Commodore 128 board profiles

Commodore 128 board profiles are service and schematic classifications for the main-board arrangements documented for the flat Commodore 128, the plastic-cased Commodore 128D, and the Commodore 128DCR. The classifications identify a printed-circuit-board assembly, component arrangement, and matching schematic set. They are therefore different from a list of marketed computer models or from the C128's software operating modes. The 1987 service manual and the Zimmers schematic archive both distinguish the earlier flat-C128/plastic-128D family from the DCR board, whose floppy-controller circuitry is integrated on the motherboard.

Scope and identifiers

The service-manual material for the earlier family is organized around PCB assembly 310379. It supplies separate layout, parts-list, and schematic material for revision 6 and revision 7; the associated schematic is numbered 310378 and is arranged in four sheets. The manual's later C128D section identifies PCB assembly 250477-01, labels its board layout “Commodore C128DCR,” and assigns schematic 252451 in five sheets. The archive descriptions make the useful service distinction explicit: the 310378 diagrams concern the flat C128 and the plastic-cased C128D, while the 252451 diagrams concern the C128DCR main board.

This is a hierarchy of documentation rather than proof that every machine in a case category used one immutable board revision. A board profile can be used to select the appropriate layout, parts list, and schematic when identifying a particular unit. It should not be treated as a serial-number chronology, a regional product ledger, or a replacement-compatibility guarantee. The manual's contents themselves separate the 310379 revision material from the 250477 material, which is the reason the assembly and schematic numbers are more informative here than a case name alone.

Shared system-board architecture

Across the documented C128 profiles, the basic architecture combines the MOS 8502 and Z80 processors with the 8722 memory-management unit (MMU) and 8721 programmable logic array (PLA). The C128 Programmer's Reference Guide identifies 128 KiB of main RAM as two 64 KiB banks, while *Commodore 128 Internals* explains that the MMU manages memory configuration, selects C64, C128, and CP/M modes, and selects between the processors. These functions explain why the processors, MMU, PLA, and RAM are useful profile-identification points without making the board comparison a duplicate account of processor switching or operating modes.

The service manual also treats the visual, sound, timing, and interface circuits as identifiable board functions. Its theory and schematic contents cover the 40-column video interface, the 80-column controller, the SID sound chip, the 8701 clock generator, reset, and the input/output, cassette, keyboard, expansion-bus, and serial-bus circuits. For the 250477 board, the component list specifically includes an 8580 SID, 8564 VIC, 8568 80-column controller, 8701, 8502, Z80B, 8722, and 8721. An independently maintained 250477 record lists the same broad host-chip complement and describes sheets 1–3 of schematic 252451 as I/O/control, processor/MMU/SID/PLA/power, and video/clock/reset, respectively.

The profile labels do not mean that every chip detail is stable across all production. For profile identification, the revision-specific manual inventory and its matching schematic set provide the narrower basis for a claim about a particular board.

The 250477 DCR integration

The 250477 profile is distinct because the DCR combines host-board circuitry with a documented floppy-drive controller section. The service-manual parts list places the drive EPROM at U102, SRAM at U103, 6522A devices at U104 and U106, the 5710 floppy-disk controller at U107, the read/write amplifier at U108, and a motor-driver-related device at U109. Sheet 5 of 252451 is the floppy-disk-drive controller sheet. The archive similarly characterizes the DCR board as having its floppy controller integrated on the motherboard, and the independent board record labels sheet 5 as the FDD controller.

Physical integration does not erase the functional boundary between the computer and drive circuitry. The component locations and dedicated controller sheet identify a drive section, but they do not establish that the DCR circuitry is electrically interchangeable with a standalone 1571. The 1571 archive expressly says that its ordinary 1571 schematic applies to the built-in 128D drive only to some extent and not to the 1571CR integrated on the C128DCR motherboard; it directs readers to the C128 schematics for that design.

The archive also reports a likely error in the reproduced right-hand half of DCR schematic page 5. That report does not undermine the broad identification of sheet 5 as the controller sheet or the manual's component inventory, but it is a reason not to infer detailed signal-level connections from that reproduction alone.

Revisions, memory, ROMs, ports, and power

The service-manual update names three revisions of the C-128 board: revision 6, revision 7-01, and revision 7-02. It also distinguishes 150 ns and 200 ns RAM in that update. These are service revisions within the earlier board family, not evidence for three additional marketed computers. A parts list preserved by Zimmers provides a concrete example of profile-qualified memory packaging: it lists sixteen 4164 DRAMs for the C128 and four 41464 DRAMs for the C128D, alongside ROM part-number alternatives and the C128/C128D SID distinction.

ROM evidence likewise needs to distinguish content claims from chip packaging. A contributor's revision analysis preserved in the Zimmers firmware archive reports that the final low-profile C128 ROM upgrade has the same content as the 128DCR version while using four 16 KiB by 8 ROMs rather than two 32 KiB by 8 chips. This is a bounded archive report about the stated revisions and packaging, not a complete catalogue of all ROMs or regional configurations.

Port and power information is also profile-qualified. The service manual documents the cassette, keyboard, expansion, and serial interfaces in its system-board theory, and it includes C128 power-supply material as well as the C128D section. The power-supply archive describes an external switching supply for the flat C128 and separately lists internal switching-supply variants for the plastic 128D and metal 128DCR. Those descriptions support a distinction among documented supply arrangements; they do not establish one universal regional configuration for every machine of each name.

C128CR evidence

The C128CR is relevant as a boundary case, not as a normal production profile. The Zimmers archive describes a photographed cost-reduced flat C128 board as an engineering sample and says that the model did not enter mass production; it records the markings 250783 and 252270 for that example. This is an archive-described observation rather than a Commodore production ledger. It supports careful separation of a non-production engineering sample from the documented 310379 family and the 250477 DCR profile, but not a claim that the C128CR was a marketed model.

See also

• MOS Technology 8502
• Commodore 128 operating modes