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{{Infobox computer
| name          = Atari 520ST
| image          = [[File:Atari 520ST.jpg]]
| manufacturer  = Atari Corporation
| type          = Home computer
| caption        = 520ST with external SF354 floppy drive and SC1224 color monitor
| release date  = {{Start date|1985|06}} <!-- limited release began June 1985; broadly available July 1985 -->
| discontinued  = 1986 <!-- superseded in production by 520STF/520STM; ST line discontinued 1993 -->
| price          = US$799 bundle (520ST + SF354 + SM124 mono), US$999 (520ST + SF354 + SC1224 RGB) at U.S. launch (Oct. 1985).<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue65/520st.php |title=Atari 520ST: A Hands-On Report |author=Tom R. Halfhill |publisher=Compute! |date=1985-10 |access-date=2025-08-13}}</ref>
| cpu            = Motorola MC68000P8 @ 8 MHz (64‑pin DIP; some later boards MC68000FN8 PLCC)<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
| memory        = 512 KB RAM standard (DRAM, 16×256‑Kbit); commonly expanded to 1 MB; third‑party upgrades up to 4 MB.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
| storage        = External 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).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
| display        = Low 320×200 (16 colors), Medium 640×200 (4 colors), High 640×400 (mono); 50/60 Hz color, 71.2 Hz mono.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Flat_Panel_Displays |title=The various Atari ST display modes |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
| sound          = Yamaha YM2149F PSG, 3 tone voices + 1 noise source, programmable envelopes; mono output via monitor connector pin 1.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari ST Interfaces / Connectors / Cables |author=Jean-Louis Guérin |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
| dimensions    = 47.6 × 24.1 × 7.0 cm (18.75 × 9.5 × 2.75 in)<ref>{{cite web |url=https://americanhistory.si.edu/collections/object/nmah_1330999 |title=Atari 520ST Microcomputer |publisher=Smithsonian National Museum of American History |access-date=2025-08-13}}</ref>
| weight        = ≈4.3 kg (9.5 lb) (unit without external PSU).<ref>{{cite web |url=https://myoldcomputers.com/museum/computers/atari_520st.htm |title=Atari 520ST |publisher=My Old Computers Museum |access-date=2025-08-13}}</ref>
| os            = Atari 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).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_TOS |title=Atari TOS |publisher=Wikipedia |date=2024-12-01 |access-date=2025-08-13}}</ref>
| predecessor    = [[Atari 8-bit family]]
| successor      = [[Atari 520STF]] / [[Atari 1040STF]]
}}


The Atari 520ST was introduced at the Winter CES in January 1985, six months before the Commodore Amiga. However, Atari took some precautions, as the operating system didn't yet exist in ROM and had to be loaded from a floppy disk upon power-up (although, of course, the Amiga did too). The 520ST also didn't have an internal floppy disk drive, but later versions included one.
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).<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue65/520st.php |title=Atari 520ST: A Hands‑On Report |author=Tom R. Halfhill |publisher=Compute! |date=1985-10 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>


It included an external 3.5-inch, single-sided 360K floppy disk drive, mouse, and monochrome monitor; it was priced at $799.
== 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).<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
* Bus: 24‑bit address bus (16 MB address space), 16‑bit data bus; unified memory architecture with video DMA contention from the Shifter.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Flat_Panel_Displays |title=The various Atari ST display modes |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts list (ST custom ICs) |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
** GLUE (bus/logic/timing): C025915‑38A; also CO70714/I10355.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts list (ST custom ICs) |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
** MMU (DRAM controller): C025912‑38; later C100109/I10353.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts list (ST custom ICs) |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
** DMA (ACSI/floppy DMA): C025913‑38 (Atari DMA controller).<ref>{{cite web |url=https://forums1128.rssing.com/chan-4289726/all_p480.html |title=Atari custom IC list (forum sale; part numbers collated) |publisher=RSSing (AtariAge content mirror) |access-date=2025-08-13}}</ref>
** MFP (interrupts/timers/serial): Motorola MC68901.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=Concise Atari ST 68000 Programmer's Reference Guide – MC68901 MFP |publisher=bighole.nl mirror |access-date=2025-08-13}}</ref>
** ACIA (keyboard and MIDI UARTs): 2× Motorola MC6850.<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue73/st_hints_tips.php |title=ST Hints & Tips (IKBD and ACIA) |publisher=Compute! |date=1986 |access-date=2025-08-13}}</ref>
** Floppy controller: Western Digital WD1772 (commonly WD1772‑00‑02; later ‑02‑02 parts tolerate 16 MHz for HD mods).<ref>{{cite web |url=https://www.exxosforum.co.uk/atari/store2/0268.html |title=WD1772 00‑02 floppy controller |publisher=exxosforum store |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Flat_Panel_Displays |title=The various Atari ST display modes |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>


