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Rewrite from Atari's Mega service manual and owner's manual (both now hosted): clock battery is two AA cells, not CR2032; supply is internal ASP34-1, not external Mitsumi; expansion connector is 64-pin; chip part numbers and locations from parts list; TOS ROM map fixed; remove uncited DTP/music claims
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{{Infobox computer
{{Infobox computer
| name = Atari Mega ST
| name = Atari Mega ST
| image = [[File:Atari-Mega-ST.jpg|250px]]
| image = [[File:Atari-Mega-ST.jpg|260px|frameless]]
| manufacturer = Atari Corporation
| caption = Mega ST with an SM124 monochrome monitor, a Megafile hard disk and the detached keyboard
| type = Professional desktop computer
| manufacturer = Atari Corporation (Atari Taiwan Manufacturing)
| caption = Atari Mega ST with detachable keyboard and SM124 monochrome monitor
| type = Desktop computer
| release date = {{Start date|1987|06}}<ref name="ces87">{{cite web |url=https://books.google.com/books?id=Bz8EAAAAMBAJ&lpg=PA42&dq=atari%20mega%20st&pg=PA42 |title=Atari Mega ST Release Coverage |publisher=InfoWorld |date=1987-10 |access-date=2026-03-27}}</ref><ref name="oldcomputers">{{cite web |url=https://oldcomputers.net/atari-mega-st.html |title=Atari Mega ST Computer |publisher=OldComputers.net |access-date=2026-03-27}}</ref>
| release_date = June 1987 (Mega ST2, West Germany)<ref name="acm-megast" />
| discontinued = 1991 (superseded by Mega STE)
| discontinued = 1991<ref name="acm-megast" />
| price = US$1,999 (Mega ST2); US$2,499 (Mega ST4)<ref name="oldcomputers"/>
| price = DM 2,998 (Mega ST2, June 1987); £995 (Mega ST2, August 1987); US$1,499.95 (Mega 2, November 1989)<ref name="acm-megast" />
| cpu = Motorola MC68000 @ 8.00 MHz
| cpu = Motorola MC68000 at 8 MHz
| memory = 1 MB (Mega ST1), 2 MB (Mega ST2), or 4 MB (Mega ST4); soldered DRAM
| memory = 1, 2 or 4 MB RAM; 192 KB ROM<ref name="om-spec" />
| storage = Internal 3.5-inch DSDD 720 KB floppy drive; external ACSI hard disks (MegaFile 20/30/60 series)
| storage = Internal 3.5-inch double-sided floppy drive, 720 KB
| graphics = Shifter video chip; BLiTTER co-processor (Atari C025912)
| graphics = Atari Shifter; Blitter
| sound = Yamaha YM2149F (PSG, 3-channel mono)
| sound = Yamaha YM2149 programmable sound generator, three voices
| os = Atari TOS 1.02 (shipped); upgradeable to TOS 1.04
| os = TOS 1.02 (1987), TOS 1.04 (1989)
| predecessor = [[Atari 1040STFM]]
| ports = MIDI in and out, RS-232 (25-pin D), parallel printer, hard disk / DMA (19-pin D), floppy (14-pin DIN), monitor (13-pin DIN), ROM cartridge, keyboard; mouse and joystick ports on the keyboard
| expansion = Internal 64-pin expansion connector (J15)
| power = Internal supply: +5 V at 3 A, +12 V at 1 A, −12 V at 30 mA; 95 W maximum<ref name="om-spec" />
| dimensions = 67 × 340 × 400 mm (keyboard 40 × 475 × 190 mm)<ref name="om-spec" />
| weight = 4.3 kg (keyboard 1.8 kg)<ref name="om-spec" />
| successor = [[Atari Mega STE]]
| successor = [[Atari Mega STE]]
| connectivity = MIDI In/Out, RS-232 (DB-25), Centronics parallel, ACSI DMA (DB-19), external floppy (DB-14), monitor (13-pin DIN), joystick &times; 2 (DE-9), MegaBus internal expansion
}}
}}


The '''Atari Mega ST''' is a professional desktop computer manufactured by Atari Corporation, released in 1987 as the business-oriented variant of the [[Atari 520ST|Atari ST]] family. Announced at the January 1987 Consumer Electronics Show (CES) in Las Vegas, the Mega ST was first available in Germany in June 1987, followed by the United Kingdom in August 1987, and the United States in October 1987.<ref name="oldcomputers"/>
The '''Atari Mega ST''' is the desktop version of the [[Atari 1040ST|Atari ST]]: a low system unit with its own power supply and fan, and a separate keyboard. Atari's service manual lists what it adds to the 520ST and 1040ST: a new TOS, more memory (with 74LS243 buffers on the memory address lines), a battery-backed real-time clock, the Blitter graphics coprocessor, an internal expansion connector, the fan and the new case.<ref name="sm-intro">Atari Corporation, ''Mega Service Manual'' (Mega 2 and Mega 4), undated, section 1, "Introduction", pp. 1.1–1.5. Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref> It was sold as the Mega ST2 and Mega ST4 from 1987, and from 1989 as the Mega 1, Mega 2 and Mega 4.<ref name="acm-megast" />


