Gigatron TTL Microcomputer
This lab guide provides instructions for building the Gigatron, an 8-bit microcomputer constructed entirely from discrete Transistor-Transistor Logic integrated circuits instead of a modern microprocessor. Students assemble a complete system from through-hole components, building up functional blocks such as the Arithmetic Logic Unit, program counter, and instruction decoder using 74HCT-series gates. Through this process, students learn the fundamentals of computer architecture, the internal data path of a CPU, and hardware-software interfacing. The project concludes with testing integrated video and audio outputs and running low-level software environments including Tiny BASIC and 6502 emulation, offering a deep dive into the foundational technology of the microcomputer revolution.
TTL
Transistor-Transistor Logic
74HCT
CPU architecture
Digital electronics
Microcomputer
8-bit computer
Through-hole soldering
ALU
Arithmetic Logic Unit
VGA
6502 emulation
Tiny BASIC
Integrated circuits
ICs
Computer engineering
Logic gates
SRAM
EPROM
Circuit troubleshooting
Uploaded by demo on 2026-07-09 · ready · Electronics · Original source ↗
Files: directions.md · assembly_manual.pdf · bom.xlsx
Readiness checks
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complete 2026-07-19 15:14 51 searches · 0 pagesview · PDF
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complete 2026-07-09 12:55 53 searches · 0 pagesview · PDF
Latest report — 2026-07-19 15:14
Readiness: Gigatron TTL Microcomputer
Verdict: Ready with caveats · 2026-07-19 · est. parts cost $115–$140
The Gigatron TTL remains buildable as of July 2026 but requires multi-vendor sourcing and DIY enclosure fabrication. The core logic components and passives are widely available, and the sourcing status for the 32K SRAM has improved since the last report due to stabilized stock of recommended substitutes. However, the 6.25 MHz crystal and the specific 16-bit EPROM remain niche items that require procurement from secondary distributors (Mouser) to ensure adequate stock. The verdict remains "Ready with caveats" as the system still requires manual firmware programming and custom case fabrication.
Action required
| Part | Problem | Recommended action |
|---|---|---|
| 74HCT04 | Out of stock at primary distributor (DigiKey). | Source SN74HCT04N from Mouser or use CD74HCT04E from DigiKey. |
| 62256 | Lyontek LY62256PL-55LLI is confirmed obsolete. | Use Alliance AS6C62256-55PCN as a direct substitute. |
| Quartz Crystal | 6.25 MHz (MP062-E) is out of stock at primary (DigiKey). | Source MP062-E from Mouser. |
| DB9 Male | 5747840-3 is out of stock at primary (DigiKey). | Use Assmann A-DS 09 A/KG-T2S as a direct substitute. |
| Wooden Enclosure | Commercial manufacturing has ceased. | Download SVG or OpenSCAD plans from gigatron.io for DIY laser cutting or woodworking. |
Parts
1. Integrated Circuits (Main Board)
| Part | Spec/Package | Status | Stock | Price | Source | Drift |
|---|---|---|---|---|---|---|