Both the Amiga and the 520ST were based on the Motorola 68000 CPU, which had a 16-bit external bus and a 32-bit internal bus; hence, the 520 "ST" stands for "sixteen/thirty-two." The operating system is "TOS" or "Tramiel Operating System."
== 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.<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari ST Interfaces / Cables – DIN‑13 monitor details |publisher=info-coach.fr |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://old.pinouts.ru/Video/AtariStMonitor_pinout.shtml |title=Atari ST monitor pinout |publisher=pinouts.ru |access-date=2025-08-13}}</ref>
* Serial (RS‑232C): DB‑25 male; line drivers/receivers typically MC1488/MC1489 on board; handled by MFP 68901 for asynchronous serial I/O.<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari 520/1040 STe Field Service Manual (I/O overview; 1488/1489 shown) |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>
* Parallel printer: DB‑25 female (Centronics‑style handshaking; unidirectional data 0–7 with Busy and Strobe).<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari 520/1040 STe Field Service Manual |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>
* MIDI: 2× 5‑pin DIN (MIDI IN and MIDI OUT/THRU), opto‑isolated input, serviced by one of the MC6850 ACIAs.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=Concise Atari ST 68000 Programmer's Reference Guide – Intelligent Keyboard & MIDI |publisher=bighole.nl |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
* ACSI hard‑disk/Laser printer: DB‑19 female (“DMA” port). Bus mastered by Atari DMA chip; nominal 10 Mbit/s transfer capability.<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix D/E Connector Pinouts |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
* Cartridge: 40‑way edge connector, 128 KB ROM address space.<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix E Specs |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>


The 520ST (and Amiga) have a graphical operating system, or GUI, similar to that of the Apple Macintosh, which was released a year earlier. The 520ST used Digital Research's GEM GUI, although it's not as good as that of the Macintosh or Amiga.
== Gallery ==
<gallery>
File:Atari 520ST.jpg|Atari 520ST (original short case with external PSU and external floppy).
File:Atari 520ST motherboard C070243.jpg|Motherboard C070243 (typical early 520ST; 6 ROM sockets visible).
File:Atari ST SC1224.jpg|SC1224 RGB monitor (15.75 kHz color) used with low/medium‑res modes.
File:Atari SM124.jpg|SM124 monochrome monitor (640×400 @ ~71.2 Hz).
</gallery>


One thing that IS very good is the built-in VT52 emulation. The 520ST can act as a dumb terminal, communicating through its serial port with another system.
== 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.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Flat_Panel_Displays |title=The various Atari ST display modes |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari ST Interfaces / Cables – DIN‑13 monitor details |publisher=info-coach.fr |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://old.pinouts.ru/Video/AtariStMonitor_pinout.shtml |title=Atari ST monitor pinout |publisher=pinouts.ru |access-date=2025-08-13}}</ref>


