PWA-IH4077C © 1992 Mitac
The PWA-IH4077C is an early-production 80486 Baby AT motherboard designed and manufactured by Taiwanese industrial OEM provider MiTAC around 1992. Serving as the direct precursor to the revised D-variant, this system board leverages the robust UMC UM8401 (Northbridge) and UM8402 (Southbridge) core logic chipset layout to route data between the CPU and high-speed expansion lines. Built with a 168-pin Socket 1/2 footprint, it is engineered to drive Intel 80486SX, DX, and AMD/Intel DX2 clock-doubled processors operating on a 25 MHz or 33 MHz Front-Side Bus (FSB). Its subsystem expansion environment includes two to three 32-bit VESA Local Bus (VLB) slots lined up alongside six legacy 16-bit ISA slots. Memory configurations feature a transitional design, offering four 30-pin SIMM sockets combined with two 72-pin SIMM sockets to accommodate Fast Page Mode (FPM) DRAM, backed by an onboard discrete external L2 cache layout utilizing 28-pin static RAM (SRAM) DIP sockets expandable up to 256 KB.
The 28-Pin Cache Layout: Unlike later, more flexible 486 boards that transitioned entirely to 32-pin DIP sockets to handle massive 512 KB cache configurations, the PWA-IH4077C features strict 28-pin SRAM sockets. This requires specific, lower-density static memory chips to fill out the L2 cache lines. Leaving these banks empty causes the system to run in a generic zero-cache real-mode state, drastically lowering data transit speed to 386 performance baselines.
The Stealth OEM Identity: Because MiTAC functioned heavily as a silent white-label manufacturer for massive computing brands and international clone distributors, the board carries no prominent retail manufacturer branding on the silk screen. Instead, it is identified solely by its structural model stamp "PWA-IH4077C BD" and a standard factory assembly notation ("ASSY NO: 4116328000XX"), making it a classic "mystery board" for vintage PC collectors.