TRS-80 & Tandy CoCo Machine Hub
Z80 (Model I, III & 4) and 6809 Color Computer (CoCo 1, 2 & 3) workbench reference — Astec PSU RIFA & 12″ CRT safety, Expansion Interface & 4116 DRAM diagnostics, CoCo 3 RGB SCART & CoCo 1/2 composite mods, and Model I power pinouts.
Model III / Model 4 Dual Astec PSU RIFA Alert & 12″ CRT Neck / High-Voltage Safety!
When opening a silver/white TRS-80 Model III, Model 4, or Model 4P, watch out for two immediate hazards: (1) Disk-equipped Model III and Model 4 units contain TWO internal open-frame Astec switching power supplies (AA11240 / AC8151 — one for the logic/CRT, one for the dual 5.25″ floppy drives), and each Astec board holds two or three amber RIFA PME 271 M mains filter capacitors (up to 6 RIFA caps per computer!) that crack and explode with thick white smoke! (2) Even worse, the glass neck of the 12-inch RCA/Clinton monochrome CRT protrudes right into the path of the top plastic case hood — lifting or dropping the top shell carelessly will snap the CRT neck off and destroy the tube!
1 Safe Model III / 4 Case Opening & CRT Discharge
- Protect the Fragile 12″ CRT Neck: Undo the bottom perimeter screws, then lift the top case shell straight up at the rear by 3 inches first so the rear lip clears the exposed CRT neck board before tilting forward!
- Discharge the 12″ CRT Anode (~12–14 kV): Clip a heavy jumper lead to the metal CRT mounting frame and slide an insulated flathead screwdriver under the rubber anode cap until it clicks against the spring clip (hold 5 seconds, repeat after 1 minute).
- Reflow the 10-Pin CRT Video Header: On Model III/4 RCA monitor boards, cracked solder joints on the edge/pin video connector and Brightness/Contrast pots on the left side of the bezel are the #1 cause of flickering or dark screens!
2 Astec PSU RIFA Replacement Checklist
Inspect both internal Astec PSU boards (Main Logic PSU on the left/rear + Floppy Drive PSU on the right/drive tower):
| Cap Position | Original RIFA Spec | Modern Safety Replacement |
|---|---|---|
Line-to-Line (C1) | 0.1 μF (100 nF) 250 V AC X2 | Kemet R46 / Vishay MKP 0.1 μF 275–310 V AC X2 polypropylene |
Line-to-Earth (C2, C3) | 0.0022 μF (2.2 nF) or 0.0047 μF Y2 | 2.2 nF / 4.7 nF 250–300 V AC Y2 safety film / disc caps |
Also check the secondary output rails (+5.0V DC, +12.0V DC, and −12.0V DC for RS-232) and replace any bulging radial caps on the Astec boards.
3 Model I & CoCo Power Supply Notes
- TRS-80 Model I (
26-1001External Brick): Outputs 14–16 V AC AND ~19 V DC unregulated over a 5-pin DIN plug! All+5V,+12V, and−5Vregulation happens inside the Model I keyboard unit (for the eight4116DRAMs!). Note: The Expansion Interface (26-1140) holds a second identical power brick inside its own case! - CoCo 1 & CoCo 2: Built-in transformer supplying internal
+5V(7805on large rear heatsink),+12V, and−5V(for 4116 RAM on early CoCo 1s) or single-rail+5V(4164RAM on late CoCo 1/2/3).
Multi-Line System Selector: Z80 Line vs. 6809 CoCo Line
Radio Shack built two completely separate, incompatible 8-bit dynasties — toggle across the Z80 Model I/III/4 line and the Motorola 6809 Color Computer 1/2/3 line.
TRS-80 Model I (Level I & Level II) 1977–1980 • Z80 @ 1.774 MHz • 64×16 Display
Designed by Steve Leininger and Don French in Fort Worth, Texas (Catalog 26-1001), the silver-and-black TRS-80 Model I outsold both the Apple II and PET in 1977–1979. The keyboard unit contains a Zilog Z80 at 1.774 MHz, 4 KB (Level I BASIC) or 12 KB (Level II Microsoft BASIC) ROM, 4 KB (4027) or 16 KB (4116 triple-rail) DRAM, and 1 KB of dedicated Static Video RAM ($3C00–$3FFF, seven 2102 1K×1 SRAMs!). Adding floppy drives, 32 KB extra RAM (48 KB total), a printer port, and RS-232 required the notorious under-monitor Expansion Interface (EI) connected via a 40-pin flat ribbon cable (Screen of Death culprit #1!).