Unlike the all-in-one 520ST and 1040ST designs, the Mega ST introduced a two-piece '''"pizza box"''' form factor comprising a separate low-profile system unit and a detachable keyboard. This design was intended to position the ST line as a viable competitor in the professional and small business markets, particularly for desktop publishing (DTP) and music production.<ref name="atarimuseum">{{cite web |url=https://atarimuseum.ctrl-alt-rees.com/computers/16bits/megast.html |title=The Atari Mega ST Personal Computer System |publisher=Atari Museum |access-date=2026-03-27}}</ref>
== History ==


The Mega ST was produced in three configurations: the '''Mega ST1''' (1 MB RAM, released spring 1989), '''Mega ST2''' (2 MB), and '''Mega ST4''' (4 MB). The Mega ST2 and Mega ST4 were the initial launch models.
Atari worked on the machine from 1986 under the names STHD, Heavy Duty ST, ST1 and ST professional. It was announced in the United States in September 1986, shown at the Fall COMDEX in November 1986 as the ST1, and launched as the Mega ST at the Winter CES in January 1987. The Mega ST2 went on sale in West Germany in June 1987 and the Mega ST4 a month later; the machines reached the United Kingdom on 18 August 1987 and the United States on 9 October 1987.<ref name="acm-megast" />


== Technical specifications ==
The first machines shipped without the Blitter, which Atari dealers fitted free of charge later. A 1 MB model, the Mega 1, followed in spring 1989 with eight 1 Mbit DRAMs; at the same time the Mega ST2 and ST4 were given a quieter, more compact floppy drive and renamed Mega 2 and Mega 4. Production ended in the last quarter of 1991, when the [[Atari Mega STE]] replaced it.<ref name="acm-megast" />


=== Processor and architecture ===
Atari's own plans for the expansion connector (an expansion box, a PC emulator, an internal laser-printer controller, a coprocessor card and networking) were never released, and the connector was left to third-party cards.<ref name="acm-megast" />


The Mega ST uses the same Motorola 68000 CPU clocked at 8 MHz as the earlier ST models. The 68000 features a 32-bit internal data path with a 16-bit external data bus and a 24-bit address bus, providing a maximum addressable memory space of 16 MB. Instruction execution is handled in a microcoded pipeline with prefetch, delivering performance of approximately 1 MIPS at 8 MHz.
== Specifications ==


=== BLiTTER chip ===
<templatestyles src="Template:StyledTable/styles.css" />
 
{| class="wikitable styled-table" style="width:100%; text-align:left;"
The Mega ST was the first ST model to ship with the Atari BLiTTER ('''BL'''ock '''I'''mage '''T'''rans'''TER''') chip as standard equipment. Designated C025912-38, the BLiTTER is a dedicated graphics co-processor that accelerates block transfer operations, line drawing, and area fills. It operates by performing memory-to-memory copy operations with logic functions (AND, OR, XOR, NOT) between source, destination, and a halftone pattern.<ref name="atarifaq">{{cite web |url=https://www.fenarinarsa.com/?p=2242 |title=Atari ST FAQ |publisher=Fenarinarsa |access-date=2026-03-27}}</ref>
|+'''Mega ST specifications (Atari)'''<ref name="om-spec" />
 
! Item !! Specification
Key BLiTTER operations include:
* '''Block moves''' — Used for window redraws, icon rendering, and font display in GEM. Accelerates desktop operations by a factor of approximately 2–5x.
* '''Line drawing''' — Hardware-accelerated Bresenham line algorithm.
* '''Area fills''' — Pattern-based fills with 16 halftone patterns.
 
The BLiTTER accesses RAM via DMA, sharing the bus with the CPU. While the BLiTTER is active, the CPU is halted. The BLiTTER is memory-mapped at address $FFFF8A00 and controlled through a set of 26 registers.
 
Software must explicitly enable and detect the BLiTTER via the ''_blit'' system variable at address $0044C. Not all software utilises the BLiTTER; compatibility with non-BLiTTER ST models was typically maintained by checking for its presence at runtime.
 
=== Memory ===
 
RAM on the Mega ST is implemented with socketed or soldered DRAM chips, depending on the board revision. The Mega ST2 uses 16 &times; 1 Mbit DRAM chips (organized as 256K &times; 4), while the Mega ST4 uses 32 such chips. The Mega ST1 uses 8 &times; 1 Mbit DRAM chips. Memory access is handled by the MMU (Memory Management Unit, Atari C025913) and the GLUE chip (Atari C025914), which together manage bus arbitration, refresh, and address decoding.
 
The memory map follows the standard ST layout:
 
{| class="wikitable"
|-
|-
! Address range !! Function
| Processor || MC68000, 32-bit internal and 16-bit external data, 24-bit address bus, 8 MHz
|-
|-
| $000000–$3FFFFF || ST RAM (up to 4 MB)
| Memory || Mega ST1: 1 MB; Mega ST2: 2 MB; Mega ST4: 4 MB. 192 KB ROM on all models
|-
|-
| $E00000–$E7FFFF || TOS ROM (192 KB)
| Display || 320 × 200 in 16 colours, 640 × 200 in 4 colours, 640 × 400 monochrome; palette of 512 colours
|-
|-
| $FA0000–$FBFFFF || Cartridge port ROM space (128 KB)
| Interfaces || MIDI in and out, monitor (RGB, monochrome and audio), parallel printer, RS-232 modem port, floppy disk port, hard disk port, 128 KB ROM cartridge port, mouse/joystick and joystick ports, internal bus extension
|-
|-
| $FF8000–$FFFFFF || I/O hardware registers
| Sound || Three voices, amplitude envelopes and noise generator
|}
 
