Atari 520ST+

Atari 520ST+
	Atari 520ST+ with SM124 monochrome monitor
Specifications
Manufacturer	Atari Corporation
Type	Home computer / personal computer
Released	January 1, 1986
Discontinued	1987 (superseded by STF/STFM variants)
Intro price	Approximately US$599–US$799 (varied by bundle and market)
CPU	Motorola MC68000 @ 8 MHz
Memory	1 MB RAM
Storage	External 3.5-inch floppy drive (Atari SF354 SSDD 360 KB or SF314 DSDD 720 KB); external ACSI hard disks
Display	320×200 (16 colours), 640×200 (4 colours), 640×400 (mono)
Sound	Yamaha YM2149F (3 voices + noise)
Dimensions	70 mm × 476 mm × 241 mm (2.75 × 18.75 × 9.5 in)
Weight	≈4.3 kg (9.5 lb)
OS / Firmware	Atari TOS 1.00 / 1.02 (192 KB ROM)
Predecessor	Atari 520ST
Successor	Atari 520STFM

The Atari 520ST+ (sometimes written 520ST Plus) is a variant of the Atari 520ST home computer that shipped with 1 MB of RAM instead of the standard 512 KB. Introduced in 1986, it was created to differentiate the higher-memory version from the base 520ST, particularly after the 1040STF established 1 MB as the standard for the ST line.

The 520ST+ retained the original "short-case" form factor with an external power supply and no internal floppy drive, making it physically identical to the standard 520ST. The "+" suffix was Atari's way of indicating the doubled RAM without changing the model number entirely. It was sold primarily in European markets.

History

When the ST line launched in 1985, the 520ST shipped with 512 KB of RAM and the 260ST had 256 KB. By 1986, Atari had introduced the 1040STF with 1 MB of RAM and an internal floppy drive, at a price point that made the 512 KB 520ST look less competitive.

Rather than discontinue the short-case design entirely, Atari upgraded some 520ST units to 1 MB of RAM and marketed them as the "520ST+". This allowed retailers to offer a 1 MB machine at a lower price point than the 1040STF (which included the internal floppy drive and, in some models, an internal power supply).

The 520ST+ was not a distinct hardware design — it used the same motherboard as the 520ST with additional or different RAM chips installed to provide 1 MB. The naming convention simply distinguished it from the 512 KB models in dealer and consumer documentation.

Architecture

The 520ST+ is architecturally identical to the Atari 520ST, differing only in RAM capacity:

CPU: Motorola MC68000P8 or MC68000FN8 @ 8.00 MHz
Architecture: 16/32-bit (32-bit internal, 16-bit external data bus, 24-bit address bus)
RAM: 1 MB, using 16× 256k×4 or equivalent DRAM configuration
ROM: 192 KB TOS (TOS 1.00 in 6× 32 KB ROMs, or TOS 1.02 in 2× 128 KB ROMs on later units)
Custom ICs: Shifter (C025914), GLUE (C025915), MMU (C025912), DMA (C025913) — identical to standard ST

Major ICs

Function	Part Number	Notes
CPU	MC68000P8 / MC68000FN8	8 MHz, DIP-64 or PLCC-68
Video Shifter	C025914-38	Pixel serialiser
GLUE	C025915-38A	Bus timing, address decode
MMU	C025912-38 / C100109	DRAM controller; later revision on some boards
DMA Controller	C025913-38	ACSI/floppy DMA
FDC	WD1772-00-02	Floppy disk controller
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

Connectors

Identical to the Atari 520ST:

Video/Monitor: 13-pin DIN (analog RGB, mono, sync, audio)
Serial (Modem): DB-25 male, RS-232C
Parallel (Printer): DB-25 female
MIDI In/Out: Two 5-pin DIN connectors
External Floppy: 14-pin DIN
ACSI (Hard Disk): DB-19 female
Cartridge: 40-pin edge connector
Joystick/Mouse: Two DE-9 male ports
Power: 7-pin DIN (external PSU: +5V, +12V, −12V)

Display

Same three display modes as all ST-family machines:

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 (requires SM124/SM125)

Sound

PSG: Yamaha YM2149F — 3 square-wave channels + noise, 5-bit envelope
MIDI: Built-in MIDI In/Out at 31.25 kbaud
Audio output: Mono line-level on monitor DIN connector

Comparison with Other ST Models

Feature	260ST	520ST	520ST+	520STFM	1040STF
RAM	256 KB	512 KB	1 MB	512 KB / 1 MB	1 MB
Internal Floppy	No	No	No	Yes (DSDD)	Yes (DSDD)
RF Modulator	No	No	No	Yes	No
Internal PSU	No	No	No	Yes	Yes
Case Style	Short	Short	Short	Long	Long

Service and Repair

The 520ST+ shares all service procedures with the standard ST family:

References