Key ICs & Memory Architecture (Model I Keyboard Unit)
- CPU (
Z40): Zilog / NEC Z80 @ 1.774 MHz (derived from 10.6445 MHz crystal) - Main RAM (
Z13–Z20): 8× 4116 (16K×1, triple-rail−5V/+12V/+5V,$4000–$7FFF) - Video SRAM (
Z45, Z46, Z47, Z48, Z61, Z62, Z63— 7 Chips!): Stock Model I displays 64×16 uppercase + 128×48 blocky semigraphics using only 7 bits per character (D0–D5&D7, omittingD6!) stored in seven 16-pin2102(1K×1) SRAMs! (Adding an 8th2102+ a toggle switch is the famous Model I Lowercase Mod!). - Character Generator (
Z29): MotorolaMCM6670P/MCM6674P(shifted vs unshifted descenders) - Memory-Mapped Keyboard (
$3800–$38FF): Read directly onto the Z80 data bus via74LS368buffers with zero PIA/VIA chip!
The Infamous Model I Expansion Interface (EI) & FD1771 Quirks
| Quirk / Upgrade | Bench Fix & Modern Solution |
|---|---|
Random Reboots & Screen of Death (Unbuffered 40-Pin Ribbon Cable) |
Early Model I Expansion Interfaces connected to the rear 40-pin keyboard edge card over an unbuffered ribbon cable that acted as an RF antenna and suffered from tin whisker/oxidation failures! Clean the 40-pin PCB edge fingers on both ends with a white eraser + DeoxIT, keep the ribbon under 15 cm, and check if the Tandy Diner / Buffered Cable Mod is installed. |
Single-Density FD1771 FDC vs Double-Density Percom Doubler / LNW |
Stock Model I EIs use a Western Digital FD1771-B01 (Single-Density FM only, 85 KB per 35-track disk, triple-rail −5V/+12V/+5V)! Plugging a Percom Doubler (FD1791) into the 40-pin FD1771 socket unlocks Double-Density (180KB) for LDOS / NewDOS/80 — or attach a modern FreHD / MISE / TRS-IO directly to the 40-pin expansion port! |
TRS-80 Model III & Model 4 / 4D / 4P 1980–1991 • All-in-One Chassis • TRSDOS / LDOS / CP/M
To eliminate the Model I's FCC RF interference and flaky Expansion Interface ribbon, the 1980 Model III integrated the 12″ CRT, keyboard, Z80 @ 2.03 MHz, 48 KB RAM, RS-232, and dual 5.25″ double-density drives (FD1793) into one shielded cabinet. In 1983, the Model 4 (and luggable Model 4P) upgraded the CPU to a 4.06 MHz Z80A, added an 80×24 screen mode (6845 CRTC), expanded RAM to 64 KB or 128 KB (4164 DRAM), and allowed bank-switching all ROM/video out of the way to run standard 0000h-based CP/M 2.2 and CP/M Plus (3.0) alongside TRSDOS 6 / LS-DOS!
Model III vs Model 4 (Non-Gate-Array vs Gate-Array)
- Model III (
1980–1983): Z80 @ 2.03 MHz, up to 48 KB RAM (24×4116triple-rail DRAMs in three 16K banks!), 14 KB Model III BASIC ROM (3× 2332/2364), 64×16 video (2× 2114SRAMs), and separate FDC/RS-232 daughterboards connected via internal flat flexible cables (FFCs). - Early Model 4 ("Non-Gate-Array" — 1983): Large mainboard with discrete TTL + PALs, 8× or 16×
4164(+5 V single-rail!) DRAMs (64K/128K), and separate FDC/RS-232 tower boards. - Late Model 4 / 4D ("Gate Array" — 1984–1991): Consolidates nearly all system glue logic AND the floppy controller/RS-232 onto a single unified PCB using two custom LSI Gate Arrays (
U65&U66) and aWD1773FDC!
The #1 Model III / Early Model 4 Fault: Brittle Internal Flat Flex Cables (FFCs)!
Inside every Model III and Non-Gate-Array Model 4, two white plastic Flat Flexible Cables (FFCs) link the main motherboard up to the Floppy Controller board and RS-232 board mounted on the vertical metal shield:
- After 40 years, the tin-plated contacts on these white flat flex strips delaminate, oxidise, and snap when unplugged — causing the computer to hang on
Cass?(indicating the ROM cannot see theFD1793floppy controller at I/O port$F0and falls back to cassette BASIC!). - Permanent Fix: Replace the crumbling flat flex strips and single-wipe friction sockets with standard IDC pin headers and grey multi-strand ribbon cables!
TRS-80 Color Computer 1 & CoCo 2 1980–1986 • Motorola 6809E • MC6883 SAM • MC6847 VDG
Born from Motorola and Tandy's 1979 "Green Thumb" Agotext terminal project in Austin, Texas, the TRS-80 Color Computer ("CoCo") pairs the advanced 8/16-bit Motorola MC6809E CPU (at 0.895 MHz, software-doubled to 1.79 MHz via the POKE 65495,0 high-speed poke!) with the MC6883 SAM and MC6847 VDG (the exact same chipset later adopted by the Welsh Dragon 32/64, though with different memory maps and PIA port assignments!).
