Lumir Vanek - old story

Introduction

Rum 80 PC was my first computer. I started work on it in 1985 and finished this in 1989. This is complete self-maded computer with famous Zilog Z80 processor, 64 kB RAM, keyboard, output to TV and printer. While building it, I get my first experiences with hardware and software development.

Hardware

Main board

Computer is constructed on self maded, dual-side universal board. This board is made very precise, because distance between chip pins is only 2.5 mm and both sides must accurate overlap.

Top side contain chips, some in sockets. Thare are some wires also. Main components are:

Address, data and control bus
8-bit processor Z80 CPU, 8 kB EPROM with monitor and BASIC, 2 x timer Z80 CTC and 8255 parallel interface for keyboard and external connector
64 kB D-RAM
Video display - black&white, 40x25 characters, connected to TV

Bottom side contain only wires and videoram chips (1 kB S-RAM). On left bottom side you may see two resistor-trimers for accurate settings horizontal and vertical TV impulses and speaker on right side.

Schemas

Keyboard

I used simple membrane keyboard. It is created from some layers of cuprextit, gum, paper with painted keys and transparent film. It doesn't work very well.

Expand board

When I consumed all free space on main board, I decided to make expand board. There are 8251 serial interface for cassette deck, 8-bit D/A converter, 8255 parallel port connected to one pin (not joke) BT100 printer. This printer parody I throw into trash.

Power supply

First I created analog power supply 5V/5A, 12V/0.7A. Lately I purchased switched power supply.

Software

I wrote my own monitor program, that allow simple things, such as display and modify registers, memory, I/O ports, run programs etc. I started only with paper, pencil, gum eraser and table of Z80 instructions. Later I have had access to machine with CP/M a moved this work to more comfortable way. Then I get access to PC/AT and used CP/M emulator here.

Software has simple two-task multitasking based on dual registries in Z80. First task operating with user, second one with printer. This allow background printing.

I have had also ported BASIC from SAPI computers.