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Atari 520ST

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Atari 520ST
Early “short-case” 520ST with external PSU and external SF354 floppy drive
Specifications
ManufacturerAtari Corporation
TypeHome computer / personal computer
ReleasedJune 1, 1985[1][2]
Discontinued1986–1987 (superseded in production by STF/STM variants; ST family continued to 1993)[3][4]
Intro priceUS$599 (CPU only, announced Jan 1985); US$699 mono bundle / US$899 color bundle (Jan 1986 SRP)[5][6]
CPUMotorola MC68000P8/MC68000FN8 @ 8.00 MHz[7][8]
Memory512 KB RAM standard; expandable to 1–4 MB via aftermarket DRAM/SIMM upgrades[9][10]
StorageExternal 3.5-inch SF354 SSDD 360 KB (standard); optional SF314 DSDD 720 KB; external ACSI hard disks (SH/MegaFile series)[11][12]
Display13‑pin DIN analog RGB and mono; 320×200 (16 colors), 640×200 (4 colors) at 50/60 Hz; 640×400 mono at ~71.2 Hz (SM124)[13][14]
SoundYamaha YM2149F 3‑voice PSG (square/noise, 5‑bit envelope); mono line out on monitor port; MIDI In/Out[15][16]
Dimensions70 mm × 476 mm × 241 mm (2.75 × 18.75 × 9.5 in)[17]
Weight≈4.3 kg (9 lb 7 oz)[18]
OS / FirmwareAtari TOS with Digital Research GEM; early units boot TOS from disk via small boot ROM, later 192 KB ROM‑TOS[19]
PredecessorAtari 65XE
SuccessorAtari 1040STE

The Atari 520ST is the first widely released model of Atari’s 16/32‑bit ST family, built around the Motorola 68000 at 8 MHz with 512 KB RAM and a rich set of built‑in interfaces (notably MIDI). It debuted in 1985 with low bundled prices and a GEM desktop, competing directly with the Macintosh and Commodore Amiga.[20][21]

Architecture and Processor

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Exact CPU and core architecture

  • CPU: Motorola MC68000 (DIP‑64 MC68000P8 on early boards; later boards sometimes PLCC‑68 MC68000FN8), nominal 8.00 MHz.[22][23]
  • Architecture: 16/32‑bit 68000 core (32‑bit registers/ALU with 16‑bit external data bus; 24‑bit address bus for 16 MB address space).[24]
  • System bus: 24 address lines, 16 data lines; unified memory (video fetches contend with CPU). Video DMA is performed by the Shifter reading DRAM under GLUE/MMU timing; the separate Atari “DMA” IC arbitrates ACSI/floppy transfers.[25]
  • Region timings: CPU runs at 8.00 MHz in all regions; display vertical refresh differs (50.05 Hz PAL, 60.04 Hz NTSC, mono ~71.5 Hz), derived from ~32.084988 MHz master clock on color modes.[26]

Major custom and support ICs (typical 520ST/520STM/early STF family)

  • Shifter (video DMA/serializer): Atari C025914‑38/‑38A; also seen as CO70713/I10356 on some boards.[27]
  • GLUE (bus/timing/GLUe logic): Atari C025915‑38A (and SMD successors).[28]
  • MMU (DRAM controller/multiplexer): Atari C025912‑38; later C100109 on later boards.[29]
  • DMA controller (ACSI/floppy DMA): Atari C025913‑38.[30]
  • Interrupt/timers/UART: Motorola MC68901 MFP (system timers, interrupt controller, 200 Hz tick, handshake lines).[31]
  • ACIAs: 2× Motorola MC6850 ACIA (keyboard/IKBD and MIDI), clocked at 500 kHz for MIDI 31.25 kbaud (÷16).[32][33]
  • Floppy controller: Western Digital WD1772, commonly WD1772‑00‑02 or ‑02‑02; later chips tolerate 16 MHz for HD‑floppy mods.[34][35]
  • PSG audio + parallel I/O: Yamaha YM2149F (AY‑3‑8910 compatible) provides 3 tone + noise channels and two 8‑bit I/O ports used by ST for printer and control lines.[36][37]

