My Nascom 2
My introduction to real computing was when I bought a Nascom 2 kit in 1979. I chose this model
partly because it came with a then massive 16k RAM when most single board computers came with only 1k.
I retired it around 1985 in favour of an Amstrad 6128. That was not actually very much more advanced than the
Nascom 2, except that it had a diskette drive on it. It was more cost-effective to buy this than upgrade the
Nascom to have disks and CP/M.
I have recently powered it up for the first time in around 15 years to see if still works and if I can recover any
of my software off tape. Surprisingly it does still work. Initially there was something not quite right because the
TV display broke up from time to time and the main board needed tapping to make it work again. After a while I found
a dry joint on the modulator; resoldering it appears to have cured it. A 6116 RAM chip failed after a few hours; thanks
to Richard Espley for a replacement.
I have had little success reading my old Nascom tapes using the original hardware. Most of these were at 4800Bd
in Biphase-M encoding,
recorded on an own-design tape deck pictured below. The custom-built decoder appears to have a problem, but despite
extensive prodding wih an oscilloscope I cannot locate the fault. The circuits I used were based on a Wireless World
article from February 1982 - a scanned copy is linked at the foot of this page.
To get around this I have written some software to decode tapes on a PC, and although in its early stages is
showing considerable promise. I have been
able to decode several tapes, including HisSoft Pascal 4, and the source code for my "Tower of Hannoi" program
that was published in one of the Nascom magazines in the early 1980s. In outline, the procedure is to record the
tape as a .wav file and chop it into separate tracks. This is done in
Audacity. My NasTape program
decodes the .wav file and writes the data to a .nas file, as if written by the NasSys T command.
It handles 300Bd Nascom, 1200Bd Nascom, 2400Bd Bi-M and 4800Bd Bi-M
encodings, and data in Nascom and Hisoft formats. It is written in Java so should work on Windows as well as Linux.
The quality of the original recodings varies considerably. Some hardly look like data at all when examined in
Audacity, and are probably irrecoverable, but others still look really good.
Pictures
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I took this picture in 1981. From the context of other pictures on the film I know it was on 29th October, a few hours
after my son James was born. The keyboard is as-now, but this picture predates making a box for the main and RAM boards.
You can see the boards in a home-made wooden card-frame, resting on an old paint tub under the table. |
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The same Nascom 2 today, now in a chipboard box. The PC to the left is what I am using to connect to the Nascom so that
I can retrieve and store programs from it. You can just see the top of PC on the floor - a K6 200MHz running
Mandrake 9.2 Linux with WindowMaker window manager, and using minicom as terminal emulator to the Nascom. |
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The Nascom in more detail. The tape deck on the top is my own design, and reliably recorded data at 4800 Bd. It is
Z80 controlled! I had intended to make it into an automated tape system with a directory structure at the start of the tape, but
by then (1985?) it became clear that it was more practical to upgrade to something else, in my case an Amstrad CPC6128.
The TV is almost as old as the Nascom - a 1981 Grundig. A nice set, from the days when B&W TVs were still designed
for quality rather than cheapness. The user manual came with a full circuit diagram which is a nice touch, and enabled me to
modify it from German to UK standard when we returned to live in the UK, and repair it when the PSU failed several years ago. |
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Inside the box. On the bottom on the left is the main board, with NAS-SYS 3, 8k Basic (normally disabled) and
16k static RAM. The RAM-A board on top has 32k dynamic RAM, ZEAP in two 2k EPROMs and monitor extensions in another
two 2k EPROMS. So the full 64k address space is occupied. (The switch at the from lets me disable the 4k monitor extensions and
another switch on the main board lets me enable Basic instead.)
On the right at the back is the 3A PSU. To the left of the transformer is an own-design board that acts as a pre-regulator for
the 5V supply on the PSU board. I found this necessary to avoid the 5V regulator from overheating. This
board also contains a zero-crossing detector for providing mains-synchronised 100Hz pulses. The monitor extensions utilise
these for an interrupt-driven real-time clock.
The device with a socket an flying lead is a 2716 EPROM emulator. I would transfer a 2k RAM chip from the main board to
here and plug it into the main board in its place. Then I would copy the EPROM image to the RAM, change the switch
to the write-protect position, unplug the emulator and plug it into an EPROM socket on the target system. |
Wireless World article "Data Recording on Audio Cassette"
My variant on this that handled Nascom2 tape encoding at 300Bd and 1200Bd, and Biphase-M at 2400Bd and 4800Bd.
Links
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It was last modified on 2005-03-28. |
