TRS-80 software I wrote
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Nick Andrew - TRS-80 - Hardware Mods
TRS-80 Hardware modifications
I owned my System-80 since about 1981 and used it actively until
around 1992. During that time I modified it extensively because
the design was simple enough for a child to understand, and
I wanted the computer to be more powerful.
The modifications I made included the following. This is not
in chronological order:
- Add lower-case using an EPROM as a character generator.
I got the character set out of one of the chip databooks and
programmed the EPROM using a programmer owned by my friend
- Upgrade memory from 16 Kbytes to 32K, then 48K.
This was done by soldering ram chips on top of the existing
socketed chips. Only pins 15 and 14 (I think) were not connected
in parallel; they were used to arbitrate between the banks of RAM.
- Speed up the CPU from 1.77 MHz to 2 MHz and then 3.54 MHz.
I never achieved 4 MHz, probably because my ROM chip was too slow.
- Replace ROM chip with an EPROM and made minor changes to the
BASIC interpreter inside. I don't recall ever trying for 4 MHz
after doing this, nor do I recall exactly what changes I made to
the 12 Kbyte ROM. I remember putting my own name into the EPROM.
I might have changed the BASIC error messages to mixed case too.
- Upgrade memory to 80 Kbytes. I did this by bank-switching
two 32K chunks (so 32 + 32 + 16 = 80). That was useful for
storing the DOS system files in the other bank.
- Add a simple interrupt counter. This was my first attempt at
a real-time clock. When DOS did disk I/O it would disable interrupts
for a long time and the CPU would lose clock interrupt ticks. So the CPU
would read this counter every time it processed a clock interrupt
and would count how many missed interrupts there were, and adjust
the clock accordingly. In practice it worked pretty well, but of
course the time was not kept across reboots.
- Add inverse video controlled by switch or I/O port. This would
invert the whole screen contents. I thought it was cool for a while.
- Install the Deakin University RS-232 controller board in my
Dick Smith System-80 Expansion Unit. This was wired up to the bus
somehow, but by the end I had completely redesigned the circuit
and I was plugging it into the 20-pin expansion connector inside
the Expansion Unit.
- Replaced the keyboard connector with a DB25 plug/socket pair
and made a 25-pin expansion cable so I could lie on my bed and
type. This was a poor attempt at portability ... I was also far
away from the screen so I couldn't see what I was typing, and
the excessive cable length caused the keyboard signals to become
corrupted. I abandoned the extension cable and kept the DB25.
The System-80 used a semi-rigid wire-frame connector for the
keyboard and to join the two main boards inside the computer.
These connectors were notoriously unreliable and any change
could only have been an improvement.
- I added a "hard reset" pushbutton. The standard reset button
causes an interrupt which the ROM passes to RAM (and can thus
be caught/ignored). My reset button forces the CPU to start
executing again at location zero.
- I added an audio output jack at the back so the internal
speaker could be plugged into a cassette recorder. I might
also have added a switch to force use of the external cassette
interface (all these details are getting murky ... I seem to
recall that it was a problem sometimes that the user wanted to
use the external cassette and the system chose the internal
- I added various keys to the keyboard ... CLEAR and TAB? I
remember by striking I, O and SPACE simultaneously that completed
the matrix so the system believed that CLEAR had been pressed.
I also added hard brackets, curly braces and the backslash key.
Or was it underline?
- I "improved" the System-80 power supply by changing the main
transformer to a bigger one which produced a higher intermediate
(AC) voltage. This was before the days of switchmode power supplies.
The computer would crash sometimes on minor power disturbances in
the house (heater use, fridge turning on, and the like) and the
bigger transformer reduced that problem. Later I put in a much
bigger Ferguson transformer which "solved" the power problems
better ... at the cost of huge heat output, which melted the
case under the transformer. I had to put the computer on a block
of wood to stop the transformer melting its way right through
- I added a simple 4-bit resistor network to output 16 different
voltage levels and thus make a better sound than the 3-level square
wave which the ordinary audio output could do.
