Atari 520STM

Atari 520STM
	Atari 520STM (short-case design with external PSU, identical appearance to the 520ST)
Specifications
Manufacturer	Atari Corporation
Type	Home computer / personal computer
Released	January 1, 1985
Discontinued	1987 (superseded by the 520STFM)
Intro price	Approximately US$599 (varied by market and bundle)
CPU	Motorola MC68000 @ 8 MHz
Memory	512 KB RAM
Storage	External 3.5-inch floppy drive (Atari SF354 or SF314); external ACSI hard disks
Display	320×200 (16 colours), 640×200 (4 colours), 640×400 (mono); RF output for television display
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 (192 KB ROM)
Predecessor	Atari 520ST
Successor	Atari 520STFM

The Atari 520STM is a variant of the Atari 520ST home computer that added a built-in RF modulator for television output. The "M" in the model name stands for "Modulator". Released in late 1985, the 520STM was designed for the home market where many users did not own a dedicated computer monitor and needed to connect the machine to a standard television set.

Apart from the addition of the RF modulator circuitry, the 520STM was identical to the standard 520ST in all other respects. It retained the short-case form factor, external power supply, and external floppy drive requirement. The 520STM was primarily sold in the United Kingdom and other European markets where TV output was a key selling point for the home computing market.

History

When the Atari ST launched in mid-1985, the original 520ST required a dedicated monitor — either the Atari SM124 monochrome monitor or the SC1224 colour monitor. This was an obstacle in markets like the UK, where home computers typically connected to the family television set.

By late 1985, Atari added an RF modulator to the 520ST design, creating the 520STM. This small change made the machine considerably more accessible to home users, as it could now produce a PAL (or NTSC) RF signal on a standard television aerial socket. The modulator was typically installed as a small shielded module on the motherboard, converting the composite video signal to a UHF/VHF RF carrier.

The 520STM was eventually superseded by the Atari 520STFM, which combined the RF modulator with an internal floppy disk drive in a redesigned "long case" with an internal power supply.

Architecture

The 520STM is architecturally identical to the Atari 520ST:

CPU: Motorola MC68000 @ 8 MHz
Architecture: 16/32-bit (32-bit internal registers, 16-bit external data bus, 24-bit address bus)
RAM: 512 KB (16× 256k×1 DRAM)
ROM: 192 KB TOS 1.00 in 6× 32 KB mask ROMs
Custom ICs: Shifter (C025914), GLUE (C025915), MMU (C025912), DMA (C025913)
RF Modulator: Internal module converting composite video to RF for television aerial input

Major ICs

Function	Part Number	Notes
CPU	MC68000P8 / MC68000FN8	8 MHz
Video Shifter	C025914-38	Pixel serialiser
GLUE	C025915-38A	Bus timing and address decode
MMU	C025912-38	DRAM controller
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 line drivers
RF Modulator	Varies (shielded module)	Converts composite video to UHF/VHF RF signal

Connectors

The 520STM has the same connector set as the 520ST, with the addition of an RF output:

Video/Monitor: 13-pin DIN (analog RGB, mono, sync, audio)
RF Output: Phono (RCA) jack or coaxial connector for television aerial input
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)

Display

The 520STM provides the standard ST display modes:

Low resolution: 320×200, 16 colours from 512, ~50/60 Hz
Medium resolution: 640×200, 4 colours from 512, ~50/60 Hz
High resolution (mono): 640×400, monochrome, ~71.2 Hz (SM124/SM125 monitor only — not available via RF)

Note: The RF modulator only outputs the colour (low and medium resolution) display modes. The monochrome high-resolution mode requires a dedicated SM124/SM125 monitor and cannot be displayed on a television.

Sound

PSG: Yamaha YM2149F — 3 square-wave channels + noise
MIDI: Built-in MIDI In/Out at 31.25 kbaud
Audio output: Mono line-level on monitor connector; also mixed into RF signal for television speakers

Known Issues

In addition to the common ST issues, the 520STM has modulator-specific concerns:

RF modulator failure: The internal RF modulator can fail or produce a poor-quality signal due to aged components. Symptoms include a snowy, unstable, or absent TV picture. The modulator module can sometimes be repaired by replacing its internal capacitors, or bypassed entirely for composite video output.
RF interference: Poor shielding on some board revisions can cause RF interference in the video output.
All standard ST issues apply: Keyboard membrane degradation, clock battery leakage, external PSU capacitor failure.

Motherboard Revisions

The 520STM used the C070243 board family (shared with later 520ST models):

Board Number	Revisions	Notes
C070243	Rev. C, H, I	Short-case board with provision for RF modulator; also used in 520ST

Service and Repair

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

References