There was a great rivalry between Amiga and Atari users, each convinced that their computer system was superior, or at least hoped it was.
== PCB Schematics & Service Manual ==
Motherboard families and service references commonly encountered in 520ST/520STF/520STFM repairs.
; Motherboard assemblies (selection)
{| class="wikitable"
! 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.<ref>{{cite web |url=https://github.com/a2ff/splitrom |title=Atari ST ROM layouts and motherboards |publisher=GitHub (splitrom) |access-date=2025-08-13}}</ref>
|-
| C070231 || 260/520ST || — || 1985 schematic reconstructed to KiCad (community).<ref>{{cite web |url=https://forums.atariage.com/topic/367081-conversion-of-the-schematics-of-the-atari-260520st-motherboard-into-kicad/ |title=Conversion of the schematics of the Atari 260/520ST motherboard into KiCad |publisher=AtariAge Forums |date=2024-05-25 |access-date=2025-08-13}}</ref>
|-
| C070243 || 520ST || Revs C/E/F/H/I || Common early 520ST board; many variants 2‑ or 6‑ROM.<ref>{{cite web |url=https://docs.sidecartridge.com/sidecartridge-tos/compatibility/ |title=Motherboard model numbers and ROM configurations |publisher=Sidecartridge Docs |access-date=2025-08-13}}</ref>
|-
| C103414‑001 || 520STFM || Rev. 1 / 1.1 || Later STFM (integrated floppy + RF); 2‑ or 6‑ROM.<ref>{{cite web |url=https://docs.sidecartridge.com/sidecartridge-tos/compatibility/ |title=Motherboard model numbers and ROM configurations |publisher=Sidecartridge Docs |access-date=2025-08-13}}</ref>
|}
; Schematics and service docs
* Atari 520ST Schematic (1985), 3 pages (core logic, video, I/O).<ref>{{cite web |url=https://archive.org/details/Atari_520ST_Schematic_1985 |title=Atari 520ST Schematic [1985] |publisher=Internet Archive |access-date=2025-08-13}}</ref>
* Atari 1040ST Schematic Rev C (C070787, 1986‑12‑26) – useful for STf/fm video/GLUE/MMU references.<ref>{{cite web |url=https://archive.org/details/Atari_1040ST_Schematic_Rev_C_C070787_Dec_26_1986 |title=Atari 1040ST Schematic Rev C C070787 [Dec 26, 1986] |publisher=Internet Archive |access-date=2025-08-13}}</ref>
* Field Service manual (STe; later architecture but same 68000, WD1772/ACSI/I/O blocks; includes port pin summaries and diagnostic flow).<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari 520/1040 STe Field Service Manual (C302481‑001 Rev. A) |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>


In addition to the 520ST, Atari released the 1040ST, with a built-in floppy disk drive and 1 MB of RAM (the 520ST has 512 KB).
== 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.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts list (MMU) |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Specs show 16,384‑byte ROM for boot units |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_TOS |title=Atari TOS |publisher=Wikipedia |date=2024-12-01 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/STINTERN.htm |title=Atari ST internals quick map (ROM base addresses) |publisher=Ctrl‑Alt‑Rees (Atari Museum mirror) |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/STINTERN.htm |title=Atari ST internals quick map |publisher=Ctrl‑Alt‑Rees |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Floppy capacity |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/040a.htm |title=DOIT ST: AJAX/WD1772 notes |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
* Hard disks: ACSI port (DB‑19), DMA‑based; SH204 and later drives supported via AHDI. Manual quotes 10 Mbit/s peak bus rate.<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix E |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>


In 1987, the new and improved Mega ST was launched.
== Related Pages ==
[[File:Atari 520 ST + with monochrome monitor SM 124.jpg|thumb]]
* [[Atari 520ST General Maintenance]]
A few years later, in 1989, the ST laptop, the Stacy, appeared.
* [[Atari 520ST Troubleshooting Guide]]
* [[Atari 520ST Capacitor Replacement Guide]]
* [[Atari 520ST Floppy Drive Upgrade (SF354→SF314)]]
* [[Atari ST ACSI Hard Disk Setup Guide]]
 
== 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.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST (sound overview) |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
* Output: monaural line/speaker feed on monitor DIN‑13 pin 1; internal amplifier in Atari monitors (SM124/SC1224).<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari ST Interfaces / Cables – audio pins |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://www.keelog.com/atari-520st-psu-modern-gray-us-replacement-atari-520st-power-supply-us-plug/ |title=Replacement Atari 520ST PSU (specs and rails) |publisher=Keelog |access-date=2025-08-13}}</ref>
* Keyboards: clean membrane and plungers; verify IKBD (HD6301) comms and MC6850 ACIA socket contacts.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_3_utilities.htm |title=Concise Atari ST 68000 Programmer's Reference – IKBD |publisher=bighole.nl |access-date=2025-08-13}}</ref>
 
See the dedicated page: [[Atari 520ST General Maintenance]].
 
