Commodore 128 Machine Hub
C128, C128D & C128DCR workbench reference — dual-CPU Z80A/8502 boot diagnostics, 80-column RGBI VDC & 64K VRAM upgrade, 5-pin square DIN PSU pinouts, and 1571 drive care.
C128 5-Pin Square DIN PSU & C128D Internal Supply Check
Unlike the C64's epoxy-potted "brick of death", the flat C128's heavy external power supply uses a much safer switching/thyristor +5 V stage rated at 4.3 A (+5 V DC) and 1.0 A (9 V AC) via a unique 5-pin Square DIN Power Connector. However, after 40 years the internal Crowbar SCR (thyristor), filter electrolytics, and bridge rectifier degrade — and on the C128D / C128DCR, the internal PSU contains RIFA-style mains filter caps and aging 5 V/12 V regulators that must be inspected before powering on!
1 5-Pin Square DIN Pinout (Flat C128)
Viewed looking into the male plug on the PSU cable (or solder side of the female motherboard socket):
| Square DIN Pin | Signal | Spec & Tolerance |
|---|---|---|
| Pin 1 (Top-Left) | +5 V DC | 4.90–5.15 V DC @ 4.3 A |
| Pin 2 (Bottom-Left) | GND (0 V) | DC Digital Return Ground |
| Pin 3 (Center) | 9 V AC (Line 1) | 9.0–10.5 V AC @ 1.0 A (TOD clock, SID/VIC +12V VR1 & User Port) |
| Pin 4 (Bottom-Right) | NC (Key / Shield) | Unconnected / Shield |
| Pin 5 (Top-Right) | 9 V AC (Line 2) | 9 V AC Return (fused inside PSU) |
2 Why Missing 9V AC Stops the C128
- Internal +12 V Regulator (VR1, 7812): On the flat C128, the 9 V AC rail is voltage-doubled and rectified on the motherboard to feed a 7812 (+12 V) regulator that powers the 6581 SID, the 8502/VIC-IIe clock circuitry, and the Z80A clock buffer!
- Blown 9V AC Fuse = Dead Machine: If the internal 1.6 A glass fuse inside the PSU brick blows on the 9 V AC winding, the C128's red power LED will still light up (from +5 V), but the machine will sit on a completely black screen with no audio or clock.
- Always measure both +5 V DC and 9 V AC before tearing down the motherboard!
3 C128D & C128DCR Internal PSU Notes
- C128D (Plastic, European): Contains a noisy 60 mm cooling fan and an internal switching PSU generating +5 V DC, +12 V DC (for the 1571 floppy drive motor & SID), and 9 V AC. Swap the noisy fan for a quiet 12 V Noctua/Sunon equivalent.
- C128DCR (Metal Case): Uses an open-frame switching PSU (with no fan installed from factory!). Inspect the X2 mains filter capacitor for cracks and recap the secondary 105 °C low-ESR smoothing caps to prevent +5 V/12 V drift during 1571 disk seeks.
Multi-Mode Architecture & Model Selector
Explore Bil Herd's three-in-one computer architecture and compare the Flat C128, C128D (Plastic), and C128DCR (Metal) hardware revisions.
C128 Native Mode (BASIC 7.0)
Runs on the MOS 8502 CPU at 1.02 MHz (40-col VIC-IIe active) or 2.04 MHz FAST mode (when the 40-column screen is blanked and using the 80-column MOS 8563/8568 VDC). Uses the MOS 8722 MMU to bank-switch 128 KB of system RAM (two 64K banks) plus 64 KB of ROM (BASIC 7.0, Kernal, Editor, Character, and internal/external Function ROMs).
C64 Compatibility Mode
Entered by holding the C= (Commodore) key at power-on, typing GO 64 in BASIC 7.0, or inserting a cartridge that pulls /GAME or /EXROM low. The 8722 MMU locks memory to Bank 0, maps in the onboard 16 KB C64 BASIC 2.0 + Kernal ROM, disables the 2 MHz clock and keypad/VDC registers, and achieves near-100% hardware-level C64 compatibility.