CoCo 1 (26-3001) vs CoCo 2 (26-3134/3127) Revisions
- Early CoCo 1 (
Rev D / E— Dark Silver "Chiclet" Case): Originally shipped with 4 KB (8× 2104), 16 KB (8× 4116triple-rail), or 32 KB/64 KB (8× 4532/4164single-rail with jumper change!). - White-Case CoCo 1 (
F2Board — 64K): Transitional white case with melted-look/full-travel keyboard and single-rail4164RAM. - CoCo 2 (
26-3134A/B& Korean/US26-3127B): Compact white case eliminating the internal +12 V/−5 V rails entirely (uses two 18-pin4416(16K×4) or4464(64K×4) DRAMs!). Late CoCo 2B models use the enhancedMC6847T1VDG (which adds true lowercase descenders!).
CoCo 1 / 2 vs Dragon 32 Compatibility Note
| Subsystem | Tandy CoCo 1 / 2 | Dragon 32 / 64 |
|---|---|---|
| Video Output | RF Channel 3/4 ONLY on stock CoCo 1/2 (needs internal Composite/S-Video mod!) | 5-Pin DIN Composite A/V standard from factory |
| Joystick Ports | 6-Pin DIN (Analogue 100 kΩ pots, Pins 1–5 match Dragon!) | 5-Pin DIN (Analogue 100 kΩ pots) |
| Serial Port | 4-Pin DIN "Bit-Banger" (600–9600 baud software UART) | Parallel Centronics (D32) + 6551 Hardware RS-232 (D64) |
| Cartridge Slot | 40-Pin Side Slot (CoCoSDC / Multi-Pak Interface works on both!) | 40-Pin Side Slot (Minor pin differences on power/reset) |
Tandy Color Computer 3 (CoCo 3) 1986–1991 • 1.79 MHz 68B09E • GIME Chip • 128KB–2MB • OS-9 Level II
Engineered to compete with the Amiga and Atari ST while maintaining backward compatibility with CoCo 1/2 software, the Tandy CoCo 3 (26-3334) replaces both the MC6883 SAM and MC6847 VDG with the custom 68-pin PLCC GIME (Graphics Interrupt Memory Enhancer, 1986 / 1987 V1/V2) ASIC! The GIME unlocks hardware 1.79 MHz double-speed operation (without losing video!), an MMU supporting 128 KB stock RAM (expandable to 512 KB or 2 MB!) for multi-user/multitasking Microware OS-9 Level II / NitrOS-9, a 64-colour RGB/Composite palette, and 320×200 (16-colour) & 640×200 (4-colour) / 80-column text modes!
Key CoCo 3 Hardware Upgrades
- CPU (
U1— 2.0MHz Rated!): Motorola MC68B09EP (or upgraded to a HitachiHD63C09EP, which adds a second 16-bit accumulatorE/F→ 32-bitQregister, hardware divide, and native mode running 30% faster in NitrOS-9!) - GIME (
U6— PLCC-68): Custom Toshiba/MotorolaTC5030(80417651986 vs80426851987 revision) - Stock 128KB RAM (
U2–U5): Four 18-pin4464(64K×4) DRAMs (upgradeable via a plug-in 512 KB / 2 MB Boomerang / Triad / Cloud-9 RAM board!) - Built-in Composite A/V + Bottom 10-Pin Analog RGB Header (
J1)!
CoCo 3 Bench Tips & GIME Heatsinking
- Put a Heatsink on the 68-Pin PLCC GIME (
U6) Immediately: The irreplaceable GIME chip runs hot inside its brown PLCC-68 socket. Clean the PLCC socket contacts with DeoxIT and attach a 25 mm×25 mm copper heatsink to the top of the GIME! - CoCoSDC + NitrOS-9 + HD63C09EP = The Ultimate CoCo 3 Trifecta: Upgrading
U1to a CMOSHD63C09EP, fitting a 512 KB RAM board, and plugging a CoCoSDC into the side cartridge port lets you boot NitrOS-9 EOU (Ease of Use) into a full multi-window GUI in seconds!
Diagnostic Checklist: Z80 Garbage Screen & CoCo Faults
Pinpointing bad 2102 / 4116 RAM chips on the Model I/III and isolating 6809E / SAM / GIME faults on the CoCo 1/2/3.
Z80 Line: "Garbage Screen" (@9@9 Pattern) & 2102 / 4116 Locator
Just like the Commodore PET, the TRS-80 Model I and Model III use hardware TTL counters to scan their 1 KB static Video RAM ($3C00–$3FFF) independently of the Z80 CPU. If the screen fills with @ / 9 / random garbage and never prompts Memory Size? (or Cass?), your video circuit is working — the Z80 CPU cannot read ROM or Main RAM!
