RetroTechCollection

Atari 520ST

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The Atari 520ST is Atari Corporation’s first 16/32‑bit ST‑series model, announced January 1985 and shipped in quantity mid‑1985. It pairs a Motorola 68000 at 8 MHz with the GEM desktop on TOS, integrated MIDI ports, and a cost‑effective graphics subsystem. Early North American bundles were priced at US$799 (mono) and US$999 (color).[12][13]

Atari 520ST
File:Atari 520ST.jpg
520ST with external SF354 floppy drive and SC1224 color monitor
Specifications
ManufacturerAtari Corporation
TypeHome computer
ReleasedJune 1, 1985
Discontinued1986
Intro priceUS$799 bundle (520ST + SF354 + SM124 mono), US$999 (520ST + SF354 + SC1224 RGB) at U.S. launch (Oct. 1985).[1]
CPUMotorola MC68000P8 @ 8 MHz (64‑pin DIP; some later boards MC68000FN8 PLCC)[2]
Memory512 KB RAM standard (DRAM, 16×256‑Kbit); commonly expanded to 1 MB; third‑party upgrades up to 4 MB.[3]
StorageExternal 3.5″ floppy (SF354 single‑sided 360 KB; later SF314 double‑sided 720 KB), ACSI (DB‑19) external hard disks (e.g., SH204), ROM cartridge slot (128 KB).[4]
DisplayLow 320×200 (16 colors), Medium 640×200 (4 colors), High 640×400 (mono); 50/60 Hz color, 71.2 Hz mono.[5]
SoundYamaha YM2149F PSG, 3 tone voices + 1 noise source, programmable envelopes; mono output via monitor connector pin 1.[6][7]
Dimensions47.6 × 24.1 × 7.0 cm (18.75 × 9.5 × 2.75 in)[8]
Weight≈4.3 kg (9.5 lb) (unit without external PSU).[9]
OS / FirmwareAtari TOS with GEM. Early 520ST units booted TOS from floppy (16 KB boot ROM); shortly after launch, TOS 1.00 shipped in 192 KB ROM (2‑ or 6‑ROM sets).[10][11]
PredecessorAtari 8-bit family
SuccessorAtari 520STF / Atari 1040STF

Architecture and Processor

At its core, the 520ST uses a 16/32‑bit Motorola 68000:

  • Exact CPU: Motorola MC68000P8 (DIP‑64) on early boards; some later STf/STfm use MC68000FN8 (PLCC‑68). Clock: 8.00 MHz (all regions).[14]
  • Bus: 24‑bit address bus (16 MB address space), 16‑bit data bus; unified memory architecture with video DMA contention from the Shifter.[15]
  • Major custom chips (exact part numbers as used on 520/1040ST family):
    • Shifter (video): C025914‑38/‑38A; also seen as CO70713/I10356 on some boards.[16]
    • GLUE (bus/logic/timing): C025915‑38A; also CO70714/I10355.[17]
    • MMU (DRAM controller): C025912‑38; later C100109/I10353.[18]
    • DMA (ACSI/floppy DMA): C025913‑38 (Atari DMA controller).[19]
    • MFP (interrupts/timers/serial): Motorola MC68901.[20]
    • ACIA (keyboard and MIDI UARTs): 2× Motorola MC6850.[21]
    • Floppy controller: Western Digital WD1772 (commonly WD1772‑00‑02; later ‑02‑02 parts tolerate 16 MHz for HD mods).[22]
  • Region differences: CPU frequency remains 8.00 MHz worldwide; video refresh differs by region (50 Hz PAL vs 60 Hz NTSC; mono at ~71.2 Hz).[23]

Input/Output and Expansion

Every external connector on the 520ST (original short case, external PSU and external floppy) is documented below. Pinouts are per Atari manuals and established community references.