=== Graphics ===
 
The video subsystem is identical to the standard ST line, using the Shifter chip (C025914) to generate three display modes:
 
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
|-
! Mode !! Resolution !! Colours !! Monitor
| Keyboard || Detachable 94-key keyboard with its own microcontroller
|-
|-
| Low resolution || 320 &times; 200 || 16 of 512 palette || SC1224 (colour)
| Power supply || Built in: +5 V at 3 A, +12 V at 1 A, −12 V at 30 mA; 95 W maximum consumption
|-
|-
| Medium resolution || 640 &times; 200 || 4 of 512 palette || SC1224 (colour)
| Internal floppy drive || 3.5-inch, 135 tpi, 360 KB per side formatted, 720 KB double-sided, 250 kbit/s
|-
|-
| High resolution || 640 &times; 400 || Monochrome (black/white) || SM124 (mono)
| Dimensions and weight || Computer 67 × 340 × 400 mm, 4.3 kg; keyboard 40 × 475 × 190 mm, 1.8 kg
|}
|}


Display mode selection is hardware-locked by the monitor type connected — the high-resolution monochrome mode requires the SM124 or SM125 monitor, which uses a separate sync frequency (71.2 Hz). Colour modes output 15 kHz horizontal sync, compatible with the SC1224 or a television via the RF modulator.
== Hardware ==


=== Sound ===
[[File:Atari Mega ST2 board layout C100167.png|thumb|right|360px|Layout of the Mega ST2 main board C100167, labelled in German (atari-computermuseum.de)]]


Audio is provided by the Yamaha YM2149F Programmable Sound Generator (PSG), producing three channels of square wave output with frequency range of approximately 30 Hz to 125 kHz, plus a single noise channel. Envelope control provides hardware volume modulation. Output is mono through the monitor speaker or the 13-pin DIN video connector.
=== Main board ===


=== Internal expansion — MegaBus ===
The service manual's silkscreen drawing is of PCB C100167, revision 5.0, printed with the assembly number CA200019.<ref name="sm-sch">Atari Corporation, ''Mega Service Manual'', section 8, "Schematics and silkscreen": main-board silkscreen (PCB C100167 Rev 5.0, assembly CA200019), power supply ASP34-1 schematic and wiring diagram. Hosted as [[:File:Atari Mega ST Service Manual.pdf]] (PDF pp. 71–81).</ref> Its parts list gives the complete main-board assemblies as CA200093-001 (Mega 2) and CA200055-001 (Mega 4).<ref name="sm-parts">Atari Corporation, ''Mega Service Manual'', section 7, "Parts list and assembly drawings" (PDF pp. 61–68). Source for the assembly numbers, the IC part numbers and locations, the crystals, the connectors and sockets, and the capacitor values. Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref> atari-computermuseum.de records a second Mega ST2/ST4 board, C100501, and a separate board for the Mega 1.<ref name="acm-megast" />


The Mega ST introduced the '''MegaBus''', an internal expansion connector providing direct access to the 68000's address bus, data bus, and control signals. This 120-pin connector is located on the motherboard and provides:
The main integrated circuits, with Atari's part numbers and locations:<ref name="sm-parts" />


* Full 68000 address and data bus access
{| class="wikitable styled-table" style="width:100%; text-align:center;"
* R/W, AS, DS, DTACK, and other control lines
|+'''Mega ST main-board ICs'''
* +5V and +12V power rails
! Location !! Device !! Atari part number !! Notes
* Interrupt lines (IPL0–IPL2)
|-
 
| U8 || MC68000 CPU, 8 MHz || C025982 ||
The MegaBus was used by third-party peripherals including:
|-
* '''Moniterm Viking''' — 19-inch high-resolution (1280 &times; 960) monochrome display adapter, widely used in DTP.
| U17 || Glue || C025915 || 68-pin LCC socket
* '''Matrix VideoBoard''' — Additional video output card.
|-
* '''68881/68882 FPU cards''' — Floating-point co-processor daughtercards via the co-processor slot.
| U30 || MMU (memory controller) || C025912 || 68-pin LCC socket
* Various network adapters and memory expansion boards.
|-
 
| U31 || Shifter || C025914, or C101608 (with built-in D/A) || 40-pin socket
Note: The MegaBus is electrically distinct from the VMEbus used in the later Atari TT030 and Mega STE, though both serve as internal expansion systems.
|-
 
| U27 || DMA controller || C025913 || 40-pin socket
=== Real-time clock ===
|-
 
| U5 || Blitter || C101643 || 68-pin LCC socket
The Mega ST was the first ST model to include a battery-backed '''real-time clock''' (RTC) using the Ricoh RP5C15 chip. This allows the system to maintain accurate date and time across power cycles. The RTC is accessed via I/O registers at $FFFC20–$FFFC3F. The backup battery is a CR2032 lithium coin cell mounted in a holder on the motherboard.
|-
 
