IBM Personal Computer XT System Board

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

Specification

• 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

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

• IBM Portable Computer 5155
• IBM Personal Computer XT
• Installing 1MB RAM in the IBM PC/XT Motherboard