| 74HCT04 | Hex Inverter, DIP-14 | Available | 1,443 | $0.88 | Mouser | +$0.20 |
| 74HCT161 | 4-bit Counter, DIP-16 | Ready | 9,661 | $1.34 | DigiKey | 0 |
| 74HCT273 | Octal Register, DIP-20 | Ready | 26,627 | $0.93 | DigiKey | 0 |
| 74HCT244 | Octal Bus Driver, DIP-20 | Ready | 4,467 | $1.52 | DigiKey | 0 |
| 74HCT139 | Dual 2-to-4 Decoder, DIP-16 | Ready | 26,330 | $1.19 | DigiKey | 0 |
| 74HCT153 | Dual 4-to-1 MUX, DIP-16 | Ready | 816 | $1.22 | DigiKey | 0 |
| 74HCT138 | 3-to-8 Decoder, DIP-16 | Ready | 3,421 | $0.89 | DigiKey | 0 |
| 74HCT240 | Octal Inv Buffer, DIP-20 | Ready | 24,075 | $0.96 | DigiKey | 0 |
| 74HCT32 | Quad OR Gate, DIP-14 | Ready | 1,773 | $0.97 | DigiKey | 0 |
| 74HCT283 | 4-bit Full Adder, DIP-16 | Ready | 915 | $1.32 | DigiKey | 0 |
| 74HCT377 | Octal Flip-Flop, DIP-20 | Ready | 10,109 | $1.43 | DigiKey | 0 |
| 74HCT157 | Quad 2-to-1 MUX, DIP-16 | Ready | 33,781 | $1.16 | DigiKey | 0 |
| 74HC595 | 8-bit Shift Reg, DIP-16 | Ready | 14,290 | $1.86 | DigiKey | 0 |
| 62256 | 32K x 8 SRAM, DIP-28 | Obsolete | 0 | — | — | — |
| 27C1024 | 1Mbit EPROM, DIP-40 | Low Stock | 96 | $4.52 | DigiKey | 0 |
| MCP100-450DI/TO | Reset IC, TO-92 | Ready | 8,796 | $0.42 | DigiKey | 0 |
2. Pluggy McPlugface (Add-on) ICs
| Part | Spec/Package | Status | Stock | Price | Source | Drift |
|---|---|---|---|---|---|---|
| ATtiny85-20PU | MCU, DIP-8 | Ready | 24,959 | $1.66 | DigiKey | 0 |
3. Passive Components (Main Board)
| Part | Spec/Package | Status | Stock | Price | Source | Drift |
|---|---|---|---|---|---|---|
| Resistor 68 Ω | Axial, 1/4W | Ready | 21,278 | $0.10 | DigiKey | 0 |
| Resistor 750 Ω | Axial, 1/4W | Ready | 20,714 | $0.10 | DigiKey | 0 |
| Resistor 1 kΩ | Axial, 1/4W | Ready | 710,261 | $0.10 | DigiKey | 0 |
| Resistor 1.5 kΩ | Axial, 1/4W | Ready | 26,620 | $0.10 | DigiKey | 0 |
| Resistor 2.2 kΩ | Axial, 1/4W | Ready | 25,751 | $0.10 | DigiKey | 0 |
| Resistor 1 MΩ | Axial, 1/4W | Ready | 534,545 | $0.10 | DigiKey | 0 |
| Resistor Array | 5-pin SIP, 2.2 kΩ | Low Stock | 32 | $0.28 | DigiKey | 0 |
| R2R Network | 6-pin SIP, 10 kΩ | Ready | 1,368 | $0.46 | DigiKey | 0 |
| Capacitor 47 pF | Ceramic, 50V | Ready | 24,936 | $0.26 | DigiKey | 0 |
| Capacitor 100 nF | Ceramic, 50V | Ready | 233,541 | $0.28 | DigiKey | +$0.01 |
| Capacitor 220 µF | Electrolytic, 25V | Ready | 7,495 | $0.60 | DigiKey | 0 |
4. Discrete Semiconductors and Protection
| Part | Spec/Package | Status | Stock | Price | Source | Drift |
|---|---|---|---|---|---|---|
| Signal Diode | BAT42, DO-35 | Ready | 23,133 | $0.50 | DigiKey | 0 |
| Zener Diode | 5.6V 5W, DO-15 | Ready | 67,621 | $1.26 | DigiKey | 0 |
| LED Red, 3mm | 1.8V, 2mA | Ready | 60,765 | $0.51 | DigiKey | 0 |
| Multifuse (PTC) | 0.75A 60V, Radial | Ready | 4,604 | $0.47 | DigiKey | 0 |
| Quartz Crystal | 6.25 MHz, HC-49/U | Available | 1,007 | $1.01 | Mouser | — |
5. Connectors and Hardware
| Part | Spec/Package | Status | Stock | Price | Source | Drift |
|---|---|---|---|---|---|---|
| Mini-USB | Type B, 5-pin | Ready | 901 | $2.07 | DigiKey | 0 |