CP/M Plus (3.0) Mode
Wait — the Zilog Z80A is actually the FIRST CPU to wake up on every C128 boot! At power-on, the Z80A runs for ~1 millisecond to check for a C= keypress or a CP/M boot sector before handing the bus over to the 8502. When booting a CP/M 3.0 disk, the Z80A takes over at 4 MHz (effective 2 MHz bus speed) while calling the 8502 for low-level I/O when needed.
Commodore 128 Flat ("Wedge") ASSY 310379 • 1985–1989
The classic single-piece desktop console with built-in numeric keypad, dedicated cursor keys, 40/80 DISPLAY locking switch, and full heavy RF shielding plate that doubles as the heatsink for the major MOS VLSI chips.
Key ICs & Board Designators
- U6 (Main CPU): MOS 8502 @ 1.02 MHz / 2.04 MHz
- U10 (CP/M CPU): Zilog / SGS Z80A @ 4.00 MHz (2 MHz effective)
- U7 (MMU): MOS 8722 Memory Management Unit (48-pin DIP)
- U11 (PLA): MOS 8721 Programmable Logic Array (48-pin DIP)
- U21 (40-Col Video): MOS 8564 (NTSC) / 8566 (PAL) VIC-IIe (48-pin DIP)
- U22 (80-Col Video): MOS 8563R7A / R8 / R9 VDC (48-pin DIP) + 16 KB VRAM (2× 4416 at U23/U25)
- U5 (Audio): MOS 6581 / 6581R4 SID (12 V filter)
- U1 & U4 (I/O): 2× MOS 6526A-1 (2 MHz-rated) CIA chips
- U38–U53 (System RAM): 16× 4164 (64K×1) DRAMs (Bank 0 = U38–U45, Bank 1 = U46–U53)
- U32–U36 (ROMs): Four 16 KB ROM sockets (U32–U35) + empty U36 Function ROM socket
Flat C128 Bench & Capacitor Notes
| Ref / Area | Spec | Notes |
|---|---|---|
| RF Shield Heatsink Fingers | Spring metal tabs | Never run a Flat C128 for extended periods with the metal RF shield removed unless you attach stick-on aluminium heatsinks to U21 (VIC-IIe), U22 (8563 VDC), U5 (6581 SID), U7 (8722 MMU), and U11 (8721 PLA)! |
| C106, C107 (Power Input) | 2200 μF 16 V, 470 μF 25 V | Main +5 V and 9 V AC/12 V rectifier filter electrolytics near VR1 (7812). |
| U28 (8701 Clock Gen) | MOS 8701 (DIP-16) | Generates dot clock and colour clock for the VIC-IIe. Shared part with C64 ASSY 250425/250466. |
6526A-1 Speed Grade: Notice the -1 (2 MHz) rating on the C128's 6526A CIA chips — because the C128 supports fast serial IEC burst transfers, standard 1 MHz 6526 pulls from early C64s may occasionally drop bits in fast burst mode, though they work fine for standard diagnostics.
Commodore 128D (Plastic Case • European Release) ASSY 310379 + 1571 Board • 1985–1986
Sold primarily in Germany and Europe, the plastic-cased C128D features a carrying handle on the left side, a detachable external keyboard (storing neatly beneath the chassis), and an internal double-sided 1571 floppy drive. Internally, it literally mounts a standard Flat C128 motherboard (ASSY 310379) alongside a standalone 1571 controller PCB!
Key Hardware Characteristics
- Main Board: Identical to Flat C128 (ASSY 310379) with 16× 4164 DRAMs and 16 KB VDC RAM (8563)
- Drive Board: Separate 1571 PCB with its own 6502A CPU, 6522 VIAs, 6526 CIA, and WD1770/1772 MFM controller
- Audio: MOS 6581 / 6581R4 SID
- Cooling: 60 mm rear exhaust fan (notoriously loud in stock configuration)
C128D Plastic Service Priorities
| Subsystem | Common Issue & Fix |
|---|---|
| 1571 Upper Read Head (Side 1) | NEVER power on or leave a C128D stored without a floppy disk or cardboard shipping protector clamped inside the drive! Without a disk separating the two spring-loaded gimbal heads, the upper ceramic head slams against the lower head, snapping the fragile beryllium-copper suspension spring. |
| Detachable Keyboard DB25 | Uses a 25-pin D-sub cable on the front/side. Broken wires at the strain relief cause dead rows or missing 40/80 key state. |
| 16K VDC VRAM | Like the Flat C128, only 16 KB (two 4416s) is fitted to the 8563 VDC by default — easily upgraded to 64 KB by swapping in two 4464s! |
Commodore 128DCR (Cost-Reduced Metal Case) ASSY 250477 • 1986–1989
The definitive desktop Commodore 128 released in both North America and Europe. Integrates the main C128 motherboard and the entire 1571 disk drive logic onto a single unified PCB (ASSY 250477) inside a stamped steel desktop case, upgrades the 80-column video chip to the HMOS-II MOS 8568 VDC with a full 64 KB of video RAM factory-installed, and fits the cleaner MOS 8580 SID!
