Mar 22

Wanted: CP/M boot disk for Amust Executive

The Amust Executive is literally a computer in a brief case. It was made in Australia (Victoria) in the early 1980′s. I purchased a non-working one (which I subsequently got working). I’ve sent some disks that came with it to somebody to investigate and image (they are looking for boots disks too). So if anybody has bootdisks (or manuals, etc) for this computer, please drop me a note, Thanks.


Amust computer in a case

Amust computer in a case

Oct 05

TRS-80 Colour Computer 2

I’m doing a bit of retro computing at the moment, having purchased a very good TRS-80 Colour Computer 2 (or coco for short). It has extended colour basic licensed by Microsoft (1981) and 16K of RAM. Yes folks, sixteen kilobytes of RAM. I’ve purchased a data cassette player to match (hasn’t arrived yet). I’ve tried loading a few programs from the PC to the coco via the cassette cable but have had only limited success.  I’ve ordered some 4 pin DIN plugs to allow me to make up a suitable serial cable so I can send/receive data over serial to the coco.


Colour computer with program listing on screen

Colour computer with program listing on screen

These computers hooked up to your tv set via the aerial socket, no separate video/sound output. As you can see from the picture, the display on the screen is a bit fuzzy. Not sure why yet, perhaps the cable is no good or the socket is dirty or out of alignment. In any event, I’m having some fun re-living the early home computer years with my coco and its really testing my memory on BASIC programming.


Update: I’ve recently purchased a TRS-80 Model I, which is really the ‘Model T Ford’ of the computing era. Here’s a nice article about them on CNN.

Sep 06

4 bit CPU – Diode ROM

I have pretty much finished the diode ROM, all that remains is to finish wiring the output to the connector.


diode_romThe purpose of this diode ROM is to be the microcode in the 4 bit CPU. That is, this software will co-ordinate the internal operation of the CPU.

Aug 27

Battlefield 4 – coming soon!

Looking forward to this one!

Aug 20

4 bit CPU – clock and program counter tested.

I’m using 2 x 74LS193 4 bit binary counters cascaded to produce an 8 bit program counter. The clock is based on the the venerable NE555  timer, although I’m using the NE556, which is basically two NE555′s in one DIP. One timer will be used as the clock source with three selectable speeds while the other half will be used to debounce the ‘write’ button for when entering programs into RAM. Below is a brief video of the test run:


Aug 19

4 bit ALU complete

I wired in the comparator  tonight, so the ALU now is basically complete.  In the pic below it’s showing that input ‘A’ is 0001 (the LEDs are reversed)  and input ‘B’ is 0000,  so the output from the comparator lights the LED representing A > B.


alu1 I’ve ordered some small toggle switches to allow me to create an interface through which to enter programs into RAM. I’ll start on that and the program counter when they arrive.

Aug 18

4 bit ALU initial test

Initial testing of the ALU shows it correctly able to add and subtract (this CPU will not do division, mulitplication). The ALU is pictured below, note I have not yet hooked up the 74HC85 (4 bit comparator, last chip top right), I’ll do that over the next few days as I expect it to work as advertised!


alu_initialNote that the LEDs and wiring on the bottom half of the board are there for the initial testing.  The first exciting step in what will be an interesting journey!

Aug 18

4 bit CPU project

I finally got tired of wondering about what really goes on inside a CPU that I decided to build my own 4 bit CPU project.  I chose a 4 bit design simply to keep the level of wiring sane as the initial hardware version will be on a bread board and a permanent version wirewrapped.

The design has been successfully tested in Logisim with a couple of simple programs. First program simple counts from 0 to 10 then starts again and the other one counts from 10 to 0 and starts again, so obviously I have kept pretty simple.

I’m using around 17 74x TTL series chips throughout. I could have cut the chip count by using a 74181 4 bit ALU but have decided to build the ALU from discreet gates.  The program RAM will consist of 2 x 2114 SRAM that I happen to have had on hand. The microcode is going to be hardwired into a diode ROM as a hat-tip to the rope memory used in the Apollo Guidance Computer.

I’ve started breadboarding the design (schematics created with Kicad) using cut lengths of solid core wire in an attempt to try and keep it tidy, as some of the other bread-boarded designs using standard jumpers gets messy and in the end all you can see is a mass of wiring.

I’ll keep this project up to date as it progresses.

Aug 07

Blue LCD keypad shield for Arduino

Just received the blue LCD keypad shield in the post today. Works great except some of the buttons didn’t seem to respond. The buttons are connected via a voltage divider network to ‘A0′ pin on the arduino (to save using up digital pins) and therefore each button returns an analog value that needs to be interpreted as off (1023) or on (<1023).



The corresponding key values are matched in the DFR_Key.cpp library.  The library had a set of default values which didn’t match what my keys were returning. The fix was to simply update the library with the correct matching values:

static int UPKEY_ARV = 132;
static int DOWNKEY_ARV = 306;
static int LEFTKEY_ARV = 478;
static int RIGHTKEY_ARV = 0;
static int SELKEY_ARV = 722;
static int NOKEY_ARV = 1023;

Then just re-load the arduino and all the keys started working as they should. I’ve setup a simple clock with my LCD module, based on an RTC module. I plan to use the keys to allow the time/date to be set as well as a sound module to generate alarms.


Apr 26

DIY LCD monitor fix

If your LCD monitor (or television) turns on but the screen is blank (or goes blank after a few seconds) chances are the back-light is not receiving sufficient power. You can check by shining a torch on the screen – you’ll see the images are still there, just to dark to be visible.  The LCD back-light is powered from the monitor’s internal power-supply and in most cases the black screen is caused by a faulty capacitor.

You can check by removing the back from the monitor and inspecting the PSU board, the faulty capacitors will be easy to spot:


They bulge at the top and in some cases, the bottom. Replacement parts can be found at stores such as Jaycar Electronics and will cost around $1. Make sure you get one with the same capacitance and with at least the same voltage rating. Unsolder the faulty capacitor (note the polarity) and solder in the replacement, reassemble the monitor, plugin and continue to enjoy.

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