Atari Mega STE

Atari Mega STE

Atari Mega STE in TT-style grey case with detachable keyboard
Specifications
Manufacturer	Atari Corporation
Type	Professional desktop computer
Released	January 1, 1991
Discontinued	1993 (entire ST line discontinued)
Intro price	US$1,799 (base configuration)
CPU	Motorola MC68000 @ 16 MHz (software-switchable to 8 MHz); 16 KB CPU cache
Memory	1 MB, 2 MB, or 4 MB ST RAM; expandable to 4 MB using 30-pin SIMMs
Storage	Internal 3.5-inch floppy (720 KB or 1.44 MB HD); optional internal SCSI hard disk (typically 48 MB Seagate ST-157N)
Display	—
Sound	Yamaha YM2149F (PSG, 3-channel mono) + National Semiconductor LMC1992 DMA stereo audio (8-bit PCM, up to 50 kHz)
OS / Firmware	Atari TOS 2.05 or 2.06 in ROM
Predecessor	Atari Mega ST
Successor	Atari Falcon030 (as final 68000-class ST)

The Atari Mega STE is the final Motorola 68000-based computer in the Atari ST series, manufactured by Atari Corporation and released in 1991. It combines the enhanced hardware of the 1040STE — including DMA stereo audio and an extended colour palette — with the professional desktop form factor of the Atari Mega ST, housed in the grey-coloured case design originally created for the Atari TT030.

The Mega STE was shown publicly at COMDEX in November 1990, where at least five units were demonstrated in the Atari booth alongside the TT030. Retail distribution began in early 1991, with an initial base price of US$1,799. It was Atari's last 68000-based ST computer; the Falcon030 with its Motorola 68030 followed in 1992.

Technical specifications

Processor and cache

The Mega STE uses the Motorola 68000 CPU clocked at 16 MHz — double the clock speed of all prior ST models. A software-switchable speed feature allows the CPU to be set to either 16 MHz or 8 MHz via the GENERAL.CPX control panel accessory. The 8 MHz mode exists for compatibility with older software that relies on timing loops calibrated for the original ST's clock speed.

The system defaults to 8 MHz with cache disabled at every cold boot. To operate at 16 MHz, the user must either load the CPX control panel or place an AUTO folder utility (such as MSTE_CC) that sets the speed and cache at boot time.

The 16 KB CPU cache is implemented using discrete SRAM chips (typically 2 × 8 KB, designated U3 and U4 on the motherboard) and cache control PAL logic. The cache stores recently accessed instructions and data, providing a significant performance boost at 16 MHz — approximately 80% faster than a standard 8 MHz ST in CPU-bound operations. The cache is automatically invalidated when the CPU relinquishes bus control (e.g., during DMA transfers or BLiTTER operations) to maintain coherency.

The CPU speed and cache are controlled via hardware registers at address $FFFF8E21:

• Bit 0: CPU speed (0 = 8 MHz, 1 = 16 MHz)
• Bit 1: Cache enable (0 = disabled, 1 = enabled)

An optional Motorola 68881 or 68882 FPU (floating-point unit) can be installed in a PLCC socket on the motherboard, providing hardware floating-point acceleration for CAD, scientific, and 3D applications.

Memory

The Mega STE uses 30-pin SIMM (Single Inline Memory Module) slots for RAM, a significant improvement over the soldered DRAM of the Mega ST. Four SIMM slots accept 256 KB or 1 MB SIMMs in matched pairs, providing configurations of 1 MB, 2 MB, or 4 MB.

Configuration	SIMMs required
1 MB	4 × 256 KB
2 MB	2 × 1 MB (+ 2 empty slots, or 2 × 256 KB in remaining slots)
4 MB	4 × 1 MB

Parity SIMMs should not be used. Multiple documented cases have shown that parity SIMMs cause instability, particularly under load or when the power supply is degraded. If parity SIMMs are installed, the parity chip should be removed from each module.

Memory access is managed by the GST MCU (GLUE-Shifter-MMU combined chip), which handles address decoding, DRAM refresh, and bus arbitration with the CPU, BLiTTER, and DMA controller.