  • RGB/mono monitor: 13‑pin DIN female, carrying analog RGB (0.7 Vp‑p), composite sync or composite video (on modulator‑equipped units), monochrome TTL video, mono audio out, and mono audio in. Includes “mono detect” (Pin 4) which forces 640×400/71 Hz mode when grounded.[24][25]
  • Serial (RS‑232C): DB‑25 male; line drivers/receivers typically MC1488/MC1489 on board; handled by MFP 68901 for asynchronous serial I/O.[26]
  • Parallel printer: DB‑25 female (Centronics‑style handshaking; unidirectional data 0–7 with Busy and Strobe).[27]
  • MIDI: 2× 5‑pin DIN (MIDI IN and MIDI OUT/THRU), opto‑isolated input, serviced by one of the MC6850 ACIAs.[28]
  • Floppy external drive: 14‑pin round DIN (external SF354/SF314). WD1772 set for 250 kbit/s DD; early systems standard single‑sided 360 KB, later double‑sided 720 KB with SF314.[29]
  • ACSI hard‑disk/Laser printer: DB‑19 female (“DMA” port). Bus mastered by Atari DMA chip; nominal 10 Mbit/s transfer capability.[30]
  • Mouse/joysticks: Two DE‑9 male (Port 0 shared with mouse). Pinout per Atari: 1 Up, 2 Down, 3 Left, 4 Right, 5 NC (mouse middle), 6 Fire, 7 +5 V, 8 GND, 9 second Fire (mouse right).[31]
  • Cartridge: 40‑way edge connector, 128 KB ROM address space.[32]
  • Power: External AC‑powered PSU feeding +5 V, +12 V, −12 V DC to a 7‑pin round DIN on the computer (+5 V @ ~2 A typical load per manual).[33]

Display and Graphics

Video is generated by the Shifter (C025914), fed by GLUE/MMU arbitration from RAM:

  • Modes and timings (region‑dependent): Low 320×200×4bpp (16 colors), Medium 640×200×2bpp (4 colors), High 640×400×1bpp (mono). Color modes: 15.75 kHz H with 50 Hz (PAL) or 60 Hz (NTSC) V; high‑res mono: ~35.7 kHz H, ~71.2 Hz V.[34]
  • Palette: 9‑bit RGB (3:3:3) = 512 possible colors; up to 16 simultaneously in low‑res, 4 in medium; mono mode is black/white at 640×400.[35]
  • Hardware features: No hardware sprites; CPU‑driven blits (pre‑STE models) though late STs may include optional BLiTTER add‑on on some boards (not standard on the 520ST).[36]
  • Video outputs (DIN‑13): RGB (pins 6/7/10), separate HSync (9)/VSync (12), Monochrome video (11), Audio Out (1), Audio In (5), “Mono detect” (4), and +12 V (pin 8; ground on early 520ST). Composite sync/video appears on pin 2 only on modulator‑equipped units; otherwise composite sync only.[37][38]

PCB Schematics & Service Manual

Motherboard families and service references commonly encountered in 520ST/520STF/520STFM repairs.

Motherboard assemblies (selection)
Assembly / PBA Typical model(s) Revision(s) seen Notes
C070115 Early 260/520ST Rev. 2 Early short‑case; 2‑ROM (boot TOS) or 6‑ROM footprints present.[39]
C070231 260/520ST — 1985 schematic reconstructed to KiCad (community).[40]
C070243 520ST Revs C/E/F/H/I Common early 520ST board; many variants 2‑ or 6‑ROM.[41]
C103414‑001 520STFM Rev. 1 / 1.1 Later STFM (integrated floppy + RF); 2‑ or 6‑ROM.[42]
Schematics and service docs
  • Atari 520ST Schematic (1985), 3 pages (core logic, video, I/O).[43]
  • Atari 1040ST Schematic Rev C (C070787, 1986‑12‑26) – useful for STf/fm video/GLUE/MMU references.[44]
  • Field Service manual (STe; later architecture but same 68000, WD1772/ACSI/I/O blocks; includes port pin summaries and diagnostic flow).[45]

Memory and Storage

RAM/ROM layout and controllers used on the 520ST:

