Editing IBM PC XT Troubleshooting Guide (section)

== Known Problems and Issues (per minuszerodegrees.net) ==

The following are well-documented 5160-specific quirks gathered from minuszerodegrees.net's long-running motherboard failure history.

=== POST looping with SW1 bit 1 ON ===

When the POST finishes, it may restart in a loop with no error displayed. One documented cause is '''Switch 1 on motherboard SW1 in the ON position'''. Verify SW1 bit 1 is set correctly for the installed RAM.

=== U19 part of Cassette BASIC on 1982 BIOSes ===

On the 11/08/82 BIOS, chip U19 contains part of [[IBM Cassette BASIC]] and is not critical to motherboard operation. If the POST detects a failed U19, it displays "F6000 ROM" but boot continues (only Cassette BASIC is lost).

On the 01/10/86 and 05/09/86 BIOSes, U19 also contains part of the BIOS itself. A failed U19 on these BIOSes produces a 'dead' motherboard. If a board with the 1986 BIOS is dead, U19 is a candidate to swap or re-burn.

=== 1986 BIOSes may not support the IBM 5161 ===

The 01/10/86 and 05/09/86 BIOS revisions may not support an IBM 5161 Expansion Unit. If the 5161 stopped being recognised after a BIOS upgrade, this is the likely cause. If the 5161 is required, stick with the 11/08/82 BIOS.

=== 1986 BIOSes have a documented symptom ===

The 01/10/86 and 05/09/86 BIOS revisions are known to cause a specific symptom documented at minuszerodegrees.net. If a 1986-BIOS XT exhibits unusual POST behaviour with no other apparent cause, consider this a candidate.

=== Slot 8 compatibility ===

Cards placed in expansion slot 8 must be '''slot-8 compatible'''. A non-compatible card in slot 8 generates an '''1801''' error at power-on. Many accelerator cards and some video cards have this requirement.

=== BIOS ROM expansion address conflicts ===

IBM reserved the address space C0000-C7FFF (32 KB) for video BIOS expansion ROMs, with HDD controller BIOSes typically at C8000. Some VGA cards extend their BIOS past C7FFF and conflict with the HDD controller. Microsoft's KB article Q63588 documents this conflict.

=== 27256 EPROM replacement compatibility ===

Early 5160 motherboards are wired for Mostek MK37000/MK38000 mask ROMs in U18 and U19. A replacement 27256 (or 27C256) EPROM '''usually''' works, but some brands may not be compatible — see [https://minuszerodegrees.net/5160/motherboard/5160_u18_u19_sockets.htm minuszerodegrees.net's U18/U19 socket compatibility notes].

=== 16-bit ISA cards in 8-bit slots ===

Same caveat as the 5150 — some 16-bit ISA cards advertised as "8-bit slot compatible" do not work in the XT's 8-bit slots. They may require reconfiguration or wider slot spacing than the XT provides.

=== V20 swap compatibility ===

Swapping the 8088 for an NEC V20 gives a 20-30% speed boost on most code, but the V20 emulates an 80186-class instruction set that breaks a small number of programs.