Graphics

The video subsystem is derived from the STE, using the enhanced Shifter chip with an expanded palette of 4,096 colours (12-bit RGB: 4 bits per channel), compared to the 512-colour palette (9-bit, 3 bits per channel) of the original ST.

Mode	Resolution	Colours	Monitor
Low resolution	320 × 200	16 of 4,096 palette	SC1435 (colour)
Medium resolution	640 × 200	4 of 4,096 palette	SC1435 (colour)
High resolution	640 × 400	Monochrome (black/white)	SM146 (mono)

The STE enhancements include:

• Hardware fine scrolling — Pixel-level horizontal scrolling via hardware registers, without the performance cost of software scrolling.
• Extended palette — 4,096 colours with STE-format palette registers at $FFFF8240–$FFFF825E.

The BLiTTER co-processor is present, functioning identically to the unit in the Mega ST and 1040STE.

Sound

The Mega STE retains the dual audio system of the STE:

• Yamaha YM2149F PSG — Three-channel mono square wave synthesis, identical to all prior ST models.
• DMA stereo audio — An 8-bit PCM DMA sound system capable of playback at 6.258 kHz, 12.517 kHz, 25.033 kHz, or 50.066 kHz sample rates. Audio output is mixed through the National Semiconductor LMC1992 programmable audio mixer, which provides software-controlled volume, bass, and treble adjustment.

Stereo audio output is available via RCA phono jacks on the rear of the system unit.

The enhanced joystick ports (DE-15) also support paddle controllers and light guns via the enhanced joystick interface, sharing the same connector pinout as the 1040STE.

Storage

Floppy disk drive

The Mega STE includes a single internal 3.5-inch floppy drive. Early production units shipped with a 720 KB double-density drive; later units include a 1.44 MB high-density drive capable of reading both DD and HD media. The floppy controller is the WD1772 (clocked at 8 MHz regardless of CPU speed setting) with DMA transfer to main memory.

SCSI hard disk

The 2 MB and 4 MB models shipped with an internal SCSI hard disk — typically a Seagate ST-157N (48 MB) or similar SCSI drive. The SCSI interface is provided by an ACSI/SCSI adapter daughterboard that bridges the Atari Computer System Interface (ACSI) DMA port to a standard SCSI bus. This daughterboard connects internally; the external ACSI DMA port on the rear panel remains available for additional ACSI peripherals.

The 1 MB base model did not include a hard disk, SCSI adapter, or high-resolution monitor.

Hard disk driver software

The internal SCSI disk requires driver software loaded at boot time. Atari shipped AHDI (Atari Hard Disk Interface) as the standard driver. Third-party alternatives include HDDriver (by Uwe Seimet) and CBHD, both of which provide improved performance, larger partition support, and broader device compatibility.

VMEbus expansion

The Mega STE includes a single 3U VMEbus expansion slot inside the case, accessible by removing the top cover. This slot conforms to a subset of the VMEbus standard and provides access to the 68000's address and data buses.

VME expansion cards used with the Mega STE include:

• ET4000-based VGA cards — Providing higher-resolution display output (e.g., 1024 × 768) via VME-to-VGA adapters.
• Ethernet adapters — PAMs NetUS and similar cards providing 10 Mbit/s Ethernet networking.
• Crazy Dots — Graphics accelerator cards by Matrix.

The VME slot is physically identical between the Mega STE and the TT030, and most VME cards are compatible with both systems, though software driver compatibility varies.

Networking — LocalTalk/RS-422 port

The Mega STE includes a dedicated Serial LAN port implementing RS-422 electrical signalling on a mini-DIN 8 connector. This port is physically and electrically compatible with Apple LocalTalk hardware, enabling connection to LocalTalk networks.

However, no AppleTalk protocol software was ever produced by Atari for TOS. Third-party solutions existed but were limited in availability. The port can also be used as a high-speed serial connection independent of networking, providing speeds up to 1 Mbit/s.

