Z80 Maximum Homebrew Computer

Bus wiring pinout

2009-07-28T10:32:00.001-07:00

I have now decided on the colour scheme of my wiring for the system

red - 5v
black - gnd
green - misc signals
white - address
blue - data

my pinout was originally going to use 2x17 way dil headers however I now realised I under allocated my backplane requirements so i have now added a second 2x10way dil header

the pinouts are now as follows

a b
1 GND 5V
2 SPARE A0
3 D0 A1
4 D1 A2
5 D2 A3
6 D3 A4
7 D4 A5
8 D5 A6
9 D6 A7
10 D7 A8
11 SPARE A9
12 SPARE A10
13 SPARE A11
14 SPARE A12
15 SPARE A13
16 SPARE A14
17 CLK A15

A B
1 /RD SPARE
2 /WR SPARE
3 /RFSH SPARE
4 /HALT SPARE
5 /WAIT SPARE
6 /INT SPARE
7 /NMI SPARE
8 /BUSRQ /IORQ
9 /BUSAK /MREQ
10 /M1 /RESET

Main Processor Board - Inital stage

2009-07-27T15:50:00.000-07:00

As can be seen from the above pics the inital pictures show the board with the chip sockets fitted (z80, buffers and inverter)

As seen in the pictures red and black follow the conventional 5v/gnd signals as well as the 2x17way pin header to carry the signals away from the board to the other boards (the bus)

One change that can be seen is between the last pic (actually pic1) i moved the bus connector from the top to the bottom. Although minimal this keeps all data/power the furthest away from the cpu as possible

Reset Circuit

2009-07-27T15:43:00.000-07:00

As the reset circuit described here is purely to reset the z80 only

I have come up with a circuit based on the southerncross z80 system which uses a single resistor and a capacitor to keep a +5v signal fed to the reset pin of the z80.

Pressing the switch pulls the signal low to ground to iniate the reset

The pictures below show a close up of the stripboard the circuit is made on as well as the completed circuit with switch

As per the clock circuit the wiring colours are as follows: -

red: 5v
black: gnd
green: reset signal

Clock Circuit

2009-07-27T09:14:00.000-07:00

Having finally decided on making a clock with 8 sets of frequencies selected via jumpers, using a 16mhz clock crystal i need to start looking at costings and ordering the kit for this. I still needed to make a circuit up to clock drive my z80 to test it in the mean time: -

I have now built and tested a circuit based on a 22k pot and a 74hc04 chip

The above picture shows where i initally placed the chip socket and pot for component placement identification.

The picture below shows the circuit with the resistors and the chip socket soldered in.

The next pictures show the same board but with 74hc04 chip mounted and the 22kpot soldered into the board.

The next two pictures show the difference in the clock frequency as shown on my oscilloscope.

For those interested the schematics for this circuit will be available for download very soon

Additionally the inital wiring for the clock circuit is as follows: -

red - 5v

black - gnd

green - clock signal for z80

Whizzkid.

Continuity tester

2009-07-27T08:55:00.001-07:00

I have also created my own continuity tester as I have misplaced my proper one

As you can see from the above picture it is a very simple device which when connected to a 5V supply (it is a 5v led) can check for circuit continuity

Inital Boards Created

2009-07-27T08:47:00.000-07:00

Over the past few days I have been creating the inital testing boards for the clock circuit and the reset circuits

I have not as of yet finalised my designs for these.

So It Starts

2009-07-25T13:10:00.000-07:00

After much thought thinking I am now at the real start of my quest to build my own Z80 computer from scratch

Having scanned the web and books for about 2 years I have now decided to put these ideas into practice

The majority of my designs will be based around the 74LS range of chips with a handful of HC/HCT chips

My inital ideas provide me with a z80 system with 1 ROM chip, 7 RAM chips and a bunch of I/O devices including ps/2 keyboard, sound, Vga Monitor output, Composite and TV output, Ethernet, rs232(multiple), parrallel to name a few as well as an ide drive or 2

Within this blog i will be including photographs showing my development, schematics as well as all my thoughts and ideas

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Initally my thoughts were to build this on eurocard pcbs but finding din41612 sockets prooved difficult and costly. i have now decided to custom make my own 34pin idc header connector sockets so that i can use standard idc cables used to connect 3.5" floppy drives to PCs

This will be a modular project.

I hope you enjoy my blog - feel free to comment at any stage that you wish to or to ask questions

BEFORE I START I WOULD LIKE TO THANK DOMINIC BEESLEY FOR ALL OF HIS HELP SO FAR AND IM SURE FOR A LOT MORE HEHE - THANKS DOM!

Whizzkid.