IBM Personal Computer XT System Board

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Ibm-xt-64-256k-system-board.jpg

The IBM PC/XT model 5160 and the IBM Portable Computer shared a system board and various other components.

Specification

Ibm-xt-64-256k-system-board-rev-1-marking.jpg
  • Intel 8088 CPU at 4.77MHz
  • Socket for Intel 8087 FPU
  • 8 ISA slots (see also IBM Personal Computer XT System Board Slot 8)
  • 4 banks of 9-bit (parity) RAM, using 64 kbit chips
  • Physically 9" x 13"
  • First version (c.1983) supported 64 to 256K RAM via 64 kbit chips (U84 is unpopulated)
  • Second version (c.1986) supported 256 to 640K RAM by supporting 256 kbit RAM chips (U84 is populated with an 74LS158 chip).

BIOS Versions

  • 16-Aug-1982 - First release, possibly never made it to production.
    • ROM SIZE = 40K
    • U18 = 32K chip labeled with IBM part number of 5000026. Contains BIOS and part of BASIC.
    • U19 = 8K chip labeled with IBM part number of 5000027. Contains remainder of BASIC.
  • 08-Nov-1982 - Second release.
    • ROM SIZE = 40K
    • U18 = 32K chip
    • U19 = 8K chip
    • Chips had part codes U18=1501512 and U19=500027, or U18=1501512 and U19=6359116
  • 10-Jan-1086
    • ROM SIZE = 64K
    • U18 = 32K chip
    • U19 = 32K chip
    • Chips had part codes U18=62X0851 and U19=62X0854 (PROM), or U18=62X0852 and U19=62X0853 (EPROM)
    • Used on 256/640K motherboards. Up to 640K RAM.
    • Enhanced POST (one known enhancement is a faster test of RAM).
    • Added support for the 101-key Enhanced keyboard.
    • Support for 720K and 1.2M floppy drives (although the PC/XT never had a high-density controller).

Configuration

DIP Switches

The system was configured using a set of DIP switches (there was no BIOS utility or NVRAM).

General System Configuration
Switch On Off
1 POST loop test Normal operation
2 Coprocessor not installed Coprocessor installed
3,4 See memory table See memory table
5,6 See video table See video table
7,8 See floppy table See floppy table


Memory Configuration
Banks Occupied Sw3 Sw4
Bank 0 only on on
Banks 0 and 1 off on
Banks 0, 1 and 2 on off
All 4 banks off off
Video Configuration
Adapter Type Sw5 Sw6
Add-in card with BIOS on on
CGA, 40x25 off on
CGA, 80x25 on off
Monochrome off off
Floppy Drive Configuration
Drives Installed Sw7 Sw8
One on on
Two off on
Three on off
Four off off

RAM

The system was fitted with either 64Kb 200ns 'IBM 4164-20' DRAM chips (first version) in four banks of nine chips (due to the RAM being in an ECC configuration), or 256 kbit chips (second version).

Various modifications were possible to increase the RAM capacity beyond 256K, including a custom decoder PROM to enable a 1MB configuration or using an ISA card.

Headers

  • P1 - Power
  • P2 - Power
  • P3 - Speaker

Beep Codes

Some problems will be signal via 'beep-codes', assuming the speaker is connected and functional:

Beep Problem
None PSU, System board, Disconnected CPU (or speaker), or Grounded Keyboard Data Line
Repeating short PSU, System Board, or Keyboard
1 short Normal POST (system is OK)
2 short POST error (code shown on screen)
1 long, 1 short System Board
1 long, 2 short Display Adapter (MDA or CGA)
1 long, 3 short Display Adapter (EGA)
3 long 3270 Keyboard Card
Continuous PSU, System Board, or Keyboard


See Also