Editing Atari 520ST

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{{Infobox computer
| name = Atari 520ST
| image = [[File:Atari 520ST system.jpg|250px]]
| manufacturer = Atari Corporation
| type = Home computer / personal computer
| caption = Early “short-case” 520ST with external PSU and external SF354 floppy drive
| release date = {{Start date|1985|06}}<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue59/next_generation.php |title=The Next Generation: New Computers at the Winter Consumer Electronics Show |publisher=Compute! |date=1985-01 |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>
| discontinued = 1986–1987 (superseded in production by STF/STM variants; ST family continued to 1993)<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue71/electronics_show.php |title=A Turning Point for Atari? Report from the Winter CES |publisher=Compute! |date=1986-02 |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>
| price = US$599 (CPU only, announced Jan 1985); US$699 mono bundle / US$899 color bundle (Jan 1986 SRP)<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue59/next_generation.php |title=The Next Generation: New Computers at the Winter CES |publisher=Compute! |date=1985-01 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.atarimagazines.com/compute/issue71/electronics_show.php |title=A Turning Point for Atari? CES 1986 price reductions |publisher=Compute! |date=1986-02 |access-date=2025-08-13}}</ref>
| cpu = Motorola MC68000P8/MC68000FN8 @ 8.00 MHz<ref>{{cite web |url=https://www.alldatasheet.net/datasheet-pdf/pdf/135463/MOTOROLA/MC68000P8.html |title=MC68000P8 8 MHz Datasheet |publisher=Motorola (via AllDataSheet) |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.datasheets360.com/part/detail/mc68000fn8/-3094971039532773301/ |title=MC68000FN8 Datasheet summary |publisher=Datasheets360 |access-date=2025-08-13}}</ref>
| memory = 512 KB RAM standard; expandable to 1–4 MB via aftermarket DRAM/SIMM upgrades<ref>{{cite web |url=https://forums.atariage.com/topic/337575-520st-ram-upgrade-question/page/2/ |title=520ST RAM upgrade discussion (C070243 board) |publisher=AtariAge Forums |date=2022-09-14 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.pcbway.com/project/shareproject/STRAM_4MB_Atari_520ST_M_Expansion_692e0348.html |title=STRAM 4MB Atari 520ST(M) Expansion |publisher=PCBWay community |date=2023-06-07 |access-date=2025-08-13}}</ref>
| storage = External 3.5-inch SF354 SSDD 360 KB (standard); optional SF314 DSDD 720 KB; external ACSI hard disks (SH/MegaFile series)<ref>{{cite web |url=https://www.atarimagazines.com/v4n4/interiors.php |title=Interiors: Opening up the 520ST (ports overview) |publisher=Antic |date=1985-08 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari ST Interfaces / Connectors / Cables Information |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
| display = 13‑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)<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><ref>{{cite web |url=https://www.oldcomputers.it/parts/atari/1040STF/docs/connettore_monitor_st.htm |title=Atari ST monitor output and levels |publisher=oldcomputers.it |access-date=2025-08-13}}</ref>
| sound = Yamaha YM2149F 3‑voice PSG (square/noise, 5‑bit envelope); mono line out on monitor port; MIDI In/Out<ref>{{cite web |url=https://en.wikipedia.org/wiki/General_Instrument_AY-3-8910 |title=General Instrument AY‑3‑8910 (Yamaha YM2149F variant) |publisher=Wikipedia |date=2025-08-13 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Yamaha_YM2149_Manual |title=Yamaha YM2149 Manual |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
| dimensions = 70 mm × 476 mm × 241 mm (2.75 × 18.75 × 9.5 in)<ref>{{cite web |url=https://americanhistory.si.edu/collections/object/nmah_1330999 |title=Atari 520ST Microcomputer (object record, measurements) |publisher=Smithsonian NMAH |access-date=2025-08-13}}</ref>
| weight = ≈4.3 kg (9 lb 7 oz)<ref>{{cite web |url=https://myoldcomputers.com/museum/computers/atari_520st.htm |title=Atari 520ST specifications (dimensions/weight) |publisher=My Old Computers |access-date=2025-08-13}}</ref>
| os = Atari TOS with Digital Research GEM; early units boot TOS from disk via small boot ROM, later 192 KB ROM‑TOS<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0f0601.htm |title=TOS ROM sizes and configurations |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
| predecessor = [[Atari 65XE]]
| successor = [[Atari 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.<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue59/next_generation.php |title=The Next Generation: New Computers at the Winter CES |publisher=Compute! |date=1985-01 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.atarimagazines.com/compute/issue71/electronics_show.php |title=CES 1986: 1040ST introduction and 520ST price changes |publisher=Compute! |date=1986-02 |access-date=2025-08-13}}</ref>

