Editing Atari 800XL Troubleshooting Guide (section)

== Component-level faults (deep dive) ==

=== Chip map ===

{| class="wikitable styled-table"
! Chip !! Role
|-
| '''SALLY''' (6502C) || CPU (6502 with HALT for DMA)
|-
| '''ANTIC''' || Display-list / DMA processor
|-
| '''GTIA''' (CTIA on early boards) || Colour, playfield and player/missile graphics
|-
| '''POKEY''' || Keyboard scan, SIO serial, sound, paddles
|-
| '''PIA''' (6520) || Joystick ports and control lines
|-
| '''MMU''' || Memory management (OS / BASIC / RAM banking)
|-
| '''FREDDIE''' || DRAM controller (later 800XL / 65XE)
|}

=== Star Raiders diagnostic ===

The Star Raiders cartridge exercises '''ANTIC, the CPU and GTIA/CTIA''' (and needs the first 8&nbsp;KB of RAM good). If it runs cleanly, those three chips and the low RAM are fine, which narrows a remaining fault to the '''PIA, OS ROM, MMU or RAM'''.<ref name="a8flt">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit (400/800/XL/XE) pictorial fault guide], 8bit-Hardware; [https://atariprojects.org/2025/01/01/label-the-6502-cpu-antic-gtia-pokey-and-pia-chips-for-easier-troubleshooting-using-a-multimeter-or-oscilloscope-10-15-mins/ Label the 6502 CPU, ANTIC, GTIA, POKEY and PIA Chips], Atari Projects; and [https://www.atarimania.com/pgefaq_chapitre.awp?id=14 What are SALLY, ANTIC, GTIA, POKEY and FREDDIE?], Atarimania. Source for the Star Raiders diagnostic coverage (ANTIC/CPU/GTIA), the POKEY fault signatures (keyboard vs SIO), the GTIA/ANTIC binary behaviour, and the reseat-first / RAM-and-socket-most-common guidance.</ref>

=== POKEY fault signatures ===

POKEY fails in tell-tale ways: the keyboard is dead but sound still works; or keyboard and sound work but '''SIO''' does not &mdash; sometimes SIO fails only at divisor&nbsp;0 (the fastest rate) yet works at divisor 4/6/8. '''GTIA and ANTIC are largely binary''' (working or dead); SALLY can show subtler faults.<ref name="a8flt">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit (400/800/XL/XE) pictorial fault guide], 8bit-Hardware; [https://atariprojects.org/2025/01/01/label-the-6502-cpu-antic-gtia-pokey-and-pia-chips-for-easier-troubleshooting-using-a-multimeter-or-oscilloscope-10-15-mins/ Label the 6502 CPU, ANTIC, GTIA, POKEY and PIA Chips], Atari Projects; and [https://www.atarimania.com/pgefaq_chapitre.awp?id=14 What are SALLY, ANTIC, GTIA, POKEY and FREDDIE?], Atarimania. Source for the Star Raiders diagnostic coverage (ANTIC/CPU/GTIA), the POKEY fault signatures (keyboard vs SIO), the GTIA/ANTIC binary behaviour, and the reseat-first / RAM-and-socket-most-common guidance.</ref>

=== Diagnostic order ===

Reseat all socketed ICs first; statistically '''RAM and socket faults are the most common, then ROM, then POKEY'''. After POKEY and the PIA, check the 74158 multiplexers, the 08/137/375, the delay line and the MMU. Use piggy-back or substitution for RAM and logic, fit sockets for any replaced chip, and recap an ageing board. The built-in self-test (RAM, audio-visual and keyboard tests) is a quick first check on a machine that boots far enough to reach it.<ref name="a8flt">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit (400/800/XL/XE) pictorial fault guide], 8bit-Hardware; [https://atariprojects.org/2025/01/01/label-the-6502-cpu-antic-gtia-pokey-and-pia-chips-for-easier-troubleshooting-using-a-multimeter-or-oscilloscope-10-15-mins/ Label the 6502 CPU, ANTIC, GTIA, POKEY and PIA Chips], Atari Projects; and [https://www.atarimania.com/pgefaq_chapitre.awp?id=14 What are SALLY, ANTIC, GTIA, POKEY and FREDDIE?], Atarimania. Source for the Star Raiders diagnostic coverage (ANTIC/CPU/GTIA), the POKEY fault signatures (keyboard vs SIO), the GTIA/ANTIC binary behaviour, and the reseat-first / RAM-and-socket-most-common guidance.</ref>