Model I Video SRAM (2102) & Main DRAM (4116) Bit Map
| Data Bit | Model I Video SRAM (2102) | Model I Main DRAM (4116) | Visual Screen Symptom (If Video 2102 Fails) |
|---|---|---|---|
| Bit 0 (D0) | Z48 | Z20 | Every space ($20) becomes ! ($21); odd/even ASCII shifted by 1 |
| Bit 1 (D1) | Z47 | Z19 | Spaces become " ($22); A becomes C |
| Bit 2 (D2) | Z46 | Z18 | Spaces become $ ($24); A becomes E |
| Bit 3 (D3) | Z45 | Z17 | Spaces become ( ($28); A becomes I |
| Bit 4 (D4) | Z61 | Z16 | Spaces become 0 ($30); numbers/letters swap |
| Bit 5 (D5) | Z62 | Z15 | Spaces become @ ($00); punctuation/letters swap |
| Bit 6 (D6) | Derived (or Mod Chip) | Z14 | Uppercase/lowercase bit (only present in Video RAM if lowercase mod installed) |
| Bit 7 (D7) | Z63 | Z13 | Entire screen turns into 2×3 semigraphics blocks (Bit 7 = 1 = Graphics)! |
Model I / III Key Bounce & Dead Keys: Early Model I and Model III keyboards use open-contact mechanical Alps switches (or capacitive/membrane on late units) that suffer from severe key bounce (MMEMORY SSIZE). Clean the switch contacts with DeoxIT D5 (or load the software KBDFIX debounce routine in Level II BASIC)!
CoCo 1 / 2 / 3: "Black Screen / Garbage Screen" & Keyboard Matrix
Isolating 6809E, SAM (MC6883), VDG (MC6847), GIME, and PIA (MC6821) failures on the Color Computer.
- 1. Green Screen with Black Border (
@Symbols or Garbage, NoOKPrompt):- On a CoCo 1 or CoCo 2, seeing the green screen with a black border proves your 14.318 MHz crystal, MC6883 SAM video counters, and MC6847 VDG are ALIVE!
- If the screen is filled with
@($00) or repeating characters, either one of the4116/4164/4416DRAMs has failed (see our Dragon 32$60RAM character decoder — CoCo Extended Color BASIC uses the exact same VDG screen codes!), OR the Color BASIC ROM (U28/IC15) has failed!
- 2. Blind
AUDIO ON : SOUND 100,5Test:- If the screen is garbled or dark, blind-type
SOUND 100,5and press ENTER. If you hear the tone through the TV, the 6809E CPU, SAM, ROM, RAM, and PIA are 100% working — only the video path (VDG/LM1889/Astec RF modulator) is faulty!
- If the screen is garbled or dark, blind-type
- 3. CoCo 1/2/3 Keyboard Mylar Ribbon &
6821 PIA #1(U8/IC19):- When an entire row or column of keys dies on a CoCo, first pull the flat Mylar keyboard ribbon out of the white PCB connector, trim 1.5 mm of cracked tip if needed, or buff the oxidised carbon/silver fingers gently on a sheet of plain white paper.
- If jumping the PCB keyboard header pins directly with a 470 Ω resistor still fails to register characters, swap the two socketed 40-pin
MC6821PPIAs (PIA #1at$FF00scans the keyboard & joysticks;PIA #2at$FF20controls VDG modes, 6-bit DAC sound, and RS-232/cassette)!
Model I Power Pinout, CoCo 1/2 Composite Mod & CoCo 3 Analog RGB
Wiring a modern bench PSU for the TRS-80 Model I, bypassing the CoCo 1/2 RF modulator, and wiring the CoCo 3's 10-pin RGB header to SCART/OSSC.
TRS-80 Model I Power (J4) & Video (J3) 2× 5-Pin DIN 180°
DANGER: The rear of the TRS-80 Model I has three identical 5-pin 180° DIN sockets right next to each other (POWER, VIDEO, and TAPE)! Plugging the power brick into the Video or Tape DIN socket will blow the output transistors immediately — always label them clearly! Viewed looking at the rear female POWER DIN socket on the Model I:
| DIN-5 Pin | Model I POWER Socket (J4) | Model I VIDEO Socket (J3) |
|---|---|---|
| Pin 1 | 14–16 V AC (Line 1) (Feeds +5V & −5V rectifiers) | NC |
| Pin 2 (Center) | GND (0 V DC Return) | +5 V DC Out (Opto-isolator supply for TRS-80 Monitor) |
| Pin 3 | 14–16 V AC (Line 2) | NC |
| Pin 4 | +19–22 V DC Unregulated (Feeds internal +12V 723 regulator) | GND (0 V Shield) |
| Pin 5 | NC | Composite Video + Sync Out (Wire Pin 5 + Pin 4 to Yellow RCA!) |
Connecting a Model I to Any Standard Composite Monitor: You don't need the original RCA-modified TRS-80 monitor! Simply wire Video DIN Pin 5 (Composite Signal) to the tip of a yellow RCA plug and Pin 4 (GND) to the outer shield (leave Pin 2 +5 V disconnected)!