| U28 || WD1772 floppy controller || C026028 ||
=== Ports and connectivity ===
 
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
|-
! Port !! Connector !! Description
| U18 || MC68901 MFP || C025984 ||
|-
|-
| MIDI In || 5-pin DIN || Standard MIDI input at 31.25 kbaud
| U16 || YM2149 sound generator || C025983 ||
|-
|-
| MIDI Out || 5-pin DIN || Standard MIDI output at 31.25 kbaud
| U14, U15 || MC6850 ACIA (keyboard, MIDI) || C025985 ||
|-
|-
| RS-232 serial || DB-25 male || Standard serial port via MFP 68901, up to 19,200 baud
| U25 || RP5C15 real-time clock || C101622 ||
|-
|-
| Centronics parallel || DB-25 female || Standard parallel printer output via PSG port B
| U37 || RTC PAL (PAL16L8) || C101625 ||
|-
|-
| ACSI DMA || DB-19 male || Atari Computer Systems Interface; hard disk and laser printer port
| U1 || TL7705A reset controller || C101621 ||
|-
|-
| External floppy || DB-14 || Additional floppy disk drive connector
| U22 || TL497A switching regulator || C070447 ||
|-
|-
| Video || 13-pin DIN || RGB analogue + mono detect + audio output
| U19, U20 || 1489 RS-232 receiver, 1488 driver || C025986, C025987 ||
|-
|-
| Joystick &times; 2 || DE-9 male || Standard Atari-compatible joystick/mouse ports
| U13 || PC900 photocoupler (MIDI in) || C025988 ||
|-
|-
| Cartridge || 128 KB ROM cartridge slot on left side of case
| U9, U10 || TOS ROM, 1 Mbit, HI-0 and LO-0 || C101629-001, C101630-001 || 28-pin sockets
|-
|-
| Power || External Mitsumi SR98/SR118 PSU
| U40–U55, U60–U75 || DRAM, 1M × 1 || C101712 || U60–U75 on the Mega 4 only
|}
|}


=== Keyboard ===
The crystals are Y1, 2.4576 MHz, for the MFP; Y2, 32.768 kHz, for the clock chip; and OSC1, 32.0424 MHz, the system oscillator.<ref name="sm-parts" /> atari-computermuseum.de lists alternative part numbers used on later boards: DMA C100110, MMU C100109 or C100601, Glue C070714 or C101602, and Shifter C301712.<ref name="acm-megast" />
 
=== Memory ===


The Mega ST features a detachable keyboard connected to the system unit via a coiled cable and RJ-11-style modular connector. The keyboard uses a dedicated Motorola HD6301V1 microcontroller for key scanning and IKBD (Intelligent Keyboard) protocol communication. The IKBD protocol also handles mouse and joystick input, as the mouse and joystick ports are located on the rear of the keyboard unit. The keyboard provides 94 keys with full numeric keypad, cursor keys, and 10 function keys.
RAM is built from 1 Mbit × 1 DRAMs, sixteen to a 2 MB bank. The Mega 2 has one bank (U40–U55) and the Mega 4 a second (U60–U75); the low bank occupies 000800–1FFFFF and the high bank 200000–3FFFFF. The 192 KB operating system sits at FC0000–FEFFFF, and the cartridge slot gives access to a further 128 KB at FA0000.<ref name="sm-theory">Atari Corporation, ''Mega Service Manual'', section 2, "Theory of operation", pp. 2.1–2.15, including main memory (p. 2.3), the real-time clock (p. 2.7), the monitor port (p. 2.8) and system start-up (p. 2.15). Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref> The DRAMs are soldered to the board. The Mega 1 uses eight 1 Mbit chips instead, and its board is laid out so that more memory has to be added on an adapter board.<ref name="acm-megast" />


== Motherboard revisions ==
=== Video and sound ===


The primary Mega ST motherboard is designated '''C100167-001'''. Known revisions include:
The Shifter produces 320 × 200 in 16 colours, 640 × 200 in 4 colours and 640 × 400 monochrome, from a palette of 512. Each colour gun is driven from three Shifter outputs summed through a resistor network and an emitter-follower transistor; the monochrome output has its own emitter follower. A monochrome monitor pulls the MONOMON line low, which is how the machine selects high resolution at start-up.<ref name="sm-theory" /> Vertical sync is 70 Hz in monochrome and 50 or 60 Hz in colour.<ref name="sm-glossary">Atari Corporation, ''Mega Service Manual'', section 9, "Glossary of part names and terms", p. 9.3. Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref>


<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:60%; text-align:center;"
{| class="wikitable styled-table"
|+'''13-pin monitor port'''<ref name="sm-theory" />
! Pin !! Signal !! Pin !! Signal
|-
| 1 || Audio out || 8 || +12 V pull-up
|-
| 2 || Composite sync || 9 || Horizontal sync
|-
| 3 || General-purpose output || 10 || Blue
|-
| 4 || Monochrome detect || 11 || Monochrome
|-
|-
! Board designation !! Notes
| 5 || Audio in || 12 || Vertical sync
|-
|-
| C100167-001 Rev.B || Standard Mega ST2/ST4 motherboard
| 6 || Green || 13 || Ground
|-
|-
| CA2000019 || Later production variant, electrically equivalent to C100167-001 Rev.B
| 7 || Red || ||
|}
|}