Input/Output and Expansion

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External connectors and electrical specs (original 520ST “short case”; all connectors are on the rear unless noted)

  • Video/monitor: 13‑pin DIN female. Signals: analog RGB (0–1.0 Vpp, 75 Ω), separate H/V sync (TTL, active low, ~3.3 kΩ source), monochrome TTL video output, composite video (only if RF modulator/composite encoder present; on pure 520ST usually composite sync only), mono audio out and audio in, +12 V pull‑up on pin 8, GPO on pin 3, and “mono detect” (pin 4) that forces 640×400/71 Hz when grounded.[38][39][40]
  • Serial (MODEM): DB‑25 male, RS‑232C subset using MC1488/1489‑style line drivers; DTR/RTS come from PSG I/O, CTS to MFP input (edge‑sensitive).[41][42]
  • Parallel (printer): DB‑25 female, 8‑bit data (pins 2–9) driven by YM2149 Port B, STROBE on pin 1 (from YM2149 Port A), BUSY on pin 11 to MFP; other Centronics status pins are not implemented on ST.[43][44]
  • MIDI: two DIN‑5 female (IN and combined OUT/THRU wiring). ACIA‑driven at 31.25 kbaud (500 kHz ÷16). Note that the ST’s MIDI OUT jack carries both OUT and THRU via nonstandard pin usage; use cables with only pins 4/5/2 wired for standard gear.[45][46]
  • External floppy: 14‑pin circular DIN female, Shugart‑style signals (Read Data, Index, Step, Direction, Write Data, Write Gate, Track 0, WP, Side 0 select, Drive selects, Motor On). Usually pin‑for‑pin straight‑through cable; model straps determine which select is B: by default.[47][48]
  • ACSI “DMA” (hard disk/laser printer): DB‑19 female. 8‑bit data bus D0..D7 plus /CS, IRQ, /RST, ACK, A1, R/W, REQ and multiple grounds; straight DB‑19M‑to‑DB‑19M cable.[49][50]
  • Cartridge: 40‑contact double‑sided edge connector supporting up to 128 KB ROM (A1–A15, D0–D15, LDS/UDS, ROM selects, +5 V).[51]
  • Joystick/mouse: two DE‑9 male (underside/front right). Port 0 is mouse/joystick; Port 1 joystick; +5 V on pin 7; ground on pin 8.[52]
  • Power (520ST/260ST external PSU): 7‑pin round DIN on computer: +5 V (multiple pins), +12 V, −12 V, grounds. Typical Atari spec: +5 V up to ~3 A, +12 V ~0.03–1 A (model‑dependent), −12 V ~0.03 A (used by RS‑232).[53]

Display and Graphics

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Video subsystem (Shifter + GLUE/MMU) characteristics:

  • Modes: 320×200 4bpp (16 colors), 640×200 2bpp (4 colors), 640×400 1bpp mono. Color modes at 15.7 kHz H with 50 Hz PAL or 60 Hz NTSC V; mono at ~35.7 kHz H and ~71.5 Hz V.[54]
  • Palette: 9‑bit RGB (3:3:3), 512 total colors; up to 16 onscreen in low‑res, 4 in medium; mono is black/white only.[55]
  • Hardware features: No hardware sprites or blitter on the original 520ST; some later STF/FM boards accept an optional BLiTTER (PLCC‑68) upgrade footprint but it was not standard on 520ST.[56]
  • Output levels and pins (DIN‑13): RGB pins 6/7/10; HSync 9, VSync 12; mono video 11; audio out 1; audio in 5; +12 V pull‑up on pin 8; ground 13; mono detect pin 4 (tie low for SM124 mode).[57][58]

Memory and Storage

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RAM organization and ROM layout