== Architecture and Processor ==
Exact CPU and core architecture
* CPU: Motorola MC68000 (DIP‑64 MC68000P8 on early boards; later boards sometimes PLCC‑68 MC68000FN8), nominal 8.00 MHz.<ref>{{cite web |url=https://www.alldatasheet.net/datasheet-pdf/pdf/135463/MOTOROLA/MC68000P8.html |title=MC68000P8 8 MHz Datasheet |publisher=Motorola |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.datasheets360.com/part/detail/mc68000fn8/-3094971039532773301/ |title=MC68000FN8 8 MHz summary |publisher=Datasheets360 |access-date=2025-08-13}}</ref>
* 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).<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>
* 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.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Atari_ST/STe/MSTe/TT/F030_Hardware_Register_Listing |title=Atari ST hardware register listing and memory map |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://www.atari-forum.com/viewtopic.php?t=32842 |title=ST/STE video frequencies derivation |publisher=Atari‑Forum |date=2017-12-24 |access-date=2025-08-13}}</ref>

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.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC part numbers list (community collation) |publisher=AtariAge Forums |date=2016-01-05 |access-date=2025-08-13}}</ref>
* GLUE (bus/timing/GLUe logic): Atari C025915‑38A (and SMD successors).<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC part numbers list |publisher=AtariAge Forums |access-date=2025-08-13}}</ref>
* MMU (DRAM controller/multiplexer): Atari C025912‑38; later C100109 on later boards.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC list |publisher=AtariAge Forums |access-date=2025-08-13}}</ref>
* DMA controller (ACSI/floppy DMA): Atari C025913‑38.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0904.htm |title=DOIT: DMA and ROM discussion with WD1772 photo |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref>
* Interrupt/timers/UART: Motorola MC68901 MFP (system timers, interrupt controller, 200 Hz tick, handshake lines).<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 Programmer’s Reference – MC68901 MFP |publisher=bighole.nl mirror |access-date=2025-08-13}}</ref>
* ACIAs: 2× Motorola MC6850 ACIA (keyboard/IKBD and MIDI), clocked at 500 kHz for MIDI 31.25 kbaud (÷16).<ref>{{cite web |url=https://www.muzines.co.uk/articles/atari-520st/9092 |title=Atari 520ST (MIDI hardware notes) |publisher=Electronics & Music Maker |date=1985-07 |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_2.htm |title=Concise Atari ST Programmer’s Reference – MIDI/IKBD ACIAs |publisher=bighole.nl mirror |access-date=2025-08-13}}</ref>
* Floppy controller: Western Digital WD1772, commonly WD1772‑00‑02 or ‑02‑02; later chips tolerate 16 MHz for HD‑floppy mods.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0b03.htm |title=HD Disk ST (WD1772 02‑02, 16 MHz HD mods) |publisher=DOIT_ST archive |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://exxosforum.co.uk/atari/last/floppy/index.htm |title=The LaST upgrade – Atari 1.44 MB floppy mod notes |publisher=exxosforum |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Yamaha_YM2149_Manual |title=Yamaha YM2149 Manual |publisher=AtariForumWiki |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/STINTERN.htm |title=Atari ST Internals – YM2149 registers and I/O |publisher=Ctrl‑Alt‑Rees Atari Museum mirror |access-date=2025-08-13}}</ref>