The board uses a combination of custom Atari ASICs (GLUE, MMU, Shifter, DMA, BLiTTER) in PLCC packages alongside discrete logic and the 68000 in a DIP-64 package.
The YM2149 sound generator is clocked at 2 MHz and has three channels, mixed and sent through a transistor amplifier to the monitor speaker. It also handles the printer port data, floppy drive and side selection, and the RS-232 RTS and DTR lines.<ref name="sm-theory" />


== Operating system ==
=== Real-time clock ===


The Mega ST shipped with '''TOS 1.02''' (commonly called "Mega TOS"), stored in two or six ROM chips depending on the board revision. TOS 1.02 was the first version to include BLiTTER support and introduced minor bug fixes over TOS 1.00.
The clock is a Ricoh RP5C15 (U25) with a 32.768 kHz crystal, trimmed by a 5–30 pF trimmer capacitor (C53) so that the CLKOUT pin gives 16.384 kHz. A 3 V battery keeps it running when the machine is off.<ref name="sm-theory" /><ref name="sm-parts" /> The battery is two AA cells in a housing on the top of the case, at the rear of the left fan vent;<ref name="om-batt">Atari Corporation, ''Computer MEGA ST Owner's Manual'', 1987, "Clock Batteries", pp. 6–7, and "The Computer's Top Panel", p. 18. Hosted as [[:File:Atari Mega ST Owner's Manual (1987).pdf]].</ref> its lead plugs into the board under the left rear corner of the top cover.<ref name="sm-disasm">Atari Corporation, ''Mega Service Manual'', section 4, "Disassembly/assembly", pp. 4.1–4.3. Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref> See [[Battery Refurbishment]].


Later Mega ST units shipped with '''TOS 1.04''' ("Rainbow TOS"), which added improved desktop functionality, faster boot times, and better hard disk support. TOS 1.04 can be retrofitted to earlier Mega ST units by replacing the ROM chips.
=== Expansion connector ===


TOS includes the GEM (Graphics Environment Manager) desktop, GEMDOS file system, the BIOS, and the XBIOS. The GEM desktop provides a graphical file manager with drag-and-drop file operations, resizable windows, and desk accessory support.
The internal expansion connector is J15, a 64-pin double-row male header (Atari part C100283-001).<ref name="sm-parts" /> Atari's expansion test fixture checks the data and address buses and the INT3, INT5 and INT7 interrupt lines through it.<ref name="sm-test">Atari Corporation, ''Mega Service Manual'', section 3, "Testing", pp. 3.1–3.19: the dead-unit procedure (pp. 3.2–3.3), the diagnostic cartridge revision 4.0, its tests and error codes, and the expansion connector test (pp. 3.16–3.17). Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref> Graphics and network cards were made for it by other companies.<ref name="acm-megast" />


== Desktop publishing ==
=== Power supply ===


The Mega ST became closely associated with desktop publishing following its launch alongside the '''Atari SLM804 laser printer''' (US$1,500). The SLM804 was unusual in that it connected via the ACSI DMA port rather than a parallel interface, using the ST's RAM as its page buffer — the printer itself contained no processor or memory of its own. This required a minimum of 2 MB RAM in the host computer and made the Mega ST2 or ST4 the natural companion.
The supply is an internal unit with its own fan, Atari assembly CA200008-001.<ref name="sm-parts" /> The service manual gives the schematic of the ASP34-1, an Atari (Japan) design: a BU508 switching transistor, TL431 reference and PC817 optocoupler, with reservoir capacitors C4 and C5 (47 µF, 250 V) and a wire link, W1, that sets the input for 110 V or 220 V. Its six-way output connector carries +5 V on pins 1 and 2, ground on pins 3 to 5 and +12 V on pin 6; the fan runs from +12 V.<ref name="sm-sch" /> The −12 V rail in Atari's specification is therefore made on the main board. atari-computermuseum.de notes four supply variants, ASPF34-1 to ASPF34-4, in Atari's documentation, two of them with extra power outlets for a hard disk.<ref name="acm-megast" />


Key DTP software on the Mega ST included:
== Operating system ==
* '''Calamus''' — Professional page layout and typesetting application by DMC. Widely used in German-speaking countries.
* '''PageStream''' — Page layout by Soft-Logik.
* '''Publishing Partner''' — Earlier DTP package by SoftLogik.
* '''Fleet Street Publisher''' — UK-oriented DTP package.


The Mega ST's high-resolution monochrome mode (640 &times; 400) on the SM124 monitor was the standard display for DTP work, and the Moniterm Viking adapter provided 1280 &times; 960 resolution via the MegaBus for true full-page WYSIWYG layout.
Machines left the factory with TOS 1.02 (ROM date 22 April 1987) and, from 1989, TOS 1.04 (6 April 1989).<ref name="acm-megast" /> The C100167 board has six 28-pin ROM sockets, U3, U4, U6, U7, U9 and U10, marked HI-0 to HI-2 and LO-0 to LO-2; a two-chip ROM set uses U9 (HI-0) and U10 (LO-0).<ref name="sm-sch" /><ref name="sm-parts" /> Atari's diagnostic ROM test prints six checksums whichever set is fitted; with two ROMs, an error in L0–L2 means the low ROM and one in H0–H2 the high ROM.<ref name="sm-test" />


