This is written for and to anyone who happens to have
an Epson Px-8 (Geneva) portable computer and who has not yet
been able to make it useful. Part of your problem may be
that you are trying to use software that comes with the
machine, e.g. "portable Wordstar" or that you are trying to
read software in from tape each time prior to execution.
In either case, one would recommend that you burn
useful software, available for free on this and other
bulletin boards, onto rom chips.
The rom chip type to use is the 27c256, made by several
manufacturers. Eli Heffron / Solid State Sales -- Hampshire
Street in Cambridge. Or order from the West Coast
discounters, JDR Microdevices, etc. Try to get 200
nanosecond chips if you can. I do not know how fast the
chips in fact have to be to work satisfactorily with the
machine. I do know that 200 ns works great. 300 probably
works as well. Beyond that I don't know. The "c" (for
Cmos) is very important. These draw less power than the
normal Eprom. You will get just under 32k of files on a
27c256 chip, the precise amount depends upon the number of
files and how you can fit them together. (The rom slot in
the expansion "wedge" will take a 27512 as well as a 27256.
I believe, as well, that two 27c256 chips can be combined in
the two carriers for a not quite 64k disk.)
You may have to erase the chips. Done using
ultraviolet light. Borrow an eraser. Or build one: You
will need a small germicidal UV bulb (G8T5 is plenty big),
which you can get from one of the big electric supply
houses, (Standard, Mass Gas & Electric, etc.) for about $20,
and a fixture with a starter and ballast, $11 at Grainger's
now in Brighton, or $20 as an "under counter fixture" at one
of the electrical supply. Wire and a darkroom timer and you
are there. Ten minutes will do most roms. Fifteen is sure.
Do not look at the light! Best: put the light in a box that
closes.
You will need to borrow or build a rom burner, or get a
nice person to do it for you. (If you ship me the software
and rom chip I will be glad to.) Build: $95 from B&C
Microsystems in the back of BYTE mag.
The only "trick" to the job is in structuring the
directory so that the PX-8 operating system understands it.
FIRST IMPORTANT PRINCIPLE: The high order address line
is inverted to the rom chip. Thus location 4000 hex is
location 0000 hex, and vice versa. The directory, which
begins at the beginning of the chip, is thus at what any
normal person would call location 4000 hex.
SECOND IMPORTANT PRINCIPLE: The 'files' begin
immediately after the directory. If the last byte of the
directory is at 404F hex, then the first byte of the first
file is at 4050 hex.
If you have those two - everything else follows.
Some PX-8 rom directories from Epson produced roms are
listed below:
Some principles and features:
1. If we don't know what to do with a particular byte
we leave it unprogrammed, i.e. at FF.
2. Number of directory entries is at 4016.
3. Number of 128 byte records in the file is at byte F
of the first line of the directory entry.
4. At byte 10 and following of the directory entry is a
list of the blocks occupied by the file. Blocks are eight
records long, i.e. 1k or 1024 bytes. Blocks are numbered
with 01 beginning immediately after the directory.
5. Every succeeding block begins at that same boundary.
If block 01 goes from 40C0 to 44BF, block 02 goes from 44C0
to 48BF, etc.
6. There can be a final block which is small in size.
It will be a maximum of 1k minus the size of the directory.
It still has to end on an even record boundary. See the
file VD.COM above.
7. If you need more than the 16 blocks you can list on
the second line of an entry, an extension entry may be
created by placing a 01 in byte C of the first line of
entry. See the WS.COM example above.
Finally, you may be wondering whether you need the
plastic chip carriers that the roms you got from Epson were
mounted in. Those are made by Molex and others and can be
bought, with some difficulty, from JDR etc. However, you
can, as an alternative, cut a piece of 1/8th inch plexigalss
the exact width of the rom chip, bend the rom leads in a
rectangular fashion around it, squeeze the chip into the
carrier socket so that the leads line up, and it will be
there through even a small earthquake. Not quite as easy to
change chips, although not that hard, and you will have to
distinguish one end from the other -- but a workable answer.
Good luck.