Serial ports

The Mega STE provides three RS-232 serial ports, all using DB-9 connectors (a change from the DB-25 connectors used on earlier ST models):

Port	Controller	Maximum speed	Notes
Serial 1 (Modem)	MFP 68901	19,200 baud	Standard ST serial port, directly compatible with existing ST serial software
Serial 2 (Serial LAN)	SCC 85C30	1 Mbit/s	RS-422; also the LocalTalk port
Serial 3	SCC 85C30	230,400 baud	Additional RS-232 port via the Zilog SCC

The Zilog 85C30 SCC (Serial Communications Controller) provides the two additional serial ports, offering significantly higher throughput than the MFP-based Serial 1.

Keyboard

The detachable keyboard connects via a coiled cable and uses the same IKBD protocol as the Mega ST. The keyboard includes mouse and joystick ports on its rear panel. The keyboard is functionally identical to the TT030 keyboard and uses the same TT-style case design. Membrane-type keyswitches are used, with occasional failure of individual key traces being a known wear issue.

Case design

The Mega STE uses the Atari TT030 case in ST grey colouring rather than the TT's off-white. The case is a two-piece modular design:

• Lower unit — Houses the motherboard, power supply, floppy drive, and optional hard disk. The power supply is a Phihong PSM-5341 unit (100–240 VAC input, universal voltage). Openings for the VME card and rear I/O panel are on the back.
• Upper lid — Slides forward for removal. The system unit sits on rubber feet and can support a monitor on top.

The case provides internal mounting for one 3.5-inch hard drive and the floppy drive. The power supply is mounted at the rear left of the chassis.

Operating system

The Mega STE shipped with TOS 2.05 in ROM, later updated to TOS 2.06 in production. TOS 2.x introduced several improvements over the 1.x series:

• New desktop — Redesigned GEM desktop with configurable icon/text views, improved file copy dialogs, and a more polished appearance. The desktop style matches the TT030's desktop.
• Improved file system — Better hard disk support with larger partition sizes (up to 512 MB with AHDI).
• Bug fixes — Numerous fixes to GEM, VDI, GEMDOS, and the BIOS.
• Cookie Jar — System information mechanism allowing software to query hardware capabilities (CPU type, machine type, FPU presence, etc.).

TOS 2.06 is the recommended version and can be installed by replacing the two ROM chips (2 × 128 KB EPROMs or mask ROMs).

Known issues

Cache instability at 16 MHz

The most commonly reported Mega STE issue is random crashing or instability when running at 16 MHz with cache enabled. This is typically caused by degraded cache SRAM chips (U3 and U4 on the motherboard) or failing cache control PAL ICs. Replacing U3 and U4 with known-good SRAM chips often resolves the issue. Machines that crash at 16 MHz but run stably at 8 MHz should have these components investigated first.

Power supply capacitor degradation

The Phihong PSM-5341 PSU uses electrolytic capacitors that degrade with age. Symptoms include video noise, screen dimming during floppy access, random crashes, and eventual failure to power on. Recapping the PSU is strongly recommended as preventive maintenance. This PSU is shared with the Atari TT030.

Video ghosting

The STE and Mega STE suffer from a video ghosting artefact where bright objects on screen produce faint shadows or echoes. This is caused by impedance mismatches in the video output circuit and can be corrected with a simple resistor modification.

Software compatibility

While the Mega STE offers better compatibility with ST software than the TT030, some programs — particularly older games and demos — may fail when run at 16 MHz or with cache enabled. Using the GENERAL.CPX control panel to switch to 8 MHz with cache disabled typically resolves compatibility issues. Software that accesses hardware directly or uses cycle-counted timing loops is most likely to be affected.

See also

• Atari Mega ST
• Atari 1040STE
• Atari TT030
• Atari Mega STE General Maintenance
• Atari Mega STE Troubleshooting Guide
• Atari Mega STE Capacitor Replacement Guide

External links

• Atari MEGA STE on Wikipedia
• Atari Mega STe at Atari Museum
• Atari Mega STE at Computing History
• Atari VME Expansion Specification (PDF)
• The LaST Upgrade — Exxos Atari ST upgrades and fixes
• Mega STE Review — ST-News, 1992