== Music production ==
== Known faults ==


With built-in MIDI In and MIDI Out ports — a feature present on all ST models — the Mega ST was widely adopted in professional music studios. The 68000's interrupt-driven MIDI timing proved exceptionally stable compared to competing platforms of the era, with jitter typically below 1 ms.
The exxos forum's list of recommended fixes starts with recapping the power supply on every ST, and names the Mega ST for four more: a 10 kΩ pull-up network on the DMA data bus, pull-up resistors on the WD1772 outputs when a drive without its own pull-ups is fitted, the reset-circuit fix for machines that need the reset button to start, and replacing the 10 kΩ bus resistors with 2.2 kΩ.<ref name="exxos-mandatory">exxos, [https://www.exxosforum.co.uk/forum/viewtopic.php?t=1585 "MANDATORY FIXES"], exxos's Atari, Amiga & retro forum, 31 January 2019. Community source; item 1 covers power supplies on all machines, items 3, 4, 11 and 12 name the Mega ST.</ref> The supply is covered in [[Atari Mega ST Capacitor Replacement Guide]]. Take the AA cells out of the clock battery housing if the machine is to be stored; see [[Atari Mega ST General Maintenance]].


Major MIDI sequencing software included:
Fault diagnosis is covered in [[Atari Mega ST Troubleshooting Guide]].
* '''Steinberg Pro-24''' and '''Cubase''' — Industry-standard sequencers. Steinberg originally developed Cubase on the Atari ST before porting to other platforms.
* '''C-Lab Creator/Notator''' — Professional sequencer and score editor.
* '''Dr. T's KCS''' (Keyboard Controlled Sequencer)
* '''Hybrid Arts SMPTE Track'''


The detachable keyboard and pizza-box form factor made the Mega ST particularly well-suited to studio environments, as the system unit could be rack-mounted or placed beneath a MIDI controller keyboard.
== Documentation ==


== Known issues ==
* [[:File:Atari Mega ST Service Manual.pdf|Atari ''Mega Service Manual'']]: theory of operation, the diagnostic cartridge and its error codes, disassembly, symptom checklist, parts list, schematics and the power supply schematic.
* [[:File:Atari Mega ST Owner's Manual (1987).pdf|Atari ''Mega ST Owner's Manual'' (1987)]]: setting up, the clock batteries, specifications and connector pinouts.


Common issues affecting the Mega ST after decades of use include:
== Related pages ==


* '''Power supply degradation''' — The Mitsumi SR98/SR118 PSU contains electrolytic capacitors that degrade over time, causing voltage ripple, random crashes, floppy read errors, video noise, and in severe cases, damage to the motherboard. PSU recapping is considered mandatory maintenance.<ref name="exxospsu">{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=Atari PSU Repair — The LaST Upgrade |publisher=exxos |access-date=2026-03-27}}</ref>
* '''Reset circuit malfunction''' — The reset circuit on ST/Mega ST boards can fail due to aged capacitors, causing the machine to require multiple power cycles or a manual reset button press to start. A modification involving the reset capacitor addresses this issue.<ref name="mandatory">{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=1585 |title=Mandatory Fixes |publisher=exxos Forum |access-date=2026-03-27}}</ref>
* '''DMA bus pull-up resistors''' — The DMA data bus lacks pull-up resistors in the original design, which can cause intermittent hard drive and floppy drive failures. Adding 10K pull-up resistors to the DMA data bus is a recommended preventive modification.<ref name="mandatory"/>
* '''1772 floppy controller pull-ups''' — Replacement floppy drives lacking internal pull-up resistors (such as the MPF920) may exhibit unreliable operation. Pull-up resistors should be added to the 1772 FDC output lines.<ref name="mandatory"/>
* '''RTC battery leakage''' — The CR2032 battery can leak over time, causing corrosion on nearby PCB traces and components. Regular battery replacement is advised.
* '''Bus resistor values''' — Stock 10K bus resistors on the address and data bus can cause instability when cartridges or certain CPU/ROM combinations are used. Replacing with 2.2K resistors improves reliability.<ref name="mandatory"/>
== See also ==
* [[Atari 520ST]]
* [[Atari 1040STFM]]
* [[Atari Mega STE]]
* [[Atari Mega ST General Maintenance]]
* [[Atari Mega ST General Maintenance]]
* [[Atari Mega ST Troubleshooting Guide]]
* [[Atari Mega ST Troubleshooting Guide]]
* [[Atari Mega ST Capacitor Replacement Guide]]
* [[Atari Mega ST Capacitor Replacement Guide]]
 
* [[Atari Mega STE]]
== External links ==
* [[Atari 1040ST]]
 
* [https://oldcomputers.net/atari-mega-st.html Atari Mega ST at OldComputers.net]
* [https://atarimuseum.ctrl-alt-rees.com/computers/16bits/megast.html Atari Mega ST at Atari Museum]
* [https://www.atari-wiki.com/index.php?title=Mega_ST Mega ST at Atari-Wiki]
* [https://www.exxosforum.co.uk/atari/last/ The LaST Upgrade — Exxos Atari ST upgrades and fixes]
* [https://docs.dev-docs.org/ Atari Document Archive (dev-docs.org)]