CoCo 3 Bottom 10-Pin Analog RGB Header (J1) to SCART / OSSC 2×5 Box Header
Look on the underside of the Tandy CoCo 3 for the recessed 10-pin (2×5, 0.1″ IDC) header (J1, designed for the Tandy CM-8 RGB monitor). It outputs razor-sharp 15.7 kHz 60 Hz 0.7 V Analog RGB + TTL Sync + Audio:
CoCo 3 J1 Pin | Signal | SCART Target Pin & Resistor |
|---|---|---|
| Pin 1 & Pin 2 | GND (0 V Return) | SCART Pins 4, 5, 9, 13, 17, 18, 21 |
| Pin 3 | RED (Analog 0.7 V) | SCART Pin 15 (Direct) |
| Pin 4 | GREEN (Analog 0.7 V) | SCART Pin 11 (Direct) |
| Pin 5 | BLUE (Analog 0.7 V) | SCART Pin 7 (Direct) |
| Pin 6 | Audio Out (Mono) | SCART Pins 2 & 6 (Right & Left Audio) |
| Pin 7 | NC (Key Pin) | — |
| Pin 8 | HSYNC (TTL Horizontal Sync) | Combine H+V via 470 Ω resistors (or tap Composite Video RCA jack for CSYNC!) to SCART Pin 20 |
| Pin 9 | VSYNC (TTL Vertical Sync) | |
| Pin 10 | +5 V / +12 V Pull-up | SCART Pin 16 (RGB Blanking via 180 Ω resistor) |
Easiest CSYNC Trick for a CoCo 3 RGB SCART Cable: Because the CoCo 3 already has a dedicated yellow Composite Video RCA jack on the back panel right next to the audio RCA jack, you can feed that rear Composite RCA output straight into SCART Pin 20 (Sync In) alongside J1 Pins 3, 4, 5 (R, G, B) — zero H+V sync combiner circuitry required!
CoCo 1 / CoCo 2 Simple Composite Video Tap: Open the silver Astec RF modulator box on a CoCo 1 or CoCo 2: the 4-pin header entering the modulator carries Pin 1 = Audio, Pin 2 = +5V, Pin 3 = Composite Video, Pin 4 = GND. Buffer Pin 3 with a single 2N3904 emitter-follower (or drop-in MC-10/CoCo UAV board) for crisp composite video!
CoCo 4-Pin RS-232 (DriveWire), Cassette & 6-Pin Joystick Pinouts
Wiring a DriveWire serial-to-PC cable on the CoCo's 4-pin DIN SERIAL I/O port, plus cassette and analogue joystick pinouts.
CoCo 4-Pin DIN SERIAL I/O Port (DriveWire Server Cable!)
Every CoCo 1, 2, and 3 has a 4-pin DIN socket on the back labelled SERIAL I/O ("Bit-Banger" port, driven by PIA #2 PA1 TX / PB0 RX). Wiring it to a PC USB-to-RS232 serial adapter lets you run DriveWire 4 to mount virtual .DSK floppies, print, and access the internet at up to 57,600–115,200 baud (on a 6309/CoCo 3):
| CoCo 4-Pin DIN | CoCo Signal | PC 9-Pin D-Sub (DE-9 Female) Target |
|---|---|---|
| Pin 1 | /CD (Carrier Detect In) | DE-9 Pin 4 (DTR) or Pin 7 (RTS) |
| Pin 2 | RS-232 DATA IN (RX) | DE-9 Pin 3 (TXD Out from PC) |
| Pin 3 | GND (0 V Return) | DE-9 Pin 5 (GND) |
| Pin 4 | RS-232 DATA OUT (TX) | DE-9 Pin 2 (RXD In to PC) |
5-Pin DIN Cassette Port (Shared by Model I, Model III/4 & CoCo 1/2/3!)
Remarkably, Radio Shack used the exact same 5-pin 180° DIN cassette cable pinout across both the Z80 line (Model I/III/4) AND the 6809 CoCo 1/2/3 (Pin 1 = Remote Motor Relay A, Pin 2 = GND, Pin 3 = Remote Motor Relay B, Pin 4 = EAR / Audio In from Tape, Pin 5 = AUX / Audio Out to Tape)! When loading .CAS files from a smartphone/PC into a Model I, set volume to ~60–70% (Model I Level II is notoriously sensitive to volume clipping).
