism which used this interface.

> the rest of the board is...:
> Silk Screened Apple Computer c 1985 with no Rev number
> socketed chips:  1st seems to be a Apple one  marked 620 VM 
> B414...2365-1239....342-0343-B... c 85 Apple   Korea  it is a 28 pin one

Sounds like a PROM or EPROM - the '342' prefix is one of those used by
Apple for PROMs.

> and it is twined by another 28 pin that is marked: 625v  
> c244....VP4060-0001....344-0041-B...c Apple 1982 1983

Probably an IWM.  I don't have anything handy which contains one (short
of dismantling a IIc) so I can't easily check the number.

Anyone else out there have ready access to a IIc or IIc+ motherboard, or
the personality card from a UniDisk 3.5, or the "Liron" 3.5" disk
controller card?

There is also a PLCC version on the IIgs motherboard, but it may have a
later part number.

> a longer 40 pin one (not socketed)  marked SR0103...341-0339-a...APple 85-
> 8622

Most likely to be a microprocessor (e.g. a 6502) with Apple labelling
for obfuscation.

> the next chip is a HM6116-3

That is a 2KB static RAM.

> and at the bottom (front of case) a socketed large square chip marked
> LIaII85...3440600-1 Apple 85.... TAE 8624 

How many pins?

This is probably an ASIC.  There is a very similarly numbered 68 pin
chip (344-0601) on the Apple II Memory Expansion Card.

> Also there is a 7.5MHz occilator

Is that a device which turns Chinese into Westerners?

Oh, you mean "oscillator".  :-)

That is an unusual frequency.  The original Mac models ran at 7.8366
MHz, and later ones (e.g. SE) were 8 MHz.

The IWM is supposed to be clocked at 8 MHz or 7 MHz, with a mode setting
to tell it which clock frequency is being used (so it uses a divide-by-7
or divide-by-8 as the basis of its timing).  The Apple II runs it in 7
MHz mode (from a 7.15909 MHz clock source, assuming an NTSC Apple II),
and the Mac runs it in 8 MHz mode.

A Mac running at 7.8 MHz produces bits which are 2.5% longer than an 8
MHz Mac.  An Apple II produces bits which are 2.2% shorter than an 8 MHz
Mac, and about 4.7% shorter than a 7.8 MHz Mac.  All of these
differences are manageable given resynchronization for each byte read
from the disk.

7.5 MHz is a rather more significant discrepancy from either 7 or 8 MHz.

-- 
David Empson
dempson@actrix.gen.nz