  • Standard RAM: 512 KB on system board, typically 16× 256k×1 DRAMs (varies by vendor), arranged as two 8‑bit banks to form the 16‑bit data path. Numerous later replacement boards allow 1–4 MB by adding new DRAM modules or SIMMs (wiring to MMU MAD/CAS/RAS lines as required).[59][60]
  • ROM/TOS: Early machines shipped with tiny 2‑chip boot ROMs that loaded TOS from disk; later 192 KB TOS 1.00 in 6× 32 KB DIPs; subsequent 192 KB TOS 1.02/1.04 appear in 2‑chip sets; part numbers for 6‑chip US TOS 1.00 include CO26160..CO26165.[61][62]
  • Floppy subsystem: WD1772 FDC at 8 MHz (250 kbit/s). Standard external single‑sided 3.5" SF354 (360 KB). The WD1772‑02‑02 variant often tolerates 16 MHz for 1.44 MB HD mods with appropriate gating and TOS support.[63][64]
  • Hard disks: External ACSI devices via DB‑19; up to eight chained targets with simple 8‑bit protocol similar to early SCSI (not electrically SCSI). Used by Atari SH204/205 and MegaFile drives and the SLM laser printer.[65]

Sound Capabilities

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  • PSG: Yamaha YM2149F (AY‑3‑8910 derivative) – 3 independent square‑wave tone channels plus 1 noise source; 5‑bit amplitude registers with 5‑bit envelope generator and 8 preset shapes; 8 octave range. No analog filter.[66]
  • Digital audio: No dedicated PCM in 520ST; CPU‑driven sample playback possible by rapid volume register updates or timer‑driven DAC on parallel port in software (common in demos and some games).[67]
  • Audio I/O: Mono line‑level out and an audio‑in on the 13‑pin monitor port (approx. 1 Vpp nominal).[68]

PCB Schematics & Service Guide

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Motherboard families and identifying service references commonly seen on 520ST/520STM:

Assembly / PBA Typical models Revisions seen Notable identifiers / notes
C070115 260/520ST (very early) Rev. 2 Earliest 520ST board reported; footprints for 2‑ROM boot TOS and 6‑ROM 192 KB TOS; minimal EMI chokes.[69]
C070231 260/520ST — Factory schematics drawing C070112 (Atari Japan); full community KiCad reconstruction available for repair work.[70][71]
C070243 520ST/520STM Rev. C, H, I Later short‑case boards; added numerous EMI chokes vs. C070115; common target for 1–4 MB RAM mods.[72][73]

Service/schematic references

  • Early factory schematics: “Atari (Japan) Corporation” drawing C070112 (1985) covering 260/520ST main logic; scans circulate online.[74]
  • 520/1040STE Field Service Manual (C302481‑001 Rev A, Aug 1991) includes many circuits compatible in concept (MFP, YM2149, DMA, WD1772) and is frequently referenced for troubleshooting I/O sections shared with ST family.[75]

Technical Details

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Comprehensive IC inventory (typical 520ST) – major functions and common part numbers:

Function Common part(s) / package Notes
CPU MC68000P8 (DIP‑64), MC68000FN8 (PLCC‑68) 8.00 MHz core.[76]
Video “Shifter” C025914‑38/‑38A; CO70713/I10356 Reads DRAM and serializes pixels.[77]
GLUE C025915‑38A Bus/timing decode, video timing assist.[78]
MMU (DRAM) C025912‑38; later C100109 Row/col multiplex/refresh to DRAM.[79]
DMA (ACSI/FDD) C025913‑38 8‑bit ACSI and FDC DMA, IRQ/DRQ handshake.[80]
Floppy controller WD1772‑00‑02 / ‑02‑02 250 kbit/s DD; many ‑02‑02 parts OK at 16 MHz for HD mods.[81]
MFP (timers/IRQs/U(S)ART) MC68901 200 Hz system tick, vectorized interrupts.[82]
ACIAs (MIDI/IKBD) 2× MC6850 7812.5 baud link to IKBD; 31.25 kbaud MIDI.[83]
Sound/PIO YM2149F 3‑voice PSG + I/O (printer strobe/data, FDD selects, RTS/DTR).[84]
RS‑232 transceivers MC1488, MC1489 (or equivalents) Level conversion for DB‑25 serial.[85]