Key Upgrades on ASSY 250477
- VDC (U22): MOS 8568 (HMOS-II) with 64 KB VRAM (2× 4464 DRAMs factory-installed)
- SID (U5): MOS 8580R5 (9 V supply, 22 nF filter caps, dramatically lower noise floor)
- System RAM: Condensed from 16× 4164 chips down to 4× 41464 (64K×4) DRAMs (U38–U41)
- System ROMs: Condensed from four 16 KB ROMs into two 32 KB ROMs (28-pin JEDEC) + revised 1986 BASIC 7.0 bug-fix ROM
- Integrated 1571CR Logic: Uses MOS 5710 gate array + WD1770/1772 right on the mainboard
C128 Flat vs C128D Plastic vs C128DCR Comparison
| Feature | C128 Flat | C128D (Plastic) | C128DCR (Metal) |
|---|---|---|---|
| Mainboard ASSY | 310379 | 310379 + 1571 PCB | 250477 (Unified) |
| System RAM Chips | 16× 4164 (64K×1) | 16× 4164 (64K×1) | 4× 41464 (64K×4) |
| VDC Chip & VRAM | 8563 + 16 KB (4416) | 8563 + 16 KB (4416) | 8568 + 64 KB (4464) |
| SID Sound Chip | 6581 (12 V) | 6581 (12 V) | 8580 (9 V) |
| ROM Socket Count | 4× 16K + U36 | 4× 16K + U36 | 2× 32K + U36 |
8563 vs 8568 Warning: Although both are 48-pin DIPs, the 8563 and 8568 are NOT pin-compatible! Never swap an 8563 into a C128DCR or vice versa.
Diagnostic Checklist & Common Faults
Tracing the Z80A-to-8502 boot handoff, 8722 MMU / 8721 PLA failures, and dual-monitor video faults.
Black Screen & Dual-CPU Boot Handoff Tree
The #1 mistake when repairing a dead C128 is forgetting that if the Z80A CPU (U10) is dead or lacks its clock, the 8502 CPU (U6) will NEVER start — not even in C64 mode!
Step-by-Step Boot Signal Trace
- 1. Verify +5 V and +12 V (VR1 output): Without +12 V from VR1 (derived from the 9 V AC PSU pins), the clock circuitry and Z80A clock driver cannot oscillate.
- 2. Check Z80A (U10) First:
- At power-on, the 8722 MMU asserts
Z80ENso the Z80A executes the boot code at$0000in the Kernal ROM. - Probe Z80A Pin 6 (CLK) for a 4 MHz gated clock and Pin 26 (/RESET) for a clean low-to-high transition.
- A dead Z80A, cracked Z80A socket, or failed 7406/74LS74 clock buffer leaves the 8502 permanently held off the bus (
AEClow).
- At power-on, the 8722 MMU asserts
- 3. Check the 8722 MMU (U7) & 8721 PLA (U11):
- Probe 8502 Pin 1 (Φ0 Clock In) for 1.02 MHz and Pin 38 (AEC). After ~1 ms, the Z80A writes to MMU register
$D505to enable the 8502 and put itself to sleep (/BUSRQlow). - Reseat the 48-pin sockets for U7 (MMU) and U11 (PLA) — oxidised single-wipe pins on U7/U11 cause 50% of intermittent C128 black screens!
