my first computer

This was my first computer. Built back in school days. Full size image.

Back row right: EPROM board (four 4K and one 2k present), containing the ‘firmware’. Monitor, assembler, etc. (I don’t really remember what the etc. consists of. The system has been used mainly as a terminal in its late days.) Left: 32k dynamic RAM, in two 16k banks. The Z80 builtin refresh counter made such things easy.

Second back row right: 6845, good old CRT controller. Middle: Video RAM (4k), with address/data multiplexers. Left: Character generator ROM and shifter.

Front board row right: Another dynamic RAM board, with whopping 4k bytes. These were almost state of the art as I built this. Next: CPU and support. 2k of RAM, also originally contained two 1k EPROMs. Not at the beginning, though.

Under the red cylindrical thingie: A 8255 for centronics interface, and in front of that a 8251 for serial comms. Under the greenish thingie there is an Z80CTC (counter/timer); outside of the image are two Z80PIO for general I/O, and some electrics for a cassette interface.

Front pieces: LEDs and dip switches for entering data directly into the RAM; easy to do thanks to the bus request feature of the Z80. Right board is data, middle two are adresses, left one shows read/write lines and status. The LEDs also display the bus states while in normal operation which looked somewhat interesting and provided a bit of feedback what it was doing.

There were about 160 bytes to enter that way to operate a simple seven-segment display and keyboard, and due to electrical instability the system was parked overnight with reset activated. Burning the first EPROM with that setup was a feat, and after that was done, the system could be turned off regularly.

The system grew slowly from front to back, starting with CPU and display boards, then adding the display/keyboard, and then further to the back.

On top of everything: A bit of magical dust. That system wasn’t in use for quite some time, and quickly photographed before it finally got scrapped. Only the firmware board was kept.

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