| 3.5mm Audio Jack | Stereo, TH | Ready | 234,430 | $1.06 | DigiKey | 0 |
| DB15 Female (VGA) | High Density, R/A | Low Stock | 114 | $2.07 | DigiKey | 0 |
| DB9 Male | Standard, R/A | Available | 18,130 | $2.87 | TTI | — |
| Mini-DIN 6-pin | PS/2, TH | Ready | 4,184 | $1.88 | DigiKey | 0 |
| DB9 Female | Standard, TH | Ready | 1,465 | $1.24 | DigiKey | 0 |
| IC Sockets | DIP Assortment | Ready | High | $1.95 | eBay | -$4.05 |
| Famicom Gamepad | Clone, DB9 | Ready | High | $4.95 | AliExpress | 0 |
| Wooden Enclosure | Custom Case | Obsolete | — | — | — | — |
| Rubber Feet | 3M Bumpons | Ready | 250,992 | $14.70 | DigiKey | 0 |
Substitutes
| For part | Substitute | Differences | Judgment |
|---|---|---|---|
| 74HCT04 | CD74HCT04E | Direct PDIP-14 substitute. | Acceptable. |
| 62256 | Alliance AS6C62256-55PCN | Direct replacement (DIP-28, 55ns). | Acceptable. |
| DB9 Male | Assmann A-DS 09 A/KG-T2S | Direct substitute footprint. | Acceptable. |
Verify any substitute against its datasheet before ordering.
Other dependencies
- Main ROM: Requires 27C1024 EPROM programmed with Gigatron ROM v5. (Verified at kervinck/gigatron-rom GitHub)
- BabelFish Firmware: Requires ATtiny85 programmed with BabelFish interface software. (Verified at kervinck/gigatron-rom Utils)
- PCB Fabrication: Requires Gerber files for Main and Pluggy boards from gigatron.io. (Verified)
- Enclosure Plans: DIY OpenSCAD/SVG files available via gigatron.io FAQ for custom fabrication. (Verified)
Watch list
- AT27C1024-70PU: Only 96 units remain at primary distributor. While 622 units exist at Mouser, this part is high-risk for future availability.
- 6.25 MHz Crystal: No stock at primary distributor; Mouser stock is stable but requires separate shipping.
- Resistor Array (4605X-101-222LF): Extremely low stock (32 units) at DigiKey. Source from Newark (8,000+ available) if primary stock fails.
- DB15 Female (L77HDE15SD1CH4R): Low stock (114 units) at DigiKey. Monitor Newark stock as a secondary source.
Snapshot What the lab depended on when it was uploaded — the baseline the readiness checks research against.
This lab builds the Gigatron TTL Microcomputer, a complete 8-bit computer system where the CPU itself is constructed from discrete 74HCT-series logic gates rather than a microprocessor chip. The system includes integrated video (VGA), audio, and a game controller interface. It is supplemented by the Pluggy McPlugface add-on board, which uses an ATtiny85 microcontroller to interface PS/2 keyboards with the main computer's game port.
Bill of Materials
1. Integrated Circuits (Main Board)
All ICs are specified in DIP (Dual In-line Package) for through-hole mounting. The logic family must be 74HCT (High-speed CMOS with TTL-compatible inputs) unless otherwise noted, as the timing and voltage thresholds of the discrete CPU design depend on this specific family.
- 74HCT04 (U1): Hex Inverter. 1 needed. Plan: distributor lookup: SN74HCT04N.
- 74HCT161 (U3, U4, U5, U6, U29, U30): 4-bit Binary Counter. 6 needed. Plan: distributor lookup: CD74HCT161E.
