Atari 260ST

Atari 260ST
	Atari 260ST (externally identical to the 520ST short-case design)
Specifications
Manufacturer	Atari Corporation
Type	Home computer / personal computer
Released	June 1, 1985
Discontinued	1985 (very limited production; quickly superseded by the 520ST)
Intro price	US$399 (announced, though retail availability was extremely limited)
CPU	Motorola MC68000 @ 8 MHz
Memory	256 KB RAM (later upgraded to 512 KB, becoming the 520ST)
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 (loaded from disk on earliest units; later 192 KB ROM TOS)
Predecessor	Atari 65XE
Successor	Atari 520ST

The Atari 260ST was the first announced model of Atari's 16/32-bit ST family, unveiled at the January 1985 Consumer Electronics Show (CES). Built around the Motorola 68000 processor at 8 MHz, the 260ST was intended to be the entry-level model with 256 KB of RAM. However, it saw extremely limited production and retail availability; Atari quickly recognised that 256 KB was insufficient — particularly since early units loaded TOS from floppy disk, consuming approximately 192 KB of the available RAM — and shifted production to the 512 KB 520ST instead.

The 260ST was physically identical to the early "short-case" 520ST and used the same motherboard designs. Most surviving 260ST units are found in Europe, where small quantities were sold through specialist dealers. Many were subsequently upgraded to 512 KB by adding or replacing RAM chips, effectively making them 520STs.

History

Jack Tramiel acquired the consumer division of Atari Inc. from Warner Communications in July 1984. He immediately set engineer Shiraz Shivji to work designing a new 68000-based computer. The machine was designed in approximately five months and announced at the January 1985 Winter CES alongside the 520ST.

The 260ST was positioned as the most affordable entry point to the new 16/32-bit ST architecture, priced at just US$399. However, by the time production began in mid-1985, the cost difference between 256 KB and 512 KB of DRAM had narrowed sufficiently that the 260ST offered poor value. The earliest units shipped with TOS loaded from a floppy disk via a small boot ROM, which consumed most of the 256 KB RAM and left very little for applications. Atari quickly consolidated production around the 512 KB 520ST model, and the 260ST was quietly discontinued after a very short production run.

The "ST" designation stands for "Sixteen/Thirty-two", referring to the Motorola 68000's 16-bit external data bus and 32-bit internal architecture.

Architecture

The 260ST uses the identical architecture as the Atari 520ST, differing only in the amount of installed RAM:

CPU: Motorola MC68000P8 (DIP-64) or MC68000FN8 (PLCC-68), running at 8.00 MHz
Architecture: 16/32-bit (32-bit registers and ALU, 16-bit external data bus, 24-bit address bus)
RAM: 256 KB, using 16× 64k×1 DRAM ICs in two 8-bit banks (vs. 16× 256k×1 on the 520ST)
ROM: Early units: small boot ROM that loads TOS from floppy disk. Later units: 192 KB TOS 1.00 in 6× 32 KB mask ROMs
Custom ICs: Shifter (C025914), GLUE (C025915), MMU (C025912), DMA controller (C025913) — identical to those used in the 520ST

Major ICs

Function	Part Number	Notes
CPU	MC68000P8 / MC68000FN8	8 MHz, DIP-64 or PLCC-68
Video Shifter	C025914-38	Pixel serialiser; reads DRAM and generates video output
GLUE	C025915-38A	Bus timing, address decode, video timing
MMU	C025912-38	DRAM row/column multiplexer and refresh controller
DMA Controller	C025913-38	ACSI hard disk and floppy DMA
FDC	WD1772-00-02	Floppy disk controller, 250 kbit/s DD
MFP	MC68901	Multi-function peripheral: timers, interrupts, UART
ACIA (×2)	MC6850	Keyboard/IKBD (7812.5 baud) and MIDI (31.25 kbaud)
Sound/PIO	YM2149F	3-voice PSG + parallel I/O ports
RS-232 drivers	MC1488 / MC1489	Line-level conversion for serial port

Connectors

The 260ST has the same connector layout as the early Atari 520ST:

Video/Monitor: 13-pin DIN (analog RGB, mono, H/V sync, audio)
Serial (Modem): DB-25 male, RS-232C
Parallel (Printer): DB-25 female, Centronics-compatible
MIDI In/Out: Two 5-pin DIN connectors
External Floppy: 14-pin DIN
ACSI (Hard Disk): DB-19 female
Cartridge: 40-pin edge connector (up to 128 KB ROM)
Joystick/Mouse: Two DE-9 male ports (Port 0: mouse/joystick, Port 1: joystick)
Power: 7-pin DIN (external PSU providing +5V, +12V, −12V)

Display

The 260ST provides the same three display modes as all ST-family machines:

Low resolution: 320×200 pixels, 16 colours from a palette of 512 (9-bit RGB), ~50/60 Hz
Medium resolution: 640×200 pixels, 4 colours from a palette of 512, ~50/60 Hz
High resolution (mono): 640×400 pixels, monochrome (requires SM124/SM125 monitor), ~71.2 Hz

Sound

PSG: Yamaha YM2149F — 3 square-wave tone channels plus 1 noise generator, 5-bit envelope generator
MIDI: Built-in MIDI In and Out ports driven by MC6850 ACIA at 31.25 kbaud — a key feature that made the ST series the standard for music production in the late 1980s
Audio output: Mono line-level on pin 1 of the 13-pin monitor DIN connector

Known Issues

Due to its extreme rarity and short production run, common issues are largely the same as for the early Atari 520ST:

Insufficient RAM for TOS-from-disk: On units without ROM TOS, loading TOS from floppy consumed ~192 KB of the 256 KB RAM, leaving very little for applications. The practical solution was to upgrade to 512 KB or install ROM TOS.
Keyboard membrane degradation: The keyboard uses a membrane assembly that degrades over time, causing unresponsive or stuck keys.
Clock battery leakage: Some units had a soldered clock battery that can leak and cause PCB corrosion.
PSU failure: The external power supply can develop failed capacitors over time.

Service and Repair

The 260ST shares motherboard designs and components with the Atari 520ST, so the same service guides apply:

Motherboard Revisions

The 260ST used the same early motherboard designs as the 520ST:

Board Number	Models	Notes
C070115	260ST / 520ST (earliest)	Very first production board; minimal EMI filtering
C070231	260ST / 520ST	Improved design; community KiCad schematics available

References