- Probe 8502 Pin 1 (Φ0 Clock In) for 1.02 MHz and Pin 38 (AEC). After ~1 ms, the Z80A writes to MMU register
- 4. Hold Down the STOP Key on Power-Up: Holding STOP during cold boot forces the C128 into the built-in Machine Language Monitor (
$B000). If the Monitor appears on screen, your hardware and Kernal ROM are working, and only the BASIC 7.0 ROM (U32/U33) or zero-page RAM is faulty!
Dual Video Troubleshooting (VIC-IIe 40-Col vs VDC 80-Col)
Because the C128 has two completely independent video subsystems, testing both the 8-pin DIN (40-col) and DB9 (80-col) ports immediately cuts your troubleshooting time in half!
| 40-Col (VIC-IIe) | 80-Col (VDC) | Diagnosis & Fault Location |
|---|---|---|
| Dead / Black | Working! (Press 40/80 + Reset) |
CPU, MMU, PLA, RAM, and ROMs are 100% healthy! Fault is isolated to U21 (8564/8566 VIC-IIe), U28 (8701 clock gen), or the RF/A/V modulator path. |
| Working! | Dead / Garbled | Main system is healthy! Fault is isolated to U22 (8563/8568 VDC), its 16 MHz oscillator circuit, or the two dedicated VDC RAM chips (U23 & U25, 4416/4464). |
| Garbage Chars | Working! | Bad Character ROM (U18), bad CIA#2 (U4, VIC bank select VA14/VA15), or bad Bank 0 DRAM (U38–U45). |
| Out of Memory / Corrupt | Out of Memory | Faulty System DRAM in Bank 0 (U38–U45) or Bank 1 (U46–U53), or failed 74LS257 multiplexers (U14/U15). |
System RAM Bit-to-IC Map (Flat C128 ASSY 310379):
• Bank 0 (Lower 64K — C64 & C128 Program RAM): Bit 0 = U38, Bit 1 = U39, Bit 2 = U40, Bit 3 = U41, Bit 4 = U42, Bit 5 = U43, Bit 6 = U44, Bit 7 = U45.
• Bank 1 (Upper 64K — C128 Variables RAM): Bit 0 = U46, Bit 1 = U47, Bit 2 = U48, Bit 3 = U49, Bit 4 = U50, Bit 5 = U51, Bit 6 = U52, Bit 7 = U53.
16K to 64K VDC Video RAM Upgrade Guide
Unlock 640×480 interlaced 80-column high-resolution colour graphics, Basic8, and GEOS 128 80-column desktop on Flat C128 & C128D Plastic boards.
1 How the Upgrade Works
The MOS 8563 VDC on the Flat C128 and C128D Plastic has its own private video memory bus (completely separate from the 128 KB system RAM) populated from the factory with two 4416 (16K×4, 18-pin DIP) DRAM chips at U23 and U25 (giving 16 KB total VRAM). Fortunately, Commodore routed all the address lines on the PCB for 4464 (64K×4, 18-pin DIP) DRAMs — making this a clean, zero-wire swap!
| Component | Stock 16K VDC | Upgraded 64K VDC |
|---|---|---|
| U23 & U25 | 2× 4416 (16K×4, DIP-18) | 2× 4464 / TMS4464 / MB81464 (64K×4, 120 ns or faster, DIP-18) |
| VDC Register 28 Bit 4 | 0 (16K Addressing Mode) | 1 (64K Addressing Mode — set via software or BASIC poke) |
2 Installation & BASIC Verification
- Desolder U23 & U25: Carefully desolder the two 18-pin 4416 DRAMs located directly beside the 8563 VDC (U22) using a vacuum desoldering pump or low-melt alloy.
- Fit Dual-Wipe / Turned-Pin 18-Pin Sockets: Solder two high-quality 18-pin IC sockets at U23 and U25, clean off flux with IPA, and insert two matching 4464 (64K×4) DRAMs (ensure the Pin 1 notch faces the same direction as the silkscreen!).
- Test in BASIC 7.0: Switch to 80-column display mode and run this 3-line test to enable 64K VDC addressing (Register 28) and verify that upper VRAM retains data without mirroring into lower VRAM:
10 SYS DEC("CDCC"), 63, 28 : REM SET VDC TO 64K MODE 20 SYS DEC("CDCC"), 85, 18 : SYS DEC("CDCC"), 0, 19 30 SYS DEC("CDCC"), 42, 31 : PRINT "64K VDC UPGRADE OK!"
