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 (`26-1001` External Brick): Outputs 14–16 V AC AND ~19 V DC unregulated over a 5-pin DIN plug! All `+5V`, `+12V`, and `−5V` regulation happens inside the Model I keyboard unit (for the eight `4116` DRAMs!). 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` (`7805` on large rear heatsink), `+12V`, and `−5V` (for 4116 RAM on early CoCo 1s) or single-rail `+5V` (`4164` RAM 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`, omitting `D6`!) stored in seven 16-pin `2102` (1K×1) SRAMs! (Adding an 8th `2102` + a toggle switch is the famous Model I Lowercase Mod!).
- Character Generator (`Z29`): Motorola `MCM6670P` / `MCM6674P` (shifted vs unshifted descenders)
- Memory-Mapped Keyboard (`$3800–$38FF`): Read directly onto the Z80 data bus via `74LS368` buffers 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× `4116` triple-rail DRAMs in three 16K banks!), 14 KB Model III BASIC ROM (`3× 2332/2364`), 64×16 video (`2× 2114` SRAMs), 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 a `WD1773` FDC!
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 the `FD1793` floppy controller at I/O port `$F0` and 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× 4116` triple-rail), or 32 KB/64 KB (`8× 4532/4164` single-rail with jumper change!).
- White-Case CoCo 1 (`F2` Board — 64K): Transitional white case with melted-look/full-travel keyboard and single-rail `4164` RAM.
- CoCo 2 (`26-3134A/B` & Korean/US `26-3127B`): Compact white case eliminating the internal +12 V/−5 V rails entirely (uses two 18-pin `4416` (`16K×4`) or `4464` (`64K×4`) DRAMs!). Late CoCo 2B models use the enhanced `MC6847T1` VDG (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 Hitachi `HD63C09EP`, which adds a second 16-bit accumulator `E/F` → 32-bit `Q` register, hardware divide, and native mode running 30% faster in NitrOS-9!)
- GIME (`U6` — PLCC-68): Custom Toshiba/Motorola `TC5030` (`8041765` 1986 vs `8042685` 1987 revision)
- Stock 128KB RAM (`U2–U5`): Four 18-pin `4464` (`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 `U1` to a CMOS `HD63C09EP`, 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, No `OK` Prompt):
- 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 the `4116`/`4164`/`4416` DRAMs has failed (see our Dragon 32 `$60` RAM character decoder — CoCo Extended Color BASIC uses the exact same VDG screen codes!), OR the Color BASIC ROM (`U28` / `IC15`) has failed!
- 2. Blind `AUDIO ON : SOUND 100,5` Test:
- If the screen is garbled or dark, blind-type
SOUND 100,5and press ENTER. If you hear the tone through the TV, the 6809E CPU, SAM, ROM, RAM, and PIA are 100% working — only the video path (`VDG` / `LM1889` / `Astec RF modulator`) is faulty!
- If the screen is garbled or dark, blind-type
- 3. CoCo 1/2/3 Keyboard Mylar Ribbon & `6821 PIA #1` (`U8`/`IC19`):
- When an entire row or column of keys dies on a CoCo, first pull the flat Mylar keyboard ribbon out of the white PCB connector, trim 1.5 mm of cracked tip if needed, or buff the oxidised carbon/silver fingers gently on a sheet of plain white paper.
- If jumping the PCB keyboard header pins directly with a 470 Ω resistor still fails to register characters, swap the two socketed 40-pin `MC6821P` PIAs (`PIA #1` at `$FF00` scans the keyboard & joysticks; `PIA #2` at `$FF20` controls 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).
View Pinout →MC6821P PIA (×2)
Two 6821 PIAs in every CoCo 1, 2, and 3 (`$FF00` Keyboard/Joystick & `$FF20` VDG/DAC/Serial/Tape).
View Pinout →Motorola MC6847 VDG
CoCo 1 & CoCo 2 Video Display Generator (`Y`, `phiA`, `phiB` component outputs).
View Pinout →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`).
View Pinout →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
- 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, `.DSK` images, 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