John R Coffman’s SBC-386EX board

The 80386EX Chip

The Intel® 80386EX™ microprocessor is an advancement on the 32-bit, 24-address-bit 80386SX™, which in turn is a cut-down version of the 32-address-bit 80386DX™.

It is fully compatible with the ‘386SX, but has 26 address bits, so can access up to 64 MiB of address space. Its main advantage, though, is that it has many on-chip peripherals that would normally be needed on the computer; so the chip is ideal for Embedded computers (which is where the “EX” designation comes from).

The on-chip peripherals are designed to be compatible with the IBM® PC™, and include:

  • Power management (including SMM)
  • DRAM Refresh Control Unit
  • Chip Select Unit
  • DMA Unit (2×8237)
  • Interrupt Control Unit (2x8259A)
  • Timer Unit (8254)
  • Asynchronous Serial (2×16450)
  • Synchronous Serial
  • GPIO

The SBC 386EX Board

John R Coffman designed a board based on the 80386EX, and added a number of other peripherals and connectors too. The board is designed to work into the whole RetroBrew Computers EuroCard Bus family of single-board computers and expansion cards.

The SBC 386EX board includes the following peripherals:

  • Socket for the 80387SX maths coprocessor
  • 32-pin DIP socket for up to 512 kiB of ROM
  • 20-pin DIP socket for up to 32 kiB of SRAM
  • 72-pin SIMM socket for up to 32 MiB of DRAM
  • 96-pin EuroCard Bus connector
  • Real-Time Clock with SuperCap power retention
  • 10-pin RS-232 serial connector
  • 1.8432 MHz crystal for precise RS-232 bit timings
  • 40-pin IDE connector
  • Micro SD card connector
  • 4x debug LEDs