Atari 520STFM

Atari 520STFM
	Atari 520STFM in the "long case" design with integrated floppy drive and RF modulator
Specifications
Manufacturer	Atari Corporation
Type	Home computer / personal computer
Released	January 1, 1986
Discontinued	1990 (superseded by the Atari 520STE)
Intro price	UK £299 (1987); US$499 (approx., varied by bundle and market)
CPU	Motorola MC68000 @ 8 MHz
Memory	512 KB RAM (some later units shipped with 1 MB)
Storage	Internal 3.5-inch DS/DD 720 KB floppy drive; external floppy via 14-pin DIN; external ACSI hard disks
Display	320×200 (16 colours), 640×200 (4 colours), 640×400 (mono); RF output for television
Sound	Yamaha YM2149F (3 voices + noise)
Dimensions	78 mm × 470 mm × 330 mm (3.1 × 18.5 × 13 in) approximately
Weight	≈5.5 kg (12 lb)
OS / Firmware	Atari TOS 1.02 / 1.04 (192 KB ROM)
Predecessor	Atari 520STM
Successor	Atari 520STE

The Atari 520STFM is one of the most popular and widely sold models in the Atari ST family. The designation stands for 520 (512 KB RAM), ST (Sixteen/Thirty-two), F (internal Floppy drive), and M (RF Modulator). Released in 1986, it combined the features that home users needed — an integrated double-sided floppy drive and television output — in a single "long case" enclosure with an internal power supply.

The 520STFM was especially successful in the United Kingdom, where it became a popular home computer and gaming machine. Its ability to connect directly to a television set, combined with its relatively low price, made it a strong competitor to the Commodore Amiga 500.

History

The original 520ST (1985) was a capable machine but required an external floppy drive and a dedicated monitor, making it expensive to set up. Atari addressed this in stages: the 520STM added an RF modulator for TV output, and the 520STF added an internal floppy drive. The 520STFM combined both enhancements and was housed in a larger "long case" that also accommodated an internal switched-mode power supply.

This made the 520STFM a complete computer in one box — users only needed to connect a television set and a floppy disk to get started. The machine was sold in several bundles, often including games and productivity software, and was aggressively priced against the Amiga 500 in the UK market.

The 520STFM typically shipped with TOS 1.02 ("Mega TOS") or later TOS 1.04 ("Rainbow TOS"), both in 2-chip 192 KB ROM configurations. Some later 520STFM units were upgraded to 1 MB of RAM at the factory, competing with the 1040STFM.

The 520STFM shares the C070789-001 motherboard with the Atari 1040STFM, differing only in the amount of RAM installed.

Architecture

CPU: Motorola MC68000 @ 8 MHz
Architecture: 16/32-bit (32-bit internal, 16-bit external data bus, 24-bit address bus)
RAM: 512 KB standard (upgradeable to 1 MB, 2.5 MB, or 4 MB with aftermarket modifications)
ROM: 192 KB TOS 1.02 or TOS 1.04 in 2× 128 KB mask ROMs
Custom ICs: Shifter (C025914 or later GST Shifter), GLUE (C025915 or later combined GLUE/MMU), MMU (C025912), DMA (C025913)
BLiTTER: Not standard; some C070789 boards have an unpopulated PLCC-68 footprint for the optional BLiTTER chip (Atari C101675)

Major ICs

Function	Part Number	Notes
CPU	MC68000P8 / MC68000FN8	8 MHz
Video Shifter	C025914-38A	Pixel serialiser; later boards may use GST Shifter
GLUE	C025915-38A	Bus timing, video timing; some boards use combined GLUE
MMU	C025912-38 / C100109	DRAM controller
DMA Controller	C025913-38	ACSI/floppy DMA
FDC	WD1772-02-02	Floppy disk controller; -02-02 variant common
MFP	MC68901	Timers, interrupts, UART
ACIA (×2)	MC6850	Keyboard/IKBD and MIDI
Sound/PIO	YM2149F	3-voice PSG + I/O
RS-232 drivers	MC1488 / MC1489	Serial port level conversion
RF Modulator	Internal shielded module	PAL or NTSC RF output

Connectors

Rear panel (left to right, typically):

