Close-up of a C64 PCBA[1]Block Diagram of the Motherboard[2]
The C64 motherboard (also mainboard, system board, logic board) is a double-sided PCB (Printed Circuit Board) featuring conductive tracks etched from copper sheets laminated onto a non-conductive substrate. The motherboard features electronic components (CPU, ROM, RAM etc) embedded directly into the substrate or seated in embedded sockets as well as connectors and interfaces for internal and external peripherals. Component identifier labels are over-printed on the PCB. Together, the boards and components are referred to as a PCBA (Printed Circuit Board Assembly). The C64 motherboard operates on TTL (Transistor-Transistor Logic), uses a composite system bus for communications and many of its components are IC chips dedicated to individual tasks.
C64 motherboards first appeared for retail in August 1982; and although they displayed the year and revision number, the year was soon omitted. In order to improve efficiency and reduce costs, designs were frequently revised. It is important to note then that there is no intended correlation between the chronological development of PCBs, components and cases. Although there may be some pattern setting during the assembly runs, this cannot be relied upon to date a motherboard. An inspection of the PCB must me made if certainty of motherboard type is required. Additionally, there were many C64s returned to Commodore for repair, and motherboards were replaced with what was available; with the replaced board likely to feature in the repair of another C64. In short, nothing should be inferred about the lineage of a motherboard.
The machines on display at the 1982 Winter Consumer Electronics Show in Las Vegas were Rev 6 boards [3]. These machines were still classed as prototypes, and so it is unlikely that previous revisions will be found.
In 1986, Commodore released the last revision to the classic C64 motherboard. It was otherwise identical to the 1984 design, except for the two 64 kilobit × 4 bit DRAM chips that replaced the original eight 64 kilobit × 1 bit ICs.
After the release of the C64C, MOS Technology began to reconfigure the C64's chipset to use HMOS production technology. The main benefit of using HMOS was that it required less current to drive the IC, which consequently generates less heat. This enhanced the overall reliability of the SID and VIC-II. The new chipset was renumbered to 85xx to reflect the change to HMOS.
The modifications to the C64 line were more than skin deep in the C64C with new versions of the SID chips being deployed—with the core voltage reduced from 12 V to 9 V, and the VIC using only 5V. The I/O chips remained functionally identical but the new 8521 which uses HMOS technology uses about half the power of the original NMOS 6526. The basic and kernal 2364 ROMs were also replaced with a single 23128 ROM.
In 1987, Commodore released C64Cs with a highly redesigned motherboard commonly known as a "short board".
The "short board" used the new HMOS chipset, featuring a new 64-pin PLA chip. The new "SuperPLA", as it was dubbed, integrated many discrete components and Transistor–Transistor Logic (TTL) chips.
In the last revision of the C64C motherboard, the 2114 color RAM was integrated into the SuperPLA.