CoCo 6-Pin DIN Analogue Joystick Ports (LEFT & RIGHT)
See also our dedicated CoCo 3 10-Pin Analog RGB (J1), CoCo 4-Pin DIN DriveWire Serial, TRS-80 Model I 5-Pin DIN Power, and 34-Pin Shugart Floppy pages. Uses a 240° 6-pin DIN socket with 100 kΩ linear potentiometers wired between +5 V and GND (Pins 1–5 are identical to the 5-pin DIN on the Dragon 32/64, with Pin 6 adding the CoCo 3's second fire button!):
| DIN-6 Pin | CoCo Signal | Function |
|---|---|---|
| Pin 1 | X-Axis Wiper | 0–5 V analogue wiper from 100 kΩ horizontal pot (Left = 0 V, Right = 5 V) |
| Pin 2 | Y-Axis Wiper | 0–5 V analogue wiper from 100 kΩ vertical pot (Up = 0 V, Down = 5 V) |
| Pin 3 | GND (0 V) | Ground return for pots & both fire buttons |
| Pin 4 | /FIRE 1 (Button 1) | Primary fire button (active-low to Pin 3 GND; read on PIA0 PA0/PA1) |
| Pin 5 | +5 V Reference | +5 V supply for top end of both 100 kΩ joystick pots |
| Pin 6 (Center) | /FIRE 2 (Button 2 — CoCo 3) | Secondary fire/mouse button supported by the CoCo 3 GIME (PA2/PA3)! |
Deluxe RS-232 Program Pak / Multi-Pak Interface (MPI) Note: Plugging a CoCoSDC or Multi-Pak Interface (26-3024 / Mega-Mini-MPI) into the 40-pin right-hand Cartridge Slot gives you SD card disk emulation plus four switchable cartridge slots!
Chip & ROM / EPROM Reference
Interactive IC Pinout Finder cards for both Z80 and 6809 families plus GIME, SAM, VDG, and ROM adapter guides.
Zilog Z80 / Z80A
Drives the TRS-80 Model I (1.77 MHz), Model III (2.03 MHz), and Model 4 / 4P (4.06 MHz).
View Pinout →MC6809E / HD63C09E
Drives the CoCo 1 & CoCo 2 (0.895 MHz) and CoCo 3 (1.79 MHz MC68B09EP / HD63C09EP upgrade).
MC6821P PIA (×2)
Two 6821 PIAs in every CoCo 1, 2, and 3 ($FF00 Keyboard/Joystick & $FF20 VDG/DAC/Serial/Tape).
Motorola MC6847 VDG
CoCo 1 & CoCo 2 Video Display Generator (Y, phiA, phiB component outputs).
MC6883 / 74LS783 SAM
CoCo 1 & CoCo 2 Synchronous Address Multiplexer, E/Q clock generator, and DRAM controller.
View Pinout →4116 DRAM (16K×1)
Triple-rail (−5 V, +12 V, +5 V) DRAM in the Model I, Expansion Interface, Model III, and early CoCo 1.
View Pinout →4164 / 4464 DRAM
Single-rail (+5 V) DRAM in the Model 4/4P, late CoCo 1 (4164), CoCo 2 (4416/4464), and CoCo 3 (4× 4464).
CoCo & TRS-80 Custom Silicon Reference
| Chip | Models | Function & Modern Substitutes |
|---|---|---|
| MC6883 / SN74LS783 (SAM) | CoCo 1 & CoCo 2 (DIP-40) | Synchronous Address Multiplexer. Divides 14.318 MHz into E/Q clocks and refreshes DRAM. Replaceable with SN74LS785 or CPLD SAM module. |
| MC6847P / MC6847T1 (VDG) | CoCo 1 & CoCo 2 (DIP-40 / DIP-28) | Video Display Generator. Note: Late CoCo 2B's 28-pin MC6847T1 adds true lowercase and is NOT pin-compatible with the 40-pin MC6847P! |
GIME (TC5030 / 8041765) |
CoCo 3 (PLCC-68) | Integrates SAM, VDG, 512K/2MB MMU, 64-colour RGB/Composite video, and dual timers. Modern FPGA drop-in: GIME-X / GIME-Z! |
| WD FD1771 / FD1793 / WD1773 | Model I EI (1771), Model III/4 & CoCo FD-502 (1793/1773) |
Floppy disk controllers. Remember: FD1771 requires −5V, +5V, and +12V! |
TRS-80 & CoCo ROM / EPROM Substitution Guide
| System & ROM | Socket Size | EPROM Substitute & Adapter Notes |
|---|---|---|
Model I Level II BASIC (12KB) |
3× 24-Pin (2332 4K) or Ribbon Daughterboard |
Three 2532 (4 KB, 24-pin) EPROMs drop into 3-socket Rev G boards, or replace all three with a single 27C128 (16KB) adapter board (e.g. XLR8er / FreHD auto-boot ROM)! |
Model III / 4 ROMs (14KB) |
U104 (8K), U105 (4K), U106 (2K) (24-Pin) |
U104 (2364 8K) = MCM68766 or 2364-to-27C64 adapter; U105 (2332 4K) = 2532 direct; U106 (2KB) = 2716 direct! |
CoCo 1 / 2 Color BASIC & Extended BASIC (2×8KB) |
2× 24-Pin (2364 8K) |
Direct 24-pin MCM68764 / 68766 or 2364-to-27C64/27C128 adapter. |
CoCo 3 Super Extended BASIC (32KB) |
1× 28-Pin JEDEC (U2) |
27C256 (32 KB, 28-Pin) is a 100% Direct Drop-In! (Great for burning the HD6309-patched Super Extended BASIC ROM!). |
Resources & Further Reading
Blog posts, interactive bench tools, and essential TRS-80 & Tandy CoCo communities.