  • Main memory: 512 KB using 16×256‑Kbit DRAMs; many 520ST+ shipped with 1 MB (16×1‑Mbit). MMU (C025912/C100109) provides RAS/CAS multiplexing and DRAM timing.[46]
  • ROM: Very early 520ST shipped with a 16 KB boot ROM (“TOS on disk”) consuming ~206 KB RAM when loaded; later machines used 192 KB TOS 1.00 in 6× 27C256 (or 2× 1‑Mbit) ROMs at $FC0000–$FEFFFF.[47][48][49]
  • Memory map highlights (ST/early TOS): TOS ROM at $FC0000–$FEFFFF (TOS 1.00/1.02/1.04); cartridge space at $FA0000; hardware registers (e.g., video base $FFFF8201, palette $FFFF8240, MFP at $FFFAxx).[50]
  • Floppy: WD1772 controller; SF354 3.5″ SSDD (360 KB) initially, SF314 3.5″ DSDD (720 KB) later. WD1772 is clocked at 8 MHz (250 kbit/s DD); certain WD1772‑02‑02 chips tolerate 16 MHz for HD mods.[51][52]
  • Hard disks: ACSI port (DB‑19), DMA‑based; SH204 and later drives supported via AHDI. Manual quotes 10 Mbit/s peak bus rate.[53]

Sound Capabilities

The 520ST uses a Yamaha YM2149F (AY‑3‑8910 compatible) PSG:

  • 3 independent square‑wave tone channels (A/B/C) plus 1 white noise generator; 16‑level per‑channel volume with shared programmable ADSR‑style envelope generator; mixer for tone/noise per channel.[54]
  • Output: monaural line/speaker feed on monitor DIN‑13 pin 1; internal amplifier in Atari monitors (SM124/SC1224).[55]
  • Connectors: audio is via the monitor cable; no separate RCA jack on the original 520ST.

General Maintenance

Routine service items for 520ST owners and technicians:

  • Clean and re‑lube SF354/SF314 drive rails; re‑belt (if applicable) and verify index sensor alignment.
  • Reseat socketed ICs (68000, Shifter can be socketed under RF can; WD1772; ROMs).
  • Inspect external PSU for ripple (replace with modern, properly regulated +5 V/+12 V/−12 V supply if unstable).[56]
  • Keyboards: clean membrane and plungers; verify IKBD (HD6301) comms and MC6850 ACIA socket contacts.[57]

See the dedicated page: Atari 520ST General Maintenance.

Technical Details

Major ICs and revisions (typical)
Function Part numbers (examples) Notes
CPU MC68000P8 / MC68000FN8 8 MHz, 24‑bit address / 16‑bit data.[58]
Shifter (video) C025914‑38/‑38A; CO70713/I10356 Generates RGB/mono from RAM fetches.[59]
GLUE C025915‑38A; CO70714/I10355 Bus/timing, interrupts, address decode.[60]
MMU C025912‑38; C100109/I10353 DRAM control, video base.[61]
DMA (ACSI/floppy) C025913‑38 ACSI/HDD and FDD DMA handshakes.[62]
Floppy controller WD1772‑00‑02 / WD1772‑02‑02 8 MHz DD operation; overclockable variants for HD mods.[63]
MFP MC68901 Timers/interrupts/serial.[64]
ACIAs 2× MC6850 Keyboard (IKBD) and MIDI UARTs.[65]
Sound Yamaha YM2149F 3‑voice PSG with noise; mono output.[66]
Power rails and consumption
  • External PSU outputs +5 V (≈2 A), +12 V (≈30 mA), −12 V (≈30 mA) to the computer (round DIN‑7); region‑specific AC mains by PSU model (120 V/60 Hz in U.S., 220–240 V/50 Hz in Europe).[67]

