Known Hardware Bugs in Early Commodore 128 Revisions

The Commodore 128 stands as a significant milestone in 8-bit computing, yet early production models suffered from specific hardware flaws. This article examines the known hardware bugs unique to the initial revisions of the Commodore 128, detailing the technical issues affecting the VDC, memory configuration, and PLA chips that collectors and enthusiasts should identify before purchasing or restoring these vintage systems.

The PLA Logic Defect

The most critical hardware bug found in the earliest Commodore 128 motherboards involves the Programmable Logic Array (PLA). In the initial assembly revision, identified as 252535, the PLA contained logic errors that disrupted memory mapping. This defect primarily manifested when users attempted to run software in Commodore 64 compatibility mode. Certain cartridges and copy-protected games would fail to load or cause the system to crash unexpectedly because the PLA did not correctly handle the bank switching signals required for these programs. Later motherboard revisions, typically marked 252535-01, corrected this logic error, making them more stable for legacy software execution.

VDC Initialization and Timing Issues

Early units also exhibited inconsistencies with the MOS 8563 Video Display Controller (VDC), which powered the 80-column text mode. Users reported flickering screens or failure to initialize the 80-column display upon booting. This was often attributed to timing discrepancies between the VDC and the system clock in the first production batches. In some cases, the capacitor values on the motherboard near the VDC circuit were insufficient to maintain stable voltage during peak processing loads. Commodore addressed this in subsequent runs by adjusting the circuitry to ensure reliable signal generation for high-resolution text modes.

Memory Refresh and System Stability

Random system freezes were another complaint associated with the early revisions of the Commodore 128. These instability issues were linked to the DRAM refresh logic on the original motherboard design. Under specific conditions, the memory refresh cycle would conflict with CPU access, leading to data corruption or a complete lockup. This bug was particularly noticeable when running complex BASIC programs or utilizing the full 128KB of RAM extensively. The revised motherboards implemented a corrected refresh sequence that eliminated these conflicts, significantly improving overall system reliability during extended use.

Identifying Early Revision Models

For collectors and restorers, identifying these early models is crucial for troubleshooting persistent issues. The primary method of identification involves opening the case to inspect the motherboard assembly number printed on the PCB. Units bearing the original 252535 number without the -01 suffix are likely to exhibit the PLA and memory bugs described. While these early boards are historically significant, enthusiasts seeking a stable experience for daily use or software compatibility are generally advised to seek out later revision boards or apply specific hardware modifications that replicate the later circuit corrections.