PCB Schematics & Service Manual

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See earlier “PCB Schematics & Service Guide” for board IDs. Early official drawing: C070112 (Atari Japan), and community KiCad reconstruction for C070231 are recommended for board‑level repair tracing.[86][87]

Technical Documentation

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  • Atari 520ST Schematic (1985) – drawing C070112 (Atari Japan).[88]
  • Conversion of the 260/520ST motherboard C070231 schematics to KiCad (community).[89]
  • Atari STe 520/1040 Field Service Manual C302481‑001 Rev A (useful for shared subsystems and diagnostics).[90]
  • Atari 520ST Owner’s Manual (1985).[91]
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References

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  1. ↑ The Next Generation: New Computers at the Winter Consumer Electronics Show, Compute!, 1985-01—link(accessed 2025-08-13)
  2. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  3. ↑ A Turning Point for Atari? Report from the Winter CES, Compute!, 1986-02—link(accessed 2025-08-13)
  4. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  5. ↑ The Next Generation: New Computers at the Winter CES, Compute!, 1985-01—link(accessed 2025-08-13)
  6. ↑ A Turning Point for Atari? CES 1986 price reductions, Compute!, 1986-02—link(accessed 2025-08-13)
  7. ↑ MC68000P8 8 MHz Datasheet, Motorola (via AllDataSheet)—link(accessed 2025-08-13)
  8. ↑ MC68000FN8 Datasheet summary, Datasheets360—link(accessed 2025-08-13)
  9. ↑ 520ST RAM upgrade discussion (C070243 board), AtariAge Forums, 2022-09-14—link(accessed 2025-08-13)
  10. ↑ STRAM 4MB Atari 520ST(M) Expansion, PCBWay community, 2023-06-07—link(accessed 2025-08-13)
  11. ↑ Interiors: Opening up the 520ST (ports overview), Antic, 1985-08—link(accessed 2025-08-13)
  12. ↑ Atari ST Interfaces / Connectors / Cables Information, info-coach.fr—link(accessed 2025-08-13)
  13. ↑ The various Atari ST display modes, AtariForumWiki—link(accessed 2025-08-13)
  14. ↑ Atari ST monitor output and levels, oldcomputers.it—link(accessed 2025-08-13)
  15. ↑ General Instrument AY‑3‑8910 (Yamaha YM2149F variant), Wikipedia, 2025-08-13—link(accessed 2025-08-13)
  16. ↑ Yamaha YM2149 Manual, AtariForumWiki—link(accessed 2025-08-13)
  17. ↑ Atari 520ST Microcomputer (object record, measurements), Smithsonian NMAH—link(accessed 2025-08-13)
  18. ↑ Atari 520ST specifications (dimensions/weight), My Old Computers—link(accessed 2025-08-13)
  19. ↑ TOS ROM sizes and configurations, DOIT_ST archive—link(accessed 2025-08-13)
  20. ↑ The Next Generation: New Computers at the Winter CES, Compute!, 1985-01—link(accessed 2025-08-13)
  21. ↑ CES 1986: 1040ST introduction and 520ST price changes, Compute!, 1986-02—link(accessed 2025-08-13)
  22. ↑ MC68000P8 8 MHz Datasheet, Motorola—link(accessed 2025-08-13)
  23. ↑ MC68000FN8 8 MHz summary, Datasheets360—link(accessed 2025-08-13)