Video, Drive & Serial Port Pinouts
DB9 80-column RGBI + Monochome composite, 8-pin DIN 40-column S-Video/Composite, Burst-Mode IEC, and internal 1571 drive headers.
80-Column VDC RGBI Output DB9 Female
See also our dedicated 5-Pin Square DIN Power, 8-Pin DIN 40-Column A/V, Fast Burst IEC Serial, and 44-Pin Cartridge Slot pages. Outputs 15.7 kHz digital TTL RGBI (16 colours, identical to IBM CGA with hardware brown fix inside Commodore 1084/1902A monitors) PLUS a convenient analog Monochrome Composite output on Pin 7 that plugs straight into any standard yellow RCA TV input!
| DB9 Pin | Signal | Level & Converter Notes |
|---|---|---|
| 1 | GND | Ground return |
| 2 | GND | Ground return |
| 3 | RED | 5 V TTL Digital Red |
| 4 | GREEN | 5 V TTL Digital Green |
| 5 | BLUE | 5 V TTL Digital Blue |
| 6 | INTENSITY (I) | 5 V TTL Brightness bit |
| 7 | MONO VIDEO | 75 Ω Monochrome Luma+Sync! Wire Pin 7 + Pin 1 directly to an RCA plug for instant 80-col greyscale on any TV! |
| 8 | HSYNC | 5 V TTL Horizontal Sync (~15.7 kHz) |
| 9 | VSYNC | 5 V TTL Vertical Sync (50/60 Hz) |
Converting Digital RGBI to SCART / HDMI: You cannot wire TTL RGBI (0–5 V) straight to an analog 0.7 V SCART TV without a resistor/DAC network. For pixel-perfect HDMI, use an RGBtoHDMI (with 12-bit/RGBI hat); for analog SCART/OSSC/RetroTINK, use a dedicated C128 RGBI-to-RGBS DAC adapter (which also implements the CGA dark-yellow → brown decoder).
40-Column VIC-IIe 8-Pin DIN A/V Port 8-Pin DIN (262° Horseshoe)
Identical to the late C64 / C64C 8-pin horseshoe DIN except Pin 7 carries a 2 MHz clock on some boards — always verify your S-Video cable uses Pin 1 (Luma) and Pin 6 (Chroma) with a 300 Ω series resistor on Chroma!
| DIN Pin | Signal | Cable Target |
|---|---|---|
| 1 | Luminance / Sync (Y) | S-Video Y (Pin 3) |
| 2 | GND | Video & Audio Ground |
| 3 | Audio Out (SID) | White/Red RCA Audio (Mono) |
| 4 | Composite Video Out | Yellow RCA Composite Video |
| 5 | Audio In (to SID Ext) | External audio input into SID filter |
| 6 | Chrominance (C) | S-Video C (Pin 4) via 300 Ω series resistor |
| 7 | NC / Clock | Leave unconnected in A/V cables |
| 8 | NC | Leave unconnected |
40/80 Column Switcher Mods: Projects like the S-Video + RGBI Auto-Switcher read the state of the C128's 40/80 key or active display flag to automatically switch your monitor between VIC-IIe and VDC outputs.
Fast Burst-Mode 6-Pin DIN IEC Serial Port & Internal 1571 Drive Interface
6-Pin DIN IEC Serial Port (Burst-Capable)
In C128 mode with a 1571 or 1581 drive, the C128 uses Pin 1 (/SRQ IN) and CIA#1's hardware synchronous serial shift register to transfer data at ~5,200 bytes/sec (over 12× faster than stock C64 IEC!):
| Pin | Signal | Burst Mode Role |
|---|---|---|
| 1 | /SRQ IN | Fast Burst Clock! Must be connected end-to-end in your 6-pin DIN cable (some cheap 5-wire C64 cables omit Pin 1, breaking 1571 Burst Mode!). |
| 2 | GND | Signal ground |
| 3 | /ATN | Attention strobe |
| 4 | /CLK | Standard IEC Clock |
| 5 | /DATA | Standard & Burst Serial Data |
| 6 | /RESET | Drive Reset |
C128D / C128DCR Internal 1571 Drive Notes
- Device ID Switching (Drive 8 vs Drive 9): On the C128D/DCR, the internal 1571 defaults to Device 8. To boot from an external SD2IEC or Pi1541 as Device 8, either run software unit swapping (`OPEN 15,8,15,"U0>"+CHR$(9):CLOSE 15`) or wire a toggle switch to the internal device-select jumper pads (J1/J2).