- I added a joystick (ATARI switch type one) connected to the
keyboard arrows and SPACE. This joystick was attached via a
round pinned connector.
- The CPU memory interface used maybe 3 D-type flip-flops to
accomplish a 3 clock-cycle delay when reading or writing memory.
I had already started using faster RAM chips (250 nS then 150 nS)
and so these long delays were no longer required and I removed
one wait state from the memory timing.
- I changed my 80 Kbytes RAM to 256 Kbytes RAM using a very
fast (13 ns?) cache ram chip on every memory lookup. It worked
- The chip was used to translate all memory accesses within the
48 Kbyte RAM address space. It didn't touch ROM or video memory
or memory-mapped peripheral areas (such as the disk controller).
- The chip mapped pages in 1 Kbyte chunks, so the 6 address
inputs on the chip were connected to address lines A10 through A16.
The 8 data outputs from the chip were connected to 8 x 256 Kbit
RAM chips, becoming A10 through A18.
- The chip was port-addressable so I could program it by
writing bytes to I/O port 0x10. To do this I actually used an
"undocumented" feature of the Z80, where an "OUT (C), A"
instruction would actually place the contents of the register
B on the top 8 bits of the address bus (allowing in effect,
a 16-bit port address space). My chip programming involved
mapping from a logical address (on the address lines) to a
physical RAM address (on the data lines).
- On power-up the system was able to load the first sector from
the disk without using more than 1 Kbyte of memory, so that part
worked even when the address translation chip was unprogrammed.
I wrote a new boot sector which did an initial mapping of the
address space and then loaded a new boot sector from sector one
of the disk (which was, by default, a copy of sector zero anyway).
Thus I achieved compatibility with the original system.
- The boot sector mapping was tricky. It would map a known
physical page into a known virtual page, then copy all the code
to that known page. If the unknown physical page which was used
to load the boot sector was the same as the known physical page,
the code would be unaffected because it would be moved to the
same memory area. After the move, the new code segment was
executed, which would map all the rest of the pages.
- I added a true real-time clock circuit using the MSM-5832RS
clock chip and 2 AA NiCad batteries for power-off retention. This
clock worked well, once the programming foibles of the chip were
- I added circuitry to the video memory to add wait states to
video reads and writes until a horizontal or vertical retrace
period occurred. This had the effect of virtually eliminating
noise on the screen during video I/O. The original circuitry
would just write at any time, and so the more I/O was done,
the more white or black streaks would run through the display.
It's possible I also added code to allow the CPU to detect when
the retrace was in effect and optionally delay its writes (as
opposed to hard-wiring the delays). I don't have the hardware
anymore to go check the circuit, so just guessing here.
- I replaced one or more 5 Volt regulators inside the CPU unit
with more powerful versions (or maybe added heatsinks).
- I unsoldered quite a few chips from the main board and soldered
sockets in their place. This was good for when I killed a chip
(which happened occasionally) but the danger was in destroying the
delicate pads on the PCB. I ended up with a few short wire jumpers
to replace broken pads.
- I have pictures of most of these works, taken on 2001-04-13.
I just have to find the time to retrieve the pics and make thumbnails
This package contains some code I wrote which was specific to my
- This would set the CPU to high speed or normal speed.
- This program would set one of the two 32K memory chunks, when
my computer had 80 Kbytes RAM. The chunks were useful primarily for
storing the DOS files in memory, so that DOS would not need to reload
parts of itself from disk for such simple operations as opening a file.
- The two programs in this directory manipulate the paged memory
- These programs set and query the hardware realtime clock.
- This program caches DOS system files in memory (as many as
needed) and hooks into DOS so that requests are filled by copying
memory rather than reading the modules from disk. I seem to recall
that the original idea was somebody else's program and my program
developed on that, to load more modules, and of course to use my
unique paged memory subsystem to enable all the modules to be
cached at once.