System Architecture at a Glance

  • 68000 @ 8 MHz; GLUE decodes I/O and arbitrates bus with Shifter and DMA.
  • 512 KB RAM (typical), 192 KB TOS 1.xx ROM on later units; earliest units boot TOS from disk via 16 KB boot ROM.
  • Video via Shifter from RAM; 512‑color palette (3:3:3), 16/4 colors on low/med, 640×400 mono on SM124.
  • I/O: DB‑25 serial and parallel, DIN‑13 video+audio, DB‑19 ACSI DMA, 14‑pin DIN floppy, two DE‑9 joystick/mouse, 2× DIN‑5 MIDI, 40‑way cartridge.
  • External PSU rails +5/+12/−12 V; computer contains only low‑voltage DC circuitry (no internal analog PSU board on 520ST).[68]

Apple Service Manual

Not applicable. The 520ST is an Atari product; for service procedures use the Atari ST schematics and field service documentation listed above, not Apple references.

Logic Board & Analog Board Schematics

The 520ST has a single main logic board; there is no separate “analog board” (that term applies to some Apple Macintosh models). For display hardware, refer to the SC1224/SM124 monitor service literature separately. Core 520ST schematics: see “PCB Schematics & Service Manual.”[69]

Troubleshooting

Common field faults on early ST systems and checks:

  • Black screen, no beep: verify 5 V at Shifter can and 68000; check reset chain; substitute GLUE/MMU as needed; inspect ROM seating (six‑ROM sets especially).[70]
  • No video in color but mono OK: DIN‑13 cable/pin 4 state; verify composite sync/video behavior (depends on modulator presence).[71]
  • Floppy errors: WD1772 socket contacts and 8 MHz clock; check drive select/jumpering on SF354/SF314; clean heads. WD1772‑02‑02 often preferred for reliability; verify motor and index sensor.[72]
  • ACSI device timeouts: Inspect DB‑19 connector solder joints; verify DMA (C025913‑38).[73]
  • Keyboard/mouse erratic: check IKBD 6301 comms at 7812.5 bps via the MC6850 ACIA; reseat keyboard cable.[74]

🧠 Architecture and Processor

Deep‑dive notes for developers and repair techs:

  • MFP 68901 base at $FFFA01 (odd‑addressed register block); timers: A/B (high‑frequency raster/VBL uses), C (200 Hz system tick), D (baud). Interrupt vectors and masks documented in ST Programmer’s guide.[75]
  • Video registers (Shifter/GLUE): base address high/med $FFFF8201/$FFFF8203; palette at $FFFF8240 (16 words); resolution reg $FFFF8260; mono detect via external pin 4 on DIN‑13.[76]
  • ROM map: TOS 1.00/1.02/1.04 at $FC0000–$FEFFFF (192 KB). STE/2.06 moves to $E00000–$E3FFFF (256 KB) – relevant for ROM upgrade wiring on some STFs.[77]