  24. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  25. ↑ Atari ST hardware register listing and memory map, AtariForumWiki—link(accessed 2025-08-13)
  26. ↑ ST/STE video frequencies derivation, Atari‑Forum, 2017-12-24—link(accessed 2025-08-13)
  27. ↑ Atari custom IC part numbers list (community collation), AtariAge Forums, 2016-01-05—link(accessed 2025-08-13)
  28. ↑ Atari custom IC part numbers list, AtariAge Forums—link(accessed 2025-08-13)
  29. ↑ Atari custom IC list, AtariAge Forums—link(accessed 2025-08-13)
  30. ↑ DOIT: DMA and ROM discussion with WD1772 photo, DOIT_ST archive—link(accessed 2025-08-13)
  31. ↑ Concise Atari ST Programmer’s Reference – MC68901 MFP, bighole.nl mirror—link(accessed 2025-08-13)
  32. ↑ Atari 520ST (MIDI hardware notes), Electronics & Music Maker, 1985-07—link(accessed 2025-08-13)
  33. ↑ Concise Atari ST Programmer’s Reference – MIDI/IKBD ACIAs, bighole.nl mirror—link(accessed 2025-08-13)
  34. ↑ HD Disk ST (WD1772 02‑02, 16 MHz HD mods), DOIT_ST archive—link(accessed 2025-08-13)
  35. ↑ The LaST upgrade – Atari 1.44 MB floppy mod notes, exxosforum—link(accessed 2025-08-13)
  36. ↑ Yamaha YM2149 Manual, AtariForumWiki—link(accessed 2025-08-13)
  37. ↑ Atari ST Internals – YM2149 registers and I/O, Ctrl‑Alt‑Rees Atari Museum mirror—link(accessed 2025-08-13)
  38. ↑ Atari ST monitor connector, levels and modes, oldcomputers.it—link(accessed 2025-08-13)
  39. ↑ Atari ST Interfaces: Video DIN‑13 pinout, info-coach.fr—link(accessed 2025-08-13)
  40. ↑ Atari ST monitor pinout, pinouts.ru—link(accessed 2025-08-13)
  41. ↑ Atari Serial Interface pinout and notes, info-coach.fr—link(accessed 2025-08-13)
  42. ↑ Atari 520/1040STE Field Service Manual (I/O schematics include 1488/1489), ManualsLib, 1991—link(accessed 2025-08-13)
  43. ↑ ST Parallel Interface (signals and rates), Concise ST Programmer’s Reference (mirror)—link(accessed 2025-08-13)
  44. ↑ Atari Parallel Interface pinout, info-coach.fr—link(accessed 2025-08-13)
  45. ↑ Atari MIDI connectors (special OUT/THRU wiring), info-coach.fr—link(accessed 2025-08-13)
  46. ↑ Atari 520ST – MIDI hardware, E&MM, 1985-07—link(accessed 2025-08-13)
  47. ↑ Floppy Disc Connector (DIN‑14), info-coach.fr—link(accessed 2025-08-13)
  48. ↑ Atari Floppy port pinout, pinouts.ru—link(accessed 2025-08-13)
  49. ↑ Atari DMA/ACSI Connector pinout, info-coach.fr—link(accessed 2025-08-13)
  50. ↑ Atari ACSI DMA pinout, pinouts.ru—link(accessed 2025-08-13)
  51. ↑ Atari Cartridge Interface pinout, info-coach.fr—link(accessed 2025-08-13)
  52. ↑ Mouse/Joystick connectors, info-coach.fr—link(accessed 2025-08-13)
  53. ↑ Power Supply FAQ – Atari 260/520ST 7‑pin DIN pinout (PS31/C070102‑3), Individual Computers Wiki—link(accessed 2025-08-13)
  54. ↑ Atari ST display modes and timings, AtariForumWiki—link(accessed 2025-08-13)
  55. ↑ Atari ST, Wikipedia, 2025-08-11—link(accessed 2025-08-13)
  56. ↑ 520STf/m Blitter upgrade experiences, AtariAge Forums, 2014-11—link(accessed 2025-08-13)
  57. ↑ ST monitor connector (pinout/levels), oldcomputers.it—link(accessed 2025-08-13)