Smiggy's Z80 & 6809 Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — Z80, 6809, 6821, 4116, 4164 & more
- Connector Pinout Directory — Model I 5-Pin DIN Power, CoCo 3 10-Pin Analog RGB, CoCo 4-Pin DriveWire Serial, 6-Pin DIN Joystick & Cassette
- Recap & Repair Reference — capacitor replacement guide
- Ohm's Law Calculator
- Dragon 32/64 Hub — Dragon 32 & 64 reference
- C64 Machine Hub — Commodore 64 reference
- VIC-20 Machine Hub — Commodore VIC-20 reference
- Commodore PET Hub — PET 2001, 4000 & 8000 reference
- Plus/4 & C16 (TED) Hub — C16, C116 & Plus/4 reference
- C128 Machine Hub — C128, C128D & DCR reference
- SX-64 Machine Hub — Executive Portable C64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
- Atari ST Hub — 520ST, 1040STFM & STE reference
- Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- Acorn Atom & Electron Hub — Atom & Electron reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Sinclair QL Hub — 68008, Microdrive & ULA reference
- Oric-1 & Atmos Hub — Oric 6502A & ULA reference
- Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
- Amstrad CPC 6128 Hub — 128K & 3″ disc reference
- MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
- Osborne 1 & Kaypro II Hub — CP/M luggable reference
- TI-99/4A Machine Hub — 16-bit TMS9900 & PEB reference
External TRS-80 & CoCo Resources
- Ira Goldklang's TRS-80 Revived Site — the premier TRS-80 Model I, III, and 4 hardware & service manual archive
- The Color Computer Archive — complete CoCo 1, 2, and 3 schematics, ROMs,
.DSKimages, and technical manuals - The NitrOS-9 Project — open-source multi-tasking 6809/6309 operating system for CoCo 1/2/3
- VCFed Tandy / Radio Shack Forum — active Z80 and CoCo hardware repair community
TRS-80 & Tandy CoCo — Capacitor & Recap Reference
Board revisions, electrolytic capacitor lists, voltage/farad ratings, and Through-Hole servicing notes.
TRS-80 & Tandy CoCo (1 / 2 / 3) — Tandy CoCo 1/2/3 & TRS-80 Model I/III Mainboards
MC6809E Color Computer & Z80 TRS-80 mainboards • Mount: Through-hole • Difficulty: Beginner friendly
Tandy Color Computer 2 / 3 Mainboard & Power Stage (7 Capacitors)
| Ref | Capacitance | Voltage | Package / Type | Circuit Location & Bench Notes |
|---|---|---|---|---|
C1 |
4700 uF | 16 V | Radial / Axial | |
C2 |
1000 uF | 25 V | Radial | |
C3 |
470 uF | 25 V | Radial | |
C4 |
100 uF | 16 V | Radial | |
C12 |
10 uF | 25 V | Radial | |
C24 |
22 uF | 25 V | Radial | |
C31 |
47 uF | 16 V | Radial |
Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))
TRS-80 & Tandy CoCo — System Bench Tools & IC Pinouts
Click any chip below to open its interactive DIP/PLCC pinout diagram, pin-by-pin signal table, and oscilloscope test waveforms.
74LS00
SN74LS00N / DM74LS00N
Quad 2-input NAND gate — the most common glue-logic part in retro computers.
Open Interactive Pinout →74LS04
SN74LS04N / DM74LS04N
Hex inverter — six independent NOT gates in one 14-pin DIP, the standard logic-level converter of the 74-series family.
Open Interactive Pinout →74LS138
SN74LS138N / DM74LS138
3-to-8 line decoder with three enable inputs and eight active-low outputs — the classic memory-address decoder.
Open Interactive Pinout →74LS153
SN74LS153N / DM74LS153 / 74HC153
Dual 4-to-1 line data selector/multiplexer — multiplexes Z80 CPU and 6845 CRTC Row/Column addresses into the 4164 DRAMs on the Amstrad CPC 464 (IC109 & IC110).