- 74HCT273 (U8, U9, U38): Octal D-type Register with Clear. 3 needed. Plan: distributor lookup: SN74HCT273N.
- 74HCT244 (U10, U28): Octal Bus Driver, Non-Inverting. 2 needed. Plan: distributor lookup: SN74HCT244N.
- 74HCT139 (U11): Dual 2-to-4 Decoder. 1 needed. Plan: distributor lookup: SN74HCT139N.
- 74HCT153 (U12, U17, U18, U19, U20, U21, U22, U23, U24): Dual 4-to-1 Multiplexer. 9 needed. Plan: distributor lookup: CD74HCT153E.
- 74HCT138 (U13, U14): 3-to-8 Decoder. 2 needed. Plan: distributor lookup: SN74HCT138N.
- 74HCT240 (U15): Octal Inverting Buffer. 1 needed. Plan: distributor lookup: SN74HCT240N.
- 74HCT32 (U16): Quad 2-input OR gate. 1 needed. Plan: distributor lookup: CD74HCT32E.
- 74HCT283 (U25, U26): 4-bit Binary Full Adder. 2 needed. Plan: distributor lookup: CD74HCT283E.
- 74HCT377 (U27, U31, U37): Octal D-type Flip-Flop with Enable. 3 needed. Plan: distributor lookup: SN74HCT377N.
- 74HCT157 (U32, U33, U34, U35): Quad 2-to-1 Multiplexer. 4 needed. Plan: distributor lookup: SN74HCT157N.
- 74HC595 (U39): 8-bit Shift Register. 1 needed. Plan: distributor lookup: SN74HC595N.
- 62256 (U36): 32K x 8 Static RAM (SRAM). Must be 55ns or faster. 1 needed. Plan: distributor lookup: LY62256PL-55LLI.
- 27C1024 (U7): 1 Mbit (64K x 16) Parallel EPROM. Must be 150ns or faster. 1 needed. Plan: distributor lookup: AT27C1024-70PU.
- MCP100-450DI/TO (U2): Supervisory Reset IC, 4.5V threshold, TO-92 package. 1 needed. Note: Sources describe this as "Supervisory circuit," but it is an active-low reset trigger. Plan: distributor lookup: MCP100-450DI/TO.
2. Pluggy McPlugface (Add-on) ICs
- ATtiny85-20PU (U1): 8-bit Microcontroller, DIP-8 package. 1 needed. Plan: distributor lookup: ATTINY85-20PU.
3. Passive Components (Main Board)
All resistors are axial 1/4W, 5% or better.
- Resistor 68 Ω (R17, R18): 2 needed. VGA sync termination. Plan: distributor lookup: CF14JT68R0.
- Resistor 750 Ω (R12, R14, R16): 3 needed. VGA DAC high bits. Plan: distributor lookup: CF14JT750R.
- Resistor 1 kΩ (R2): 1 needed. Clock damping. Plan: distributor lookup: CF14JT1K00.
- Resistor 1.5 kΩ (R11, R13, R15): 3 needed. VGA DAC low bits. Plan: distributor lookup: CF14JT1K50.
- Resistor 2.2 kΩ (R3, R6, R7, R10, R19): 5 needed. General pull-up/audio. Plan: distributor lookup: CF14JT2K20.
- Resistor 1 MΩ (R1): 1 needed. Clock feedback. Plan: distributor lookup: CF14JT1M00.
- Resistor Array, 5-pin SIP, 2.2 kΩ (R4, R5, R8): 3 needed. Bourns 4605X-101-222LF or equivalent. Plan: distributor lookup: 4605X-101-222LF.
- R2R Network, 6-pin SIP, 10 kΩ (R9): 1 needed. Bourns 4606X-R2R-103LF. Plan: distributor lookup: 4606X-R2R-103LF.