== Input/Output and Expansion ==
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.<ref>{{cite web |url=https://www.oldcomputers.it/parts/atari/1040STF/docs/connettore_monitor_st.htm |title=Atari ST monitor connector, levels and modes |publisher=oldcomputers.it |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari ST Interfaces: Video DIN‑13 pinout |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 (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).<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari Serial Interface pinout and notes |publisher=info-coach.fr |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari 520/1040STE Field Service Manual (I/O schematics include 1488/1489) |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>
* 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.<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 Parallel Interface (signals and rates) |publisher=Concise ST Programmer’s Reference (mirror) |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari Parallel Interface pinout |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari MIDI connectors (special OUT/THRU wiring) |publisher=info-coach.fr |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://www.muzines.co.uk/articles/atari-520st/9092 |title=Atari 520ST – MIDI hardware |publisher=E&MM |date=1985-07 |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Floppy Disc Connector (DIN‑14) |publisher=info-coach.fr |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://old.pinouts.ru/HD/AtariFloppyPort_pinout.shtml |title=Atari Floppy port pinout |publisher=pinouts.ru |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari DMA/ACSI Connector pinout |publisher=info-coach.fr |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://old.pinouts.ru/ParallelPorts/AtariAcsiDma_pinout.shtml |title=Atari ACSI DMA pinout |publisher=pinouts.ru |access-date=2025-08-13}}</ref>
* Cartridge: 40‑contact double‑sided edge connector supporting up to 128 KB ROM (A1–A15, D0–D15, LDS/UDS, ROM selects, +5 V).<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Atari Cartridge Interface pinout |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web |url=https://info-coach.fr/atari/hardware/interfaces.php |title=Mouse/Joystick connectors |publisher=info-coach.fr |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://wiki.icomp.de/wiki/Powersupply_FAQ |title=Power Supply FAQ – Atari 260/520ST 7‑pin DIN pinout (PS31/C070102‑3) |publisher=Individual Computers Wiki |access-date=2025-08-13}}</ref>

== Display and Graphics ==
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.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Flat_Panel_Displays |title=Atari ST display modes and timings |publisher=AtariForumWiki |access-date=2025-08-13}}</ref>
* 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.<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 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.<ref>{{cite web |url=https://forums.atariage.com/topic/232054-520stfm-blitter-upgrade/ |title=520STf/m Blitter upgrade experiences |publisher=AtariAge Forums |date=2014-11 |access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web |url=https://www.oldcomputers.it/parts/atari/1040STF/docs/connettore_monitor_st.htm |title=ST monitor connector (pinout/levels) |publisher=oldcomputers.it |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>

== Memory and Storage ==
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).<ref>{{cite web|url=https://forums.atariage.com/topic/337575-520st-ram-upgrade-question/page/2/|title=520ST RAM upgrade discussion, C070243 boards|publisher=AtariAge Forums|date=2022-09-14|access-date=2025-08-13}}</ref><ref>{{cite web|url=https://www.pcbway.com/project/shareproject/STRAM_4MB_Atari_520ST_M_Expansion_692e0348.html|title=STRAM 4MB Atari 520ST(M) Expansion|publisher=PCBWay|date=2023-06-07|access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web|url=https://mikrosk.github.io/doitarchive/doit_st/0f0601.htm|title=TOS ROM sizes and families|publisher=DOIT_ST archive|access-date=2025-08-13}}</ref><ref>{{cite web|url=https://www.best-electronics-ca.com/custom-i.htm|title=Best Electronics – Atari TOS ROM part numbers|publisher=Best Electronics|access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web|url=https://mikrosk.github.io/doitarchive/doit_st/0b03.htm|title=HD Disk ST (WD1772 02‑02 at 16 MHz)|publisher=DOIT_ST archive|access-date=2025-08-13}}</ref><ref>{{cite web|url=https://exxosforum.co.uk/atari/last/floppy/index.htm|title=LaST 1.44 MB mod notes|publisher=exxosforum|access-date=2025-08-13}}</ref>
* 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.<ref>{{cite web|url=https://info-coach.fr/atari/hardware/interfaces.php|title=Atari DMA/ACSI interface|publisher=info-coach.fr|access-date=2025-08-13}}</ref>

