Z80-PC1 - A Personal Computer on a Single Board, 1976 CPU Z80

What it is:

A personal computer on a single board, built around a CPU from 1976.

The board is designed to be small because this is my first PCB motherboard project and I don't have a lot of experience with the expenses and complications of creating larger boards. Start with a 5 V supply, plug in a VGA monitor and a PS/2 keyboard, and theoretically a few seconds later you should have a command prompt. No operating system to install, no updates, no network. It boots into CP/M, the operating system that ran most serious 8-bit machines before MS-DOS, and from there you write programs, edit files, and save your work to a microSD card.

What you'd actually do with it:

Boot to an A> prompt. Write a "BASIC" program and you can run it immediately. You will be able to write Z80 assembly and assemble it on the machine itself, or cross-compile C on your main laptop and drop the binary on the SD card. Real software of the era should run on it, such as WordStar, Turbo Pascal, or simple games. Thousands of CP/M programs exist free today. 

And because a 40-pin connector exposes the complete CPU bus, anyone can design their own card - more memory, a sound chip, an instrument interface - and plug it in. That makes it easy to learn what a bus actually is and what it lets the system do.

What's on the board:

The three big black chips down the left are the heart: the Z80 processor at 8 MHz, 128 KB of RAM (bank-switched, because the Z80 can only see 64 KB at a time), and an ATmega microcontroller acting as the entire support chipset.

The small green board in the middle is a designed to be the GPU (Graphics Processing Unit): a Raspberry Pi Pico that generates the video and reads the keyboard. Around the edge: VGA, keyboard, serial, SD card, clock, power, expansion. The empty patch top-right is deliberately reserved so sound can be added later without redesigning everything.

A modern approach:

A 1983 machine needed a dozen support chips around its CPU - boot ROM, serial controller, disk controller, timer, video. This replaces nearly all of that with two microcontrollers running software! 

The computer is real, though: the Z80 is genuinely in charge. It fetches and executes every instruction, owns the memory, and runs CP/M natively - not in an emulator. Nothing you write is simulated. The clever chips are peripherals, and peripherals outperforming the CPU isn't new; a 1983 floppy controller often held a microprocessor doing more work than the host.

What it isn't:

It's not that fast. Whomp whomp. It won't browse the web. This is a simple, but complete-ish board. Everything is through-hole with socketed chips, so it's a reasonable first or second board to build.

Sadly, it isn't finished:

The schematic is complete and checked, the components are placed, but more work needs to be done.

The traces are not routed yet. A manufacturing files does not yet exist. The firmware is not written yet, 

The video firmware is recognized and noted as being a substantial project.

FAQ:

Why not discrete video logic or a TMS9918A?

Discrete character-cell video is a dozen more chips and a much harder board. Sourcing TMS9918A/V9958 parts may be hard to source at fair prices, and the 9918 outputs composite, not VGA. Neither fits the with the direction of this PC.

Parts list

Ref	Part	Package	Notes
U1	Z84C0008PEG	DIP-40	CMOS Z80, 8 MHz.
U2	ATmega1284P-PU	DIP-40	128 KB flash, 16 KB RAM
U3	AS6C1008-55PCN	DIP-32	128 KB SRAM (TC551001 equivalent)
U4	74HC32	DIP-14	Bank select - 2 OR gates used, 2 spare
U5	74HC00	DIP-14	WAIT latch (IOS sheet)
U6	74LVC245AN	DIP-20	Z80 data → Pico, LVC at 3.3 V
U7	74HCT245N	DIP-20	Pico data → Z80, HCT at 5 V
U8	74LVC245AN	DIP-20	Control lines, 5 V → 3.3 V
M1	Raspberry Pi Pico	2×20 header	Video + keyboard. Socket it.
M2	Micro SD module	6-pin header	Same style as MBC2
M3	DS3231 RTC module	6-pin header	Optional

Real Time Clock (RTC as a module): J2 six-pin header accepts a real time clock add-on module to enable a system clock with a battery backup. DS3231 RTC module

FAQ:

Why not discrete video logic or a TMS9918A?

Discrete character-cell video is a dozen more chips and a much harder board. Sourcing TMS9918A/V9958 parts may be hard to source at fair prices, and the 9918 outputs composite, not VGA. Neither fits the with the direction of this PC.

Why three bus buffers rather than one

The obvious design is a single bidirectional 74LVC245 between the Z80 bus and the Pico. It does not work. An LVC245 running from 3.3 V drives its outputs to 3.3 V, and a CMOS Z80's input threshold can be as high as 0.7 × V_CC = 3.5 V. The Z80 might read the Pico's data as a logic low. Some Z84C00 datasheets quote a TTL-level 2.2 V V_IH, in which case it could work.

So the two directions are split, each using a part that is unambiguously correct:

• Z80 → Pico (U6): 74LVC245 at 3.3 V. Its inputs are 5 V-tolerant, and it outputs a clean 3.3 V into the Pico.
• Pico → Z80 (U7): 74HCT245 at 5 V. HCT inputs use TTL thresholds (V_IH = 2.0 V), so the Pico's 3.3 V is comfortably a logic high, and the outputs swing to a full 5 V for the Z80.
• Control lines (U8): 74LVC245 at 3.3 V, one direction only — /VID_CS, /RD, /WR, A0, A1. Five of eight channels used; the spare inputs are grounded.

U6 and U7 are never enabled at once: VID_WR_N and VID_RD_N, generated by the two spare OR gates of U4, enable exactly one of them per bus cycle. Those two gates are physically on the Memory sheet's 74HC32 but drawn on the Video sheet, which is where they are used.

LED Lights explanation:

D1 - HALT - Wired to the Z80's /HALT pin. The Z80 pulls that line low when it executes a HALT instruction, meaning it has deliberately stopped and is waiting for an interrupt. This could indicate a program crash.

D2 - DMA - Wired to /BUSACK pin. Flashes while board microcontroller loads code into memory. If it stays on, this indicates the machine is frozen.

D3 - USER - Driven from microcontroller. The firmware controls this, and it can be used for a "heartbeat", disk activity, and/or boot progress indicator.

D6 - PWR - Power light sits after the reverse-polarity diode. Indicates power is wired correctly. On = Powered!

Buttons explanation:

SW1 - RESET - System reset button. It resets the microcontroller on the board. The microcontroller then holds the Z80 processor in reset, reloads the boot code into RAM, then releases the Z80.

SW2 - USER - General purpose. Can be used to change what the machine boots into. Can be sensed in programs as "The operator pressed the button".

SW3 - PICO_RST - Resets the Raspberry Pi Pico only. Z80 CPU remains running. Useful for video firmware development and restarting graphics/keyboard input. Additional use: Put R. Pi Pico into USB drive mode.

Updated 8/1/2026