Power: IEC C14 mains inlet (internal PSU) — CAUTION: contains mains-voltage components
Floppy (external): 14-pin DIN
ACSI (Hard Disk): DB-19 female
Serial (Modem): DB-25 male, RS-232C
Parallel (Printer): DB-25 female
Video/Monitor: 13-pin DIN (analog RGB, mono, sync, audio)
RF Output: Phono (RCA) or coaxial connector for television

Left side:

MIDI In: 5-pin DIN
MIDI Out: 5-pin DIN
Cartridge: 40-pin edge connector
Reset button (some models)

Under keyboard:

Port 0: DE-9 male (mouse/joystick)
Port 1: DE-9 male (joystick)

Display

Low resolution: 320×200, 16 colours from 512-colour palette, ~50/60 Hz
Medium resolution: 640×200, 4 colours, ~50/60 Hz
High resolution (mono): 640×400, monochrome, ~71.2 Hz (SM124/SM125 only)

The RF modulator outputs the colour display modes for television viewing. The monochrome high-resolution mode requires a dedicated monitor and cannot be viewed via RF.

Sound

PSG: Yamaha YM2149F — 3 square-wave channels + noise, 5-bit envelope generator
MIDI: Built-in MIDI In/Out at 31.25 kbaud
Audio: Mono line-level on monitor DIN; also mixed into RF signal

Internal Floppy Drive

The 520STFM includes an internal 3.5-inch double-sided, double-density (DS/DD) floppy drive providing 720 KB formatted capacity. The drive is connected to the WD1772 floppy disk controller via the DMA chip. Common internal drive models include:

Epson SMD-340/360
Citizen OSDA series
Alps DFxx series

An external floppy drive can be connected via the 14-pin DIN port for a second drive (drive B:).

Internal Power Supply

The 520STFM uses an internal switched-mode power supply (SMPS), a significant change from the external "brick" PSU of the 520ST/520STM. The internal PSU provides:

Rail	Voltage	Typical Current	Purpose
+5V	5.0V ±5%	~2.5A	Logic, RAM, ICs
+12V	12.0V ±10%	~0.5A	Floppy drive motor, RS-232 drivers
−12V	−12.0V ±10%	~0.05A	RS-232 negative rail

WARNING: The internal PSU contains mains-voltage components. Capacitors may retain dangerous charge even after the machine is unplugged. Always disconnect from the mains and wait at least 5 minutes before opening the case.

Motherboard Revisions

Board Number	Notes
C070789-001	Most common 520STFM/1040STFM board; also used in the 1040STFM with different RAM population
C103175 (Rev. D)	Later revision with some component changes

BLiTTER Upgrade

Some C070789-001 boards include an unpopulated PLCC-68 socket position for the Atari BLiTTER chip (C101675). When installed, the BLiTTER provides hardware-accelerated block transfer operations, improving performance in GEM desktop operations and some applications. The BLiTTER upgrade requires:

Atari C101675 BLiTTER IC (PLCC-68)
TOS 1.02 or later (earlier TOS versions do not support the BLiTTER)
Soldering a socket or the chip directly to the unpopulated pads

Known Issues

PSU capacitor failure: The internal switched-mode PSU is a common failure point. Dried-out or bulging capacitors can cause instability, failure to power on, or intermittent crashes. Recapping the PSU is a recommended preventive measure.
Floppy drive failure: The internal drive mechanism can develop worn heads, dirty head assemblies, or failed drive belts. Cleaning or replacement may be required.
Keyboard membrane degradation: Same issue as all ST models.
RF modulator quality: The RF output provides a soft, somewhat blurry picture typical of the era. For best display quality, use the 13-pin DIN monitor output with a dedicated monitor or an RGB-to-SCART adapter.
RAM issues: DRAM chips can develop faults over time, causing crashes, garbled displays, or failure to boot.

Service and Repair

Software Compatibility

The 520STFM ran the full ST software library. Its 512 KB RAM was sufficient for most games and many productivity applications, though some later software required 1 MB. Key software categories included:

Games: Dungeon Master, Carrier Command, Starglider, Xenon, Populous, Captive, Stunt Car Racer
Music production: Steinberg Pro-24, C-Lab Creator/Notator — the built-in MIDI ports made the ST the standard for music production
Desktop publishing: Calamus, PageStream
Word processing: 1st Word Plus, WordPerfect
Programming: GFA BASIC, Lattice C, Mark Williams C, Devpac assembler

References