== Sound Capabilities ==
* 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.<ref>{{cite web|url=https://temlib.org/AtariForumWiki/index.php/Yamaha_YM2149_Manual|title=YM2149 manual (features)|publisher=AtariForumWiki|access-date=2025-08-13}}</ref>
* 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).<ref>{{cite web|url=https://www.goto10retro.com/p/sounds-of-the-atari-st|title=Sounds of the Atari ST (historical overview)|author=Paul Lefebvre|publisher=GoTo10Retro|date=2024-01-10|access-date=2025-08-13}}</ref>
* Audio I/O: Mono line‑level out and an audio‑in on the 13‑pin monitor port (approx. 1 Vpp nominal).<ref>{{cite web|url=https://www.oldcomputers.it/parts/atari/1040STF/docs/connettore_monitor_st.htm|title=Monitor connector – audio in/out levels|publisher=oldcomputers.it|access-date=2025-08-13}}</ref>

== PCB Schematics & Service Guide ==
Motherboard families and identifying service references commonly seen on 520ST/520STM:
{| class="wikitable"
! 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.<ref>{{cite web |url=https://forums.atariage.com/topic/319794-almost-unknown-atari-st-schematics/ |title=(Almost) Unknown Atari ST schematics |publisher=AtariAge Forums |date=2021-04-27 |access-date=2025-08-13}}</ref>
|-
| C070231 || 260/520ST || — || Factory schematics drawing C070112 (Atari Japan); full community KiCad reconstruction available for repair work.<ref>{{cite web |url=https://archive.org/details/Atari_520ST_Schematic_1985 |title=Atari 520ST Schematic (1985), drawing set |publisher=Internet Archive |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://forums.atariage.com/topic/367081-conversion-of-the-schematics-of-the-atari-260520st-motherboard-into-kicad/ |title=KiCad conversion of Atari 260/520ST C070231 |publisher=AtariAge Forums |date=2024-05-25 |access-date=2025-08-13}}</ref>
|-
| 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.<ref>{{cite web |url=https://retrocomputing.stackexchange.com/questions/21618/why-do-some-atari-st-motherboards-include-many-inductors |title=Why do some Atari ST motherboards include many inductors? |publisher=Retrocomputing StackExchange |access-date=2025-08-13}}</ref><ref>{{cite web |url=https://wiki.console5.com/wiki/Atari_ST |title=Atari ST recap notes by board (C070243 Rev C/H) |publisher=Console5 TechWiki |access-date=2025-08-13}}</ref>
|}
Service/schematic references
* Early factory schematics: “Atari (Japan) Corporation” drawing C070112 (1985) covering 260/520ST main logic; scans circulate online.<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 520STE/1040STE Field Service Manual|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.<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari STe 520/1040 Field Service Manual, C302481‑001 |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>