== References ==
== References ==
<references />
<references>
<ref name="acm-megast">"Desktop-Modelle: Der Atari Mega ST", atari-computermuseum.de, last edited 13 July 2024, https://www.atari-computermuseum.de/megast.htm. Secondary source for the development names, announcement, show and release dates, prices, the late Blitter and its free retrofit, the Mega 1 and the renamed Mega 2 and Mega 4, the end of production, the unreleased expansion plans, the board numbers, the alternative chip part numbers, the ASPF34 supply variants and the factory TOS versions.</ref>
<ref name="om-spec">Atari Corporation, ''Computer MEGA ST Owner's Manual'', 1987, appendix C, "MEGA ST Computer Specifications", pp. 109–110. Hosted as [[:File:Atari Mega ST Owner's Manual (1987).pdf]].</ref>
</references>


{{Navbox-AtariVintageComputers}}
{{Navbox-AtariVintageComputers|state=collapsed}}


[[Category:Atari Systems]]
[[Category:Atari Systems]]

Revision as of 00:22, 1 October 2026

Atari Mega ST
Mega ST with an SM124 monochrome monitor, a Megafile hard disk and the detached keyboard
Specifications
ManufacturerAtari Corporation (Atari Taiwan Manufacturing)
TypeDesktop computer
ReleasedJune 1987 (Mega ST2, West Germany)[1]
Discontinued1991[1]
Intro priceDM 2,998 (Mega ST2, June 1987); £995 (Mega ST2, August 1987); US$1,499.95 (Mega 2, November 1989)[1]
CPUMotorola MC68000 at 8 MHz
Memory1, 2 or 4 MB RAM; 192 KB ROM[2]
StorageInternal 3.5-inch double-sided floppy drive, 720 KB
Display—
SoundYamaha YM2149 programmable sound generator, three voices
GraphicsAtari Shifter; Blitter
Ports / interfaceMIDI in and out, RS-232 (25-pin D), parallel printer, hard disk / DMA (19-pin D), floppy (14-pin DIN), monitor (13-pin DIN), ROM cartridge, keyboard; mouse and joystick ports on the keyboard
ExpansionInternal 64-pin expansion connector (J15)
PowerInternal supply: +5 V at 3 A, +12 V at 1 A, −12 V at 30 mA; 95 W maximum[2]
Dimensions67 × 340 × 400 mm (keyboard 40 × 475 × 190 mm)[2]
Weight4.3 kg (keyboard 1.8 kg)[2]
OS / FirmwareTOS 1.02 (1987), TOS 1.04 (1989)
SuccessorAtari Mega STE

The Atari Mega ST is the desktop version of the Atari ST: a low system unit with its own power supply and fan, and a separate keyboard. Atari's service manual lists what it adds to the 520ST and 1040ST: a new TOS, more memory (with 74LS243 buffers on the memory address lines), a battery-backed real-time clock, the Blitter graphics coprocessor, an internal expansion connector, the fan and the new case.[3] It was sold as the Mega ST2 and Mega ST4 from 1987, and from 1989 as the Mega 1, Mega 2 and Mega 4.[1]

History

Atari worked on the machine from 1986 under the names STHD, Heavy Duty ST, ST1 and ST professional. It was announced in the United States in September 1986, shown at the Fall COMDEX in November 1986 as the ST1, and launched as the Mega ST at the Winter CES in January 1987. The Mega ST2 went on sale in West Germany in June 1987 and the Mega ST4 a month later; the machines reached the United Kingdom on 18 August 1987 and the United States on 9 October 1987.[1]

The first machines shipped without the Blitter, which Atari dealers fitted free of charge later. A 1 MB model, the Mega 1, followed in spring 1989 with eight 1 Mbit DRAMs; at the same time the Mega ST2 and ST4 were given a quieter, more compact floppy drive and renamed Mega 2 and Mega 4. Production ended in the last quarter of 1991, when the Atari Mega STE replaced it.[1]

Atari's own plans for the expansion connector (an expansion box, a PC emulator, an internal laser-printer controller, a coprocessor card and networking) were never released, and the connector was left to third-party cards.[1]

Specifications

Mega ST specifications (Atari)[2]
Item Specification
Processor MC68000, 32-bit internal and 16-bit external data, 24-bit address bus, 8 MHz
Memory Mega ST1: 1 MB; Mega ST2: 2 MB; Mega ST4: 4 MB. 192 KB ROM on all models
Display 320 × 200 in 16 colours, 640 × 200 in 4 colours, 640 × 400 monochrome; palette of 512 colours
Interfaces MIDI in and out, monitor (RGB, monochrome and audio), parallel printer, RS-232 modem port, floppy disk port, hard disk port, 128 KB ROM cartridge port, mouse/joystick and joystick ports, internal bus extension
Sound Three voices, amplitude envelopes and noise generator
Keyboard Detachable 94-key keyboard with its own microcontroller
Power supply Built in: +5 V at 3 A, +12 V at 1 A, −12 V at 30 mA; 95 W maximum consumption
Internal floppy drive 3.5-inch, 135 tpi, 360 KB per side formatted, 720 KB double-sided, 250 kbit/s
Dimensions and weight Computer 67 × 340 × 400 mm, 4.3 kg; keyboard 40 × 475 × 190 mm, 1.8 kg