- 300 RPM Spindle Speed & Index Sensor: The 1571 uses a direct-drive brushless motor (on Alps/Newtronics mechanisms) and an optical index hole sensor required for reading CP/M MFM disks. Clean the index LED/phototransistor window with a dry cotton bud if CP/M disks fail to boot while standard GCR C64 disks still load.
- JiffyDOS 128 Upgrade: Requires upgrading both the C128 Kernal ROM (plus C64-mode Kernal ROM) AND the 1571 drive DOS ROM (32 KB 27C256).
Chip & Function ROM Reference
Interactive IC Pinout Finder cards plus C128 custom VLSI and U36 Internal Function ROM EPROM guide.
MOS 8502 CPU
U6 — 1 MHz / 2 MHz 6510-compatible processor with a 7-bit built-in I/O port (bit 6 drives the C128 CAPS LOCK / ASCII/DIN sense).
View Pinout →Zilog Z80A CPU
U10 — Co-processor handling initial cold-boot mode detection and CP/M Plus 3.0 execution.
View Pinout →MOS 6526A-1 CIA
U1 & U4 — 2 MHz Complex Interface Adapters managing keyboard, extended keypad, joysticks, User Port, and Burst IEC.
View Pinout →MOS 6581 SID
U5 (Flat C128 & C128D Plastic) — 3-voice analog/digital hybrid sound synthesizer (+12 V VDD).
View Pinout →MOS 8580R5 SID
U5 (C128DCR Metal) — HMOS-II revised SID (+9 V VDD) with improved combined waveforms and clean filter.
View Pinout →4164 DRAM (×16)
U38–U53 — Single-rail +5 V 64K×1 DRAMs organized into Bank 0 (U38–U45) and Bank 1 (U46–U53).
View Pinout →MOS 8563 / 8568 VDC
U22 — 80-Column RGBI Video Display Controller mapped at $D600/$D601 with 16K/64K private VRAM.
MOS 8722 MMU
U7 — Memory Management Unit controlling Z80/8502/C64 mode handover (/Z80EN) and 128 KB RAM banking.
MOS 8701 Clock Gen
U28 — Master clock multiplier/divider generating Color Clock and Dot Clock.
View Pinout →WD1770 / 1772 FDC
Used in the C128D internal 1571 drive (and 1581) to read/write CP/M MFM double-density disks.
View Pinout →Custom C128 48-Pin VLSI Reference
| Chip | Part Number | Function & Substitution Notes |
|---|---|---|
| U7 (MMU) | MOS 8722R1 / R2 (DIP-48) | Controls 128K RAM banking, zero-page/stack relocation, common RAM areas, and Z80/8502/C64 mode selection at $D500–$D50B. |
| U11 (PLA) | MOS 8721R3 (DIP-48) | Large 48-pin gate array decoding all chip selects and bus phases. Modern CPLD replacements (e.g. PLAnkton 128 / UAV) are available. |
| U21 (VIC-IIe) | MOS 8564 (NTSC) / 8566 (PAL) | 48-pin enhanced VIC-II adding the 2 MHz clock generator, keypad scan lines (K0–K2), and CHROMA/LUMA separation. |
| U22 (VDC) | MOS 8563R7A/R8/R9 (Flat/D) vs 8568 (DCR) | 80-column RGBI controller accessed via two indirect registers at $D600 (Index/Status) and $D601 (Data). Revision R9 fixes earlier R7A block-copy bugs. |
U36 Function ROM & System EPROM Guide
Every C128 motherboard includes an empty 28-pin JEDEC socket at U36 (Internal Function ROM) that maps directly into memory bank 4 ($8000–$FFFF) and auto-boots on startup (unless bypassed by holding CONTROL)!
