Editing Atari 520ST (section)

== 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>