Subject: HP 48SX hardware etc
Date: 16 Mar 90 03:29:39 GMT
Organization: Imperial College Computer Center, London, UK
Lines: 62

Some HP 48SX hardware details from one of the "independent reviewers"
might be of interest, and may even answer some questions. Part of
the information here may look out of date as I wrote it a few days
ago, but only found the time to post it now - sorry.  I have not
[ yet ;-) ] taken an HP 48 apart, there seem to be less than 10
here in the UK at the moment, but I have talked with people who
should know about the hardware.

First a word about the matrix bug.  I have not seen the printed bug
list, so I do not know what solution HP have suggested, but anyone who
simply HAS to invert matrices larger than 7x7 on a handheld can divide
the matrix into a unit matrix.  This is easily done - duplicate the
matrix, convert the copy into an identity matrix, swap levels 1 and 2,
and divide :
               <<  DUP  IDN  SWAP  /  >>  'INVMAT'  STO

Now the hardware.  I have been told that HP48 is held together with
screws at the corners, instead of the plastic posts in the HP28.
You would still have to remove the keyboard to get at these screws,
almost certainly damaging it, and the HP48 keyboard has the keys built
into it, as on the smaller HP calculators, so opening up the HP48 is
best left to people with access to spare keyboards.

Inside the HP48 there are supposed to be two display drivers and a chip
carrying the CPU.  The CPU is the now-standard Saturn, with a 4-bit bus
for communication OUTSIDE the CPU, but with wider data paths inside. The
hybrid chip carrying the CPU is a new version, called the Lewis chip.
Presumably it carries the ROM as well. As HP now have a policy of using
standard components where possible, the 32k RAM is most likely to be a
separate industry standard chip, as in the HP28S.  The self-test checks
the UART (chip to support I/O) separately, so this is most likely a
separate component too. A fair deal of information can be gleaned from
this diagnostic self-test, already described by Alonzo Gariepy and others.
BE WARNED: when you use ON and the [C] menu key to leave the diagnostic
mode this clears the stack and the graphic display too.

The Saturn CPU communicates with all memory (ROM and RAM) beyond the CPU
by means of special memory controllers.  These were built into memory units
for the HP-71B - and only chips with such memory controllers could be
used to expand the HP-28C memory.  The Epson cards used with the HP48SX
do not have such memory controllers built into them; general purpose
memory controllers are built into the HP48SX itself, connected to each
of the plug-in ports.  When a RAM or ROM card is plugged into either port,
the memory controller recognises the card and acts as an interface
between the CPU and the plug-in. HP say that the Epson RAM cards
need to satisfy special voltage conditions so not every Epson card will
work.  I think this is motivated by a genuine concern, not just a desire
to sell only HP-labelled RAM cards. Does anyone here have access to a
store full of Epson RAM cards?  Would you care to check what percentage
of them work on an HP 48SX?  ;-)

After carrying my HP 48SX around in a pocket for a few days I find that
the paint around the display can get scratched. As this was a review unit
it came without a case, but it was pointed out to me that an HP-41 case
(with the foam padding removed) fits the HP 48SX very well.  The only
problem is that the HP 48SX does not then fit in my pocket - pity!

Wlodek Mier-Jedrzejowicz,
Space Physics, Imperial College, London
BITNET : MIER @ SPVA.PH.IC.AC.UK
Disclaimer - neither my employer nor I claim anything, but the above
may prove to be of interest to someone.