| Socket | Flat C128 (310379) | C128DCR (250477) | Direct EPROM Drop-In |
|---|---|---|---|
| U36 (Internal Function ROM) | Empty 28-pin socket (up to 32 KB) | Empty 28-pin socket (up to 32 KB) | 27C256 (32 KB) or 27C128 (16 KB) drops straight in with zero adapters! Popular U36 ROMs: Servant 128, GEOS 128 Boot ROM, KeyDOS, SuperChip, or C128 DiagROM. |
| U32 (C64 BASIC + Kernal) | 251913-01 (16 KB, 28-pin) | 251913-01 (16 KB, 28-pin) | 27C128 (16 KB) direct drop-in (or 27C256 with A14 toggle switch for JiffyDOS 64). |
| U33 / U34 / U35 (C128 BASIC & Kernal) | Three separate 16 KB ROMs (U33/U34 BASIC 7.0, U35 Editor/Z80/Kernal) | Condensed into two 32 KB ROMs (U32 = 32K BASIC 7.0, U34 = 32K C64+C128 Kernal) | 27C128 (for 16K sockets) or 27C256 (for 32K DCR sockets) — 100% standard JEDEC 28-pin pinout! |
Why C128 ROM Sockets Are a Joy: Unlike the breadbin C64 (which used 24-pin 2364 ROMs requiring adapters), every ROM socket on the C128 uses a standard 28-pin JEDEC pinout — meaning standard 27C128 and 27C256 EPROMs plug straight in without any adapter boards!
Resources & Further Reading
Blog posts, interactive bench tools, and essential Commodore 128 archives.
Smiggy's Commodore Posts
Bench Tools & Machine Hubs
- Retro File Utilities (Rust/Wasm) — 1541/1571
.D64parser, 6502/8502 disassembler & U36 Function ROM.BINpatcher - IC Pinout Finder — 8502, Z80, 6526, 6581, 8580 & more
- Connector Pinout Directory — DB9 80-Column RGBI, 5-Pin Square DIN Power, 8-Pin DIN A/V, Fast Burst IEC Serial
- Recap & Repair Reference — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
- Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
- SX-64 Machine Hub — Executive Portable C64 reference
- Atari ST Hub — 520ST, 1040STFM & STE reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
- Sinclair QL Hub — 68008, Microdrive & ULA reference
- Acorn Atom & Electron Hub — Atom & Electron reference
- Osborne 1 & Kaypro II Hub — CP/M luggable reference
- Oric-1 & Atmos Hub — Oric 6502A & ULA reference
- Amstrad CPC 6128 Hub — 128K & 3″ disc reference
- MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
- Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
- Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
- VIC-20 Machine Hub — Commodore VIC-20 reference
- Commodore PET Hub — PET 2001, 4000 & 8000 reference
- Plus/4 & C16 (TED) Hub — 264 series reference
- TRS-80 & CoCo Hub — Model I/III/4 & CoCo 1/2/3 reference
- TI-99/4A Machine Hub — 16-bit TMS9900 & PEB reference
External C128 Resources
- Zimmers.net C128 Schematics — official Commodore service manuals & PCB layouts
- C64-Wiki: Commodore 128 — technical overview, memory map, and VDC register docs
- Ray Carlsen's C128 Repair Notes — chip-by-chip symptom & diagnostic guide
- C128.com (Bil Herd) — engineering history and design notes from the lead C128 designer
Commodore 128 / 128D — Capacitor & Recap Reference
Board revisions, electrolytic capacitor lists, voltage/farad ratings, and Through-Hole servicing notes.