Technical Documentation

References

  1. ↑ Atari 520ST: A Hands-On Report– Tom R. Halfhill, Compute!, 1985-10—link(accessed 2025-08-13)
  2. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  3. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  4. ↑ Atari 520ST Owner's Manual (Rev. B), Atari Corporation, 1985—link(accessed 2025-08-13)
  5. ↑ The various Atari ST display modes, AtariForumWiki—link(accessed 2025-08-13)
  6. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  7. ↑ Atari ST Interfaces / Connectors / Cables– Jean-Louis Guérin, info-coach.fr—link(accessed 2025-08-13)
  8. ↑ Atari 520ST Microcomputer, Smithsonian National Museum of American History—link(accessed 2025-08-13)
  9. ↑ Atari 520ST, My Old Computers Museum—link(accessed 2025-08-13)
  10. ↑ Atari 520ST Owner's Manual (Rev. B), Atari Corporation, 1985—link(accessed 2025-08-13)
  11. ↑ Atari TOS, Wikipedia, 2024-12-01—link(accessed 2025-08-13)
  12. ↑ Atari 520ST: A Hands‑On Report– Tom R. Halfhill, Compute!, 1985-10—link(accessed 2025-08-13)
  13. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  14. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  15. ↑ The various Atari ST display modes, AtariForumWiki—link(accessed 2025-08-13)
  16. ↑ DOIT ST: Parts list (ST custom ICs), DOIT_ST archive—link(accessed 2025-08-13)
  17. ↑ DOIT ST: Parts list (ST custom ICs), DOIT_ST archive—link(accessed 2025-08-13)
  18. ↑ DOIT ST: Parts list (ST custom ICs), DOIT_ST archive—link(accessed 2025-08-13)
  19. ↑ Atari custom IC list (forum sale; part numbers collated), RSSing (AtariAge content mirror)—link(accessed 2025-08-13)
  20. ↑ Concise Atari ST 68000 Programmer's Reference Guide – MC68901 MFP, bighole.nl mirror—link(accessed 2025-08-13)
  21. ↑ ST Hints & Tips (IKBD and ACIA), Compute!, 1986—link(accessed 2025-08-13)
  22. ↑ WD1772 00‑02 floppy controller, exxosforum store—link(accessed 2025-08-13)
  23. ↑ The various Atari ST display modes, AtariForumWiki—link(accessed 2025-08-13)
  24. ↑ Atari ST Interfaces / Cables – DIN‑13 monitor details, info-coach.fr—link(accessed 2025-08-13)
  25. ↑ Atari ST monitor pinout, pinouts.ru—link(accessed 2025-08-13)
  26. ↑ Atari 520/1040 STe Field Service Manual (I/O overview; 1488/1489 shown), ManualsLib, 1991—link(accessed 2025-08-13)
  27. ↑ Atari 520/1040 STe Field Service Manual, ManualsLib, 1991—link(accessed 2025-08-13)
  28. ↑ Concise Atari ST 68000 Programmer's Reference Guide – Intelligent Keyboard & MIDI, bighole.nl—link(accessed 2025-08-13)
  29. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs, Atari Corporation, 1985—link(accessed 2025-08-13)
  30. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs, Atari Corporation, 1985—link(accessed 2025-08-13)
  31. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix D/E Connector Pinouts, Atari Corporation, 1985—link(accessed 2025-08-13)
  32. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs, Atari Corporation, 1985—link(accessed 2025-08-13)
  33. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs, Atari Corporation, 1985—link(accessed 2025-08-13)
  34. ↑ The various Atari ST display modes, AtariForumWiki—link(accessed 2025-08-13)
  35. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  36. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  37. ↑ Atari ST Interfaces / Cables – DIN‑13 monitor details, info-coach.fr—link(accessed 2025-08-13)
  38. ↑ Atari ST monitor pinout, pinouts.ru—link(accessed 2025-08-13)
  39. ↑ Atari ST ROM layouts and motherboards, GitHub (splitrom)—link(accessed 2025-08-13)
  40. ↑ Conversion of the schematics of the Atari 260/520ST motherboard into KiCad, AtariAge Forums, 2024-05-25—link(accessed 2025-08-13)