Open Interactive Pinout →74LS32
SN74LS32N / DM74LS32N
Quad 2-input OR gate — same 14-pin pinout as the 74LS00, with outputs high when either input is high.
Open Interactive Pinout →74LS74
SN74LS74AN / DM74LS74A
Dual D flip-flop with asynchronous preset and clear (both active low) — the standard edge-triggered latch and divider of the 74-series.
Open Interactive Pinout →CD4017
CD4017BC / MC14017 / HEF4017
Johnson decade counter with 10 decoded outputs — Q0-Q9 scattered around the package, not in pin order.
Open Interactive Pinout →Motorola MC6809
MC6809 / MC68A09 / MC68B09 / HD6809
The most capable 8-bit CPU of its generation — position-independent code, hardware multiply, and a clean contiguous bus.
Open Interactive Pinout →Zilog Z80
Z8400 / Z84C00
The 8-bit CPU behind the ZX Spectrum, MSX, Amstrad CPC and countless CP/M machines.
Open Interactive Pinout →Motorola MC6821 PIA
MC6821 / EF6821 / MC6820 / MOS 6520
Two 8-bit I/O ports with two interrupt lines each — the 6800-family peripheral chip, half-shared with the 6522 and 6526.
Open Interactive Pinout →Motorola MC6883 / 74LS783 SAM
MC6883P / SN74LS783N / SN74LS785N
The Synchronous Address Multiplexer of the Dragon 32/64 and CoCo 1/2 — generates E/Q CPU clocks, interleaved VDG/CPU DRAM cycles, and S0-S2 device chip selects.
Open Interactive Pinout →Motorola MC6845 CRTC
MC6845 / HD6845 / HD6845S / UM6845 / MC6835
The CRT controller behind the Amstrad CPC, BBC Micro, IBM MDA and CGA — two opposing address buses and one register-select pin.
Open Interactive Pinout →Motorola MC6847 VDG
MC6847P / MC6847Y / S68047
The Video Display Generator behind the Dragon 32/64, Tandy Color Computer 1/2, Acorn Atom and Mattel Aquarius — outputs native Y, R-Y and B-Y component video.
Open Interactive Pinout →2114 1K x 4 SRAM
2114 / 2114L / F2114 / TMM314 / UM2114
1024 x 4 static RAM — data pins numbered from ONE (DQ1-DQ4, no DQ0), used in pairs for byte-wide storage.
Open Interactive Pinout →4116 16K x 1 DRAM
4116 / MK4116 / TMS4116 / MCM4116 / uPD416
16K x 1 DRAM with FOUR supply pins and THREE rails. A 4164 fitted here receives +12 V on its 5 V pin and will not survive.
Open Interactive Pinout →4164 64K x 1 DRAM
4164 / MT4264 / TMS4164 / MB8264 / KM4164 / 41464 (x4)
64K x 1 DRAM, single +5 V. VCC is pin 8 and GND is pin 16 — the reverse of almost every other 16-pin DIP. Apply 5 V to pin 16 and it dies.
Open Interactive Pinout →4416 / 4464 (16K/64K x 4) DRAM
TMS4416 / MB81416 / TMS4464 / MT4067 / KM41464
The 18-pin nibble-wide (x4) DRAM family — just TWO chips replace eight 4116/4164 DRAMs! Upgrading 4416 to 4464 unlocks 64KB on the C16, C128 VDC, CoCo 2 and Atari 600XL.
Open Interactive Pinout →6264 8K x 8 SRAM
6264 / HM6264 / 5564 / TC5565 / UM6264
8K x 8 SRAM, the JEDEC 28-pin byte-wide standard. Two chip selects of opposite polarity — leave CS2 floating and it never responds.
Open Interactive Pinout →Maxim / TI MAX232
MAX232 / MAX232A / MAX232I / SP232 / ICL232
Dual RS-232 driver/receiver — generates +/-10 V from a single +5 V rail. Six of sixteen pins carry no signal (charge pump and caps).
Open Interactive Pinout →Signal & Trace Visualizer (Scope)
Compare healthy vs. faulty CPU clocks, reset ramps, PLA strobes, and 5V PSU ripple waveforms.
Disassembler & ROM LabRetro File Utilities (Rust/Wasm)
6502/6510 disassembler with IC register links, C64 .D64 parser, and EPROM CRC32 verifier/patcher.
Logic & PinoutsConnector & Port Pinout Reference
59 interactive RGB SCART, DIN A/V, PSU polarity, joystick, floppy, and expansion bus pinouts.
Logic & PinoutsLogic & RAM Substitution Guide
Cross-reference 4116→4164 DRAM mods, 2364→27C256 EPROM adapters, and 74LS/74HCT swaps.