== Technical Details ==
; Major ICs and revisions (typical)
{| class="wikitable"
! Function !! Part numbers (examples) !! Notes
|-
| CPU || MC68000P8 / MC68000FN8 || 8 MHz, 24‑bit address / 16‑bit data.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
|-
| Shifter (video) || C025914‑38/‑38A; CO70713/I10356 || Generates RGB/mono from RAM fetches.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
|-
| GLUE || C025915‑38A; CO70714/I10355 || Bus/timing, interrupts, address decode.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
|-
| MMU || C025912‑38; C100109/I10353 || DRAM control, video base.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0303.htm |title=DOIT ST: Parts |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
|-
| DMA (ACSI/floppy) || C025913‑38 || ACSI/HDD and FDD DMA handshakes.<ref>{{cite web |url=https://forums1128.rssing.com/chan-4289726/all_p480.html |title=Atari custom IC list |publisher=RSSing (AtariAge mirror) |access-date=2025-08-13}}</ref>
|-
| Floppy controller || WD1772‑00‑02 / WD1772‑02‑02 || 8 MHz DD operation; overclockable variants for HD mods.<ref>{{cite web |url=https://www.exxosforum.co.uk/atari/store2/0268.html |title=WD1772 00‑02 floppy controller |publisher=exxosforum store |access-date=2025-08-13}}</ref>
|-
| MFP || MC68901 || Timers/interrupts/serial.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=ST Programmer's Guide – MFP |publisher=bighole.nl |access-date=2025-08-13}}</ref>
|-
| ACIAs || 2× MC6850 || Keyboard (IKBD) and MIDI UARTs.<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue73/st_hints_tips.php |title=ST Hints & Tips – 6301 IKBD and ACIA |publisher=Compute! |access-date=2025-08-13}}</ref>
|-
| Sound || Yamaha YM2149F || 3‑voice PSG with noise; mono output.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_ST |title=Atari ST |publisher=Wikipedia |date=2025-08-11 |access-date=2025-08-13}}</ref>
|}
 
; 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).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) – Appendix E |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
 
== 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).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
 
== 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.”<ref>{{cite web |url=https://archive.org/details/Atari_520ST_Schematic_1985 |title=Atari 520ST Schematic [1985] |publisher=Internet Archive |access-date=2025-08-13}}</ref>
 
== 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).<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari STe Field Service Manual – Symptom and flowcharts (applicable concepts) |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>
* No video in color but mono OK: DIN‑13 cable/pin 4 state; verify composite sync/video behavior (depends on modulator presence).<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari Video Connector (details on composite vs sync) |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/080103.htm |title=DOIT ST: WD1772 notes |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
* ACSI device timeouts: Inspect DB‑19 connector solder joints; verify DMA (C025913‑38).<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual – ACSI overview |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
* Keyboard/mouse erratic: check IKBD 6301 comms at 7812.5 bps via the MC6850 ACIA; reseat keyboard cable.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=ST Programmer's Guide – IKBD/ACIA |publisher=bighole.nl |access-date=2025-08-13}}</ref>
 
== 🧠 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.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=Concise ST Programmer’s Guide – MFP registers |publisher=bighole.nl |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/STINTERN.htm |title=Atari ST internals quick map |publisher=Ctrl‑Alt‑Rees |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_TOS |title=Atari TOS |publisher=Wikipedia |date=2024-12-01 |access-date=2025-08-13}}</ref>
 
== Technical Documentation ==
* [[Atari 520ST Owner’s Manual (Rev. B, 1985)]].<ref>{{cite web |url=https://archive.org/download/Atari_520ST_Owners_Manual_Rev._B_1985/Atari_520ST_Owners_Manual_Rev._B_1985.pdf |title=Atari 520ST Owner's Manual (Rev. B) |publisher=Atari Corporation |date=1985 |access-date=2025-08-13}}</ref>
* [[Atari 520ST Schematic (1985)]].<ref>{{cite web |url=https://archive.org/details/Atari_520ST_Schematic_1985 |title=Atari 520ST Schematic [1985] |publisher=Internet Archive |access-date=2025-08-13}}</ref>
* [[Atari 1040ST Schematic Rev C (C070787) – 1986]].<ref>{{cite web |url=https://archive.org/details/Atari_1040ST_Schematic_Rev_C_C070787_Dec_26_1986 |title=Atari 1040ST Schematic Rev C C070787 [Dec 26, 1986] |publisher=Internet Archive |access-date=2025-08-13}}</ref>
* [[Atari TOS Versions and ROM Configurations]].<ref>{{cite web |url=https://en.wikipedia.org/wiki/Atari_TOS |title=Atari TOS |publisher=Wikipedia |date=2024-12-01 |access-date=2025-08-13}}</ref>
* [[Atari ST Interfaces and Pinouts]].<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari Video/Interfaces |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
 
== References ==
<references/>
 
[[Category:Computers]]
[[Category:Home computers]]
[[Category:Atari ST]]
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