- Capacitor 47 pF (C1, C2, C3): Ceramic disc, 50V. 3 needed. Plan: distributor lookup: K470J15C0GF5TL2.
- Capacitor 100 nF (0.1 µF) (C5 to C44): Ceramic disc, 50V. 40 needed. Decoupling for every IC. Plan: distributor lookup: K104K10X7RF5UH5.
- Capacitor 220 µF (C4): Electrolytic, radial, ≥25V. 1 needed. Plan: distributor lookup: EEU-FC1E221.
4. Discrete Semiconductors and Protection
- Signal Diode 1N60P or BAT42 (D3, D4, D9 to D36): Schottky signaling diodes, DO-35 package. 30 needed. Plan: distributor lookup: BAT42-TR.
- Zener Diode 5.6V 5W (D1): Power protection, DO-15 package. 1 needed. Plan: distributor lookup: 1N5339B-TP.
- LED Red, 3mm (D2, D5, D6, D7, D8): 1.8V, 2mA. 5 needed. Plan: distributor lookup: HLMP1700.
- Multifuse (PTC), 0.75A 60V (F1): Resettable fuse, radial leaded. 1 needed. Plan: distributor lookup: MF-R075.
- Quartz Crystal, 6.25 MHz (Y1): HC-49/U or through-hole equivalent, 30pF load capacitance. 1 needed. Plan: distributor lookup: MP062-E.
5. Connectors and Hardware
- Mini-USB Receptacle (J1): Type B, 5-pin, through-hole. 1 needed. Plan: distributor lookup: 54819-0519.
- 3.5mm Audio Jack (J2): Stereo jack, through-hole. 1 needed. Plan: distributor lookup: SJ1-3523N.
- DB15 Female (VGA) (J3): High density 15-pin D-sub, right-angle, PCB mount. 1 needed. Plan: distributor lookup: L77HDE15SD1CH4R.
- DB9 Male (Game Port) (J4): Standard 9-pin D-sub, right-angle, PCB mount. 1 needed. Plan: distributor lookup: 5747840-3.
- Mini-DIN 6-pin (Pluggy J1): PS/2 keyboard connector, through-hole. 1 needed. Plan: distributor lookup: KMDGX-6S-AS.
- DB9 Female (Pluggy J2): Standard 9-pin D-sub, through-hole. 1 needed. Plan: distributor lookup: L77SDE09SOL2.
- IC Sockets: 40-pin DIP (1), 28-pin DIP (1), 8-pin DIP (1). Plan: retail shopping search: DIP IC socket assortment.
- Famicom-style Gamepad: Clone controller with DB9 connector. 1 needed. Plan: retail shopping search: Famicom DB9 clone controller.
- Wooden Enclosure: Custom-made case for the PCB. Note: Sources state this is "no longer available." Plan: general web research: "Gigatron wooden case alternatives or DIY plans."
- Rubber Feet: Self-adhesive bumpons. 8 needed. Plan: distributor lookup: SJ-5302-CLEAR.
Additional Dependencies
Firmware and Software
- Main ROM: The 27C1024 EPROM must be programmed with the Gigatron system ROM. This firmware is temporal and version-dependent (current reference is ROM v5). Source: github.com/kervinck/gigatron-rom.
- BabelFish Firmware: The ATtiny85 for the PS/2 adapter must be programmed with the BabelFish interface software. Source: github.com/kervinck/gigatron-rom/tree/master/Utils/BabelFish.
Fabrication and PCB
- PCB Files: The design requires two separate PCBs (Main and Pluggy). Gerber/EDA files are provided at gigatron.io/pcb and gigatron.io/pluggy.
- Substitutions: The manual explicitly warns that if substituting 74HCT series with 74HC, the system may fail due to timing or voltage threshold differences. The 27C1024 EPROM is a specific 16-bit wide memory that is increasingly rare; substitutes must maintain the 16-bit parallel interface and access time ≤ 150ns.