Commodore 128 — ASSY 310379 (C128 Flat) & ASSY 250477 (C128DCR)
Dual-CPU 8502 + Z80A 40/80-column mainboard • Mount: Through-hole • Difficulty: Beginner friendly
Mainboard (ASSY 310379) (12 Capacitors)
| Ref | Capacitance | Voltage | Package / Type | Circuit Location & Bench Notes |
|---|---|---|---|---|
C12 |
10 uF | 25 V | Radial | |
C20 |
10 uF | 25 V | Radial | |
C40 |
4.7 uF | 25 V | Radial | |
C43 |
10 uF | 25 V | Radial | |
C59 |
1 uF | 50 V | Radial | |
C61 |
10 uF | 25 V | Radial | |
C92 |
100 uF | 16 V | Radial | |
C100 |
100 uF | 16 V | Radial | |
C104 |
10 uF | 25 V | Radial | |
C106 |
2200 uF | 25 V | Axial | |
C107 |
1000 uF | 25 V | Axial | |
C109 |
470 uF | 16 V | Radial |
Complete Diagnostic Flowchart: Commodore 64 Blank / Black Screen Fault Isolation (Commodore 64 (ASSY 326298 / 250407 / 250425 / 250466 / 250469))
Commodore 128 / 128D — 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.
74LS08
SN74LS08N / DM74LS08N
Quad 2-input AND gate — same 14-pin pinout as the 74LS00, with outputs high only when both inputs are high.
Open Interactive Pinout →74LS139
SN74LS139N / DM74LS139A
Dual 2-to-4 line decoder — two independent decoders, each with one active-low enable and four active-low outputs.
Open Interactive Pinout →74LS14
SN74LS14N / DM74LS14N
Hex Schmitt-trigger inverter — six NOT gates with hysteresis, the standard cure for slow or noisy input edges.
Open Interactive Pinout →CD4066
CD4066B / MC14066 / HEF4066
Quad bilateral switch — four analogue/digital switches with independent controls; control pins are not adjacent to the switches they operate.
Open Interactive Pinout →MOS 8502
MOS 8502
The Commodore 128 CPU — a 6510 with a 7-bit I/O port, running at 1 MHz or 2 MHz.
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 →MOS 6526 CIA
MOS 6526 / 6526A / 8521
Two 8-bit I/O ports, two timers, a time-of-day clock and a serial port — the C64 peripheral chip.
Open Interactive Pinout →MOS 8722 MMU
MOS 8722R1 / 8722R2
The Commodore 128's Memory Management Unit — switches between Z80, C128 (8502) and C64 modes, banks 128KB RAM, and relocates Zero-Page and Stack.
Open Interactive Pinout →MOS 8701 Clock Generator
MOS 8701 (251527-01)
The 16-pin master clock multiplier/divider in the Commodore 64 (ASSY 250425/250466) and Commodore 128 — generates Color Clock and Dot Clock from a single crystal.
Open Interactive Pinout →WD1770 / WD1772 Floppy Controller
WD1770-PH / WD1772-PH / VL1772-02PC
The compact 28-pin single-rail (+5V) FM/MFM floppy controller with built-in digital data separator and motor-on timer — used in the Atari ST, BBC Micro/Master and C128D/1571/1581.
Open Interactive Pinout →MOS 6567 / 6569 VIC-II
MOS 6567 / 6569 / 8562 / 8565
The Commodore 64 video chip — generates the display, the system clock and the DRAM timing.
Open Interactive Pinout →MOS 6581 SID
MOS 6581 / 6581R4
The Commodore 64's sound chip — three-voice synthesiser with filter, envelope and paddle ADC.
Open Interactive Pinout →MOS 8580 SID
MOS 8580 / 6582
The later C64/C128 sound chip — same pinout as the 6581 but 9 V supply and a cleaner filter.
Open Interactive Pinout →MOS 8563 / 8568 VDC
MOS 8563R9 / 8568 (C128DCR)
The Commodore 128's 80-column RGBI Video Display Controller — accessed through just two I/O registers ($D600/$D601) with its own private 16KB or 64KB VRAM.
Open Interactive Pinout →2764 8K x 8 EPROM
2764 / 2764A / 27C64 / M2764A / HN482764
8K x 8 UV-erasable EPROM, the JEDEC 28-pin byte-wide standard. Same pinout as the 6264 SRAM except for four pins.
Open Interactive Pinout →27128 / 27256 EPROM & CPC 40009 System ROM
Amstrad 40009 / 27C128 (16KB) / 27C256 (32KB) / 27C512
16KB (27128) and 32KB (27256) JEDEC 28-pin ROM/EPROM — direct drop-in replacement for the Amstrad CPC 464's 32KB OS + Locomotive BASIC ROM (IC103, part 40009).
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 →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.