  41. ↑ Motherboard model numbers and ROM configurations, Sidecartridge Docs—link(accessed 2025-08-13)
  42. ↑ Motherboard model numbers and ROM configurations, Sidecartridge Docs—link(accessed 2025-08-13)
  43. ↑ Atari 520ST Schematic [1985], Internet Archive—link(accessed 2025-08-13)
  44. ↑ Atari 1040ST Schematic Rev C C070787 [Dec 26, 1986], Internet Archive—link(accessed 2025-08-13)
  45. ↑ Atari 520/1040 STe Field Service Manual (C302481‑001 Rev. A), ManualsLib, 1991—link(accessed 2025-08-13)
  46. ↑ DOIT ST: Parts list (MMU), DOIT_ST archive—link(accessed 2025-08-13)
  47. ↑ Atari 520ST Owner's Manual (Rev. B) – Specs show 16,384‑byte ROM for boot units, Atari Corporation, 1985—link(accessed 2025-08-13)
  48. ↑ Atari TOS, Wikipedia, 2024-12-01—link(accessed 2025-08-13)
  49. ↑ Atari ST internals quick map (ROM base addresses), Ctrl‑Alt‑Rees (Atari Museum mirror)—link(accessed 2025-08-13)
  50. ↑ Atari ST internals quick map, Ctrl‑Alt‑Rees—link(accessed 2025-08-13)
  51. ↑ Atari 520ST Owner's Manual (Rev. B) – Floppy capacity, Atari Corporation, 1985—link(accessed 2025-08-13)
  52. ↑ DOIT ST: AJAX/WD1772 notes, DOIT_ST archive—link(accessed 2025-08-13)
  53. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix E, Atari Corporation, 1985—link(accessed 2025-08-13)
  54. ↑ Atari ST (sound overview), Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  55. ↑ Atari ST Interfaces / Cables – audio pins, info-coach.fr—link(accessed 2025-08-13)
  56. ↑ Replacement Atari 520ST PSU (specs and rails), Keelog—link(accessed 2025-08-13)
  57. ↑ Concise Atari ST 68000 Programmer's Reference – IKBD, bighole.nl—link(accessed 2025-08-13)
  58. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  59. ↑ DOIT ST: Parts, DOIT_ST archive—link(accessed 2025-08-13)
  60. ↑ DOIT ST: Parts, DOIT_ST archive—link(accessed 2025-08-13)
  61. ↑ DOIT ST: Parts, DOIT_ST archive—link(accessed 2025-08-13)
  62. ↑ Atari custom IC list, RSSing (AtariAge mirror)—link(accessed 2025-08-13)
  63. ↑ WD1772 00‑02 floppy controller, exxosforum store—link(accessed 2025-08-13)
  64. ↑ ST Programmer's Guide – MFP, bighole.nl—link(accessed 2025-08-13)
  65. ↑ ST Hints & Tips – 6301 IKBD and ACIA, Compute!—link(accessed 2025-08-13)
  66. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  67. ↑ Atari 520ST Owner's Manual (Rev. B) – Appendix E, Atari Corporation, 1985—link(accessed 2025-08-13)
  68. ↑ Atari 520ST Owner's Manual (Rev. B), Atari Corporation, 1985—link(accessed 2025-08-13)
  69. ↑ Atari 520ST Schematic [1985], Internet Archive—link(accessed 2025-08-13)
  70. ↑ Atari STe Field Service Manual – Symptom and flowcharts (applicable concepts), ManualsLib, 1991—link(accessed 2025-08-13)
  71. ↑ Atari Video Connector (details on composite vs sync), info-coach.fr—link(accessed 2025-08-13)
  72. ↑ DOIT ST: WD1772 notes, DOIT_ST archive—link(accessed 2025-08-13)
  73. ↑ Atari 520ST Owner's Manual – ACSI overview, Atari Corporation, 1985—link(accessed 2025-08-13)
  74. ↑ ST Programmer's Guide – IKBD/ACIA, bighole.nl—link(accessed 2025-08-13)
  75. ↑ Concise ST Programmer’s Guide – MFP registers, bighole.nl—link(accessed 2025-08-13)
  76. ↑ Atari ST internals quick map, Ctrl‑Alt‑Rees—link(accessed 2025-08-13)
  77. ↑ Atari TOS, Wikipedia, 2024-12-01—link(accessed 2025-08-13)
  78. ↑ Atari 520ST Owner's Manual (Rev. B), Atari Corporation, 1985—link(accessed 2025-08-13)
  79. ↑ Atari 520ST Schematic [1985], Internet Archive—link(accessed 2025-08-13)
  80. ↑ Atari 1040ST Schematic Rev C C070787 [Dec 26, 1986], Internet Archive—link(accessed 2025-08-13)
  81. ↑ Atari TOS, Wikipedia, 2024-12-01—link(accessed 2025-08-13)
  82. ↑ Atari Video/Interfaces, info-coach.fr—link(accessed 2025-08-13)
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