  58. ↑ Atari ST monitor pinout, pinouts.ru—link(accessed 2025-08-13)
  59. ↑ 520ST RAM upgrade discussion, C070243 boards, AtariAge Forums, 2022-09-14—link(accessed 2025-08-13)
  60. ↑ STRAM 4MB Atari 520ST(M) Expansion, PCBWay, 2023-06-07—link(accessed 2025-08-13)
  61. ↑ TOS ROM sizes and families, DOIT_ST archive—link(accessed 2025-08-13)
  62. ↑ Best Electronics – Atari TOS ROM part numbers, Best Electronics—link(accessed 2025-08-13)
  63. ↑ HD Disk ST (WD1772 02‑02 at 16 MHz), DOIT_ST archive—link(accessed 2025-08-13)
  64. ↑ LaST 1.44 MB mod notes, exxosforum—link(accessed 2025-08-13)
  65. ↑ Atari DMA/ACSI interface, info-coach.fr—link(accessed 2025-08-13)
  66. ↑ YM2149 manual (features), AtariForumWiki—link(accessed 2025-08-13)
  67. ↑ Sounds of the Atari ST (historical overview)– Paul Lefebvre, GoTo10Retro, 2024-01-10—link(accessed 2025-08-13)
  68. ↑ Monitor connector – audio in/out levels, oldcomputers.it—link(accessed 2025-08-13)
  69. ↑ (Almost) Unknown Atari ST schematics, AtariAge Forums, 2021-04-27—link(accessed 2025-08-13)
  70. ↑ Atari 520ST Schematic (1985), drawing set, Internet Archive—link(accessed 2025-08-13)
  71. ↑ KiCad conversion of Atari 260/520ST C070231, AtariAge Forums, 2024-05-25—link(accessed 2025-08-13)
  72. ↑ Why do some Atari ST motherboards include many inductors?, Retrocomputing StackExchange—link(accessed 2025-08-13)
  73. ↑ Atari ST recap notes by board (C070243 Rev C/H), Console5 TechWiki—link(accessed 2025-08-13)
  74. ↑ Atari 520ST Schematic (1985), Internet Archive—link(accessed 2025-08-13)
  75. ↑ Atari STe 520/1040 Field Service Manual, C302481‑001, ManualsLib, 1991—link(accessed 2025-08-13)
  76. ↑ MC68000P8 datasheet, Motorola—link(accessed 2025-08-13)
  77. ↑ Atari custom IC list, AtariAge—link(accessed 2025-08-13)
  78. ↑ Atari custom IC list, AtariAge—link(accessed 2025-08-13)
  79. ↑ Atari custom IC list, AtariAge—link(accessed 2025-08-13)
  80. ↑ DMA, ROM, and WD1772 photo, DOIT_ST—link(accessed 2025-08-13)
  81. ↑ HD Disk ST, DOIT_ST—link(accessed 2025-08-13)
  82. ↑ MFP details, Concise ST Prog Ref—link(accessed 2025-08-13)
  83. ↑ Atari 520ST (MIDI hardware), E&MM, 1985-07—link(accessed 2025-08-13)
  84. ↑ ST Internals – YM2149 regs, Ctrl‑Alt‑Rees—link(accessed 2025-08-13)
  85. ↑ STE Service Manual (I/O section), ManualsLib, 1991—link(accessed 2025-08-13)
  86. ↑ Atari 520ST Schematic (1985), Internet Archive—link(accessed 2025-08-13)
  87. ↑ KiCad conversion for 260/520ST mainboard, AtariAge, 2024-05-25—link(accessed 2025-08-13)
  88. ↑ Atari 520ST Schematic (1985), Internet Archive—link(accessed 2025-08-13)
  89. ↑ KiCad conversion for 260/520ST, AtariAge Forums, 2024-05-25—link(accessed 2025-08-13)
  90. ↑ Atari STe Field Service Manual, ManualsLib, 1991—link(accessed 2025-08-13)
  91. ↑ Atari 520ST Owner’s Manual, Rev. B (1985), Internet Archive—link(accessed 2025-08-13)
  92. ↑ 1040ST announcement and bundles, Compute!, 1986-02—link(accessed 2025-08-13)
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