== Technical Details ==
Comprehensive IC inventory (typical 520ST) – major functions and common part numbers:
{| class="wikitable"
! Function !! Common part(s) / package !! Notes
|-
| CPU || MC68000P8 (DIP‑64), MC68000FN8 (PLCC‑68) || 8.00 MHz core.<ref>{{cite web |url=https://www.alldatasheet.net/datasheet-pdf/pdf/135463/MOTOROLA/MC68000P8.html |title=MC68000P8 datasheet |publisher=Motorola |access-date=2025-08-13}}</ref>
|-
| Video “Shifter” || C025914‑38/‑38A; CO70713/I10356 || Reads DRAM and serializes pixels.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC list |publisher=AtariAge |access-date=2025-08-13}}</ref>
|-
| GLUE || C025915‑38A || Bus/timing decode, video timing assist.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC list |publisher=AtariAge |access-date=2025-08-13}}</ref>
|-
| MMU (DRAM) || C025912‑38; later C100109 || Row/col multiplex/refresh to DRAM.<ref>{{cite web |url=https://forums.atariage.com/topic/247569-huge-list-of-new-original-atari-custom-ic-chips-transistors-etc/ |title=Atari custom IC list |publisher=AtariAge |access-date=2025-08-13}}</ref>
|-
| DMA (ACSI/FDD) || C025913‑38 || 8‑bit ACSI and FDC DMA, IRQ/DRQ handshake.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0904.htm |title=DMA, ROM, and WD1772 photo |publisher=DOIT_ST |access-date=2025-08-13}}</ref>
|-
| Floppy controller || WD1772‑00‑02 / ‑02‑02 || 250 kbit/s DD; many ‑02‑02 parts OK at 16 MHz for HD mods.<ref>{{cite web |url=https://mikrosk.github.io/doitarchive/doit_st/0b03.htm |title=HD Disk ST |publisher=DOIT_ST |access-date=2025-08-13}}</ref>
|-
| MFP (timers/IRQs/U(S)ART) || MC68901 || 200 Hz system tick, vectorized interrupts.<ref>{{cite web |url=https://www.bighole.nl/pub/mirror/homepage.ntlworld.com/kryten_droid/Atari/ST/spg/st_prog_guide_1.htm |title=MFP details |publisher=Concise ST Prog Ref |access-date=2025-08-13}}</ref>
|-
| ACIAs (MIDI/IKBD) || 2× MC6850 || 7812.5 baud link to IKBD; 31.25 kbaud MIDI.<ref>{{cite web |url=https://www.muzines.co.uk/articles/atari-520st/9092 |title=Atari 520ST (MIDI hardware) |publisher=E&MM |date=1985-07 |access-date=2025-08-13}}</ref>
|-
| Sound/PIO || YM2149F || 3‑voice PSG + I/O (printer strobe/data, FDD selects, RTS/DTR).<ref>{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/STINTERN.htm |title=ST Internals – YM2149 regs |publisher=Ctrl‑Alt‑Rees |access-date=2025-08-13}}</ref>
|-
| RS‑232 transceivers || MC1488, MC1489 (or equivalents) || Level conversion for DB‑25 serial.<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=STE Service Manual (I/O section) |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>
|}
*

== PCB Schematics & Service Manual ==
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.<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><ref>{{cite web |url=https://forums.atariage.com/topic/367081-conversion-of-the-schematics-of-the-atari-260520st-motherboard-into-kicad/ |title=KiCad conversion for 260/520ST mainboard |publisher=AtariAge |date=2024-05-25 |access-date=2025-08-13}}</ref>
*

== Technical Documentation ==
* Atari 520ST Schematic (1985) – drawing C070112 (Atari Japan).<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>
* Conversion of the 260/520ST motherboard C070231 schematics 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=KiCad conversion for 260/520ST |publisher=AtariAge Forums |date=2024-05-25 |access-date=2025-08-13}}</ref>
* [[Atari 520STE/1040STE Field Service Manual|Atari STe 520/1040 Field Service Manual]] C302481‑001 Rev A (useful for shared subsystems and diagnostics).<ref>{{cite web |url=https://www.manualslib.com/manual/2170364/Atari-520ste.html |title=Atari STe Field Service Manual |publisher=ManualsLib |date=1991 |access-date=2025-08-13}}</ref>
* Atari 520ST Owner’s Manual (1985).<ref>{{cite web |url=https://archive.org/details/Atari_520ST_Owners_Manual_Rev._B_1985 |title=Atari 520ST Owner’s Manual, Rev. B (1985) |publisher=Internet Archive |access-date=2025-08-13}}</ref>

== Related Pages ==
* [[Atari 520STF]] – internal floppy version (later case/PSU variants)  
* [[Atari 520STFM]] – internal floppy and RF modulator version  
* [[Atari 1040STE]] – 1 MB model with integrated floppy/PSU (1986)<ref>{{cite web |url=https://www.atarimagazines.com/compute/issue71/electronics_show.php |title=1040ST announcement and bundles |publisher=Compute! |date=1986-02 |access-date=2025-08-13}}</ref>  
* [[Mega ST]] – enhanced chassis and expansion bus  
* [[Atari SC1224]] and [[Atari SM124]] monitors (display specs/repairs)

== References ==
<references/>
[[Category:Atari Systems]]