Hardware

Layout of the Mega ST2 main board C100167, labelled in German (atari-computermuseum.de)

Main board

The service manual's silkscreen drawing is of PCB C100167, revision 5.0, printed with the assembly number CA200019.[4] Its parts list gives the complete main-board assemblies as CA200093-001 (Mega 2) and CA200055-001 (Mega 4).[5] atari-computermuseum.de records a second Mega ST2/ST4 board, C100501, and a separate board for the Mega 1.[1]

The main integrated circuits, with Atari's part numbers and locations:[5]

Mega ST main-board ICs
Location Device Atari part number Notes
U8 MC68000 CPU, 8 MHz C025982
U17 Glue C025915 68-pin LCC socket
U30 MMU (memory controller) C025912 68-pin LCC socket
U31 Shifter C025914, or C101608 (with built-in D/A) 40-pin socket
U27 DMA controller C025913 40-pin socket
U5 Blitter C101643 68-pin LCC socket
U28 WD1772 floppy controller C026028
U18 MC68901 MFP C025984
U16 YM2149 sound generator C025983
U14, U15 MC6850 ACIA (keyboard, MIDI) C025985
U25 RP5C15 real-time clock C101622
U37 RTC PAL (PAL16L8) C101625
U1 TL7705A reset controller C101621
U22 TL497A switching regulator C070447
U19, U20 1489 RS-232 receiver, 1488 driver C025986, C025987
U13 PC900 photocoupler (MIDI in) C025988
U9, U10 TOS ROM, 1 Mbit, HI-0 and LO-0 C101629-001, C101630-001 28-pin sockets
U40–U55, U60–U75 DRAM, 1M × 1 C101712 U60–U75 on the Mega 4 only

The crystals are Y1, 2.4576 MHz, for the MFP; Y2, 32.768 kHz, for the clock chip; and OSC1, 32.0424 MHz, the system oscillator.[5] atari-computermuseum.de lists alternative part numbers used on later boards: DMA C100110, MMU C100109 or C100601, Glue C070714 or C101602, and Shifter C301712.[1]

Memory

RAM is built from 1 Mbit × 1 DRAMs, sixteen to a 2 MB bank. The Mega 2 has one bank (U40–U55) and the Mega 4 a second (U60–U75); the low bank occupies 000800–1FFFFF and the high bank 200000–3FFFFF. The 192 KB operating system sits at FC0000–FEFFFF, and the cartridge slot gives access to a further 128 KB at FA0000.[6] The DRAMs are soldered to the board. The Mega 1 uses eight 1 Mbit chips instead, and its board is laid out so that more memory has to be added on an adapter board.[1]

Video and sound

The Shifter produces 320 × 200 in 16 colours, 640 × 200 in 4 colours and 640 × 400 monochrome, from a palette of 512. Each colour gun is driven from three Shifter outputs summed through a resistor network and an emitter-follower transistor; the monochrome output has its own emitter follower. A monochrome monitor pulls the MONOMON line low, which is how the machine selects high resolution at start-up.[6] Vertical sync is 70 Hz in monochrome and 50 or 60 Hz in colour.[7]

13-pin monitor port[6]
Pin Signal Pin Signal
1 Audio out 8 +12 V pull-up
2 Composite sync 9 Horizontal sync
3 General-purpose output 10 Blue
4 Monochrome detect 11 Monochrome
5 Audio in 12 Vertical sync
6 Green 13 Ground
7 Red

The YM2149 sound generator is clocked at 2 MHz and has three channels, mixed and sent through a transistor amplifier to the monitor speaker. It also handles the printer port data, floppy drive and side selection, and the RS-232 RTS and DTR lines.[6]

Real-time clock

The clock is a Ricoh RP5C15 (U25) with a 32.768 kHz crystal, trimmed by a 5–30 pF trimmer capacitor (C53) so that the CLKOUT pin gives 16.384 kHz. A 3 V battery keeps it running when the machine is off.[6][5] The battery is two AA cells in a housing on the top of the case, at the rear of the left fan vent;[8] its lead plugs into the board under the left rear corner of the top cover.[9] See Battery Refurbishment.

Expansion connector

The internal expansion connector is J15, a 64-pin double-row male header (Atari part C100283-001).[5] Atari's expansion test fixture checks the data and address buses and the INT3, INT5 and INT7 interrupt lines through it.[10] Graphics and network cards were made for it by other companies.[1]

Power supply

The supply is an internal unit with its own fan, Atari assembly CA200008-001.[5] The service manual gives the schematic of the ASP34-1, an Atari (Japan) design: a BU508 switching transistor, TL431 reference and PC817 optocoupler, with reservoir capacitors C4 and C5 (47 µF, 250 V) and a wire link, W1, that sets the input for 110 V or 220 V. Its six-way output connector carries +5 V on pins 1 and 2, ground on pins 3 to 5 and +12 V on pin 6; the fan runs from +12 V.[4] The −12 V rail in Atari's specification is therefore made on the main board. atari-computermuseum.de notes four supply variants, ASPF34-1 to ASPF34-4, in Atari's documentation, two of them with extra power outlets for a hard disk.[1]

Operating system

Machines left the factory with TOS 1.02 (ROM date 22 April 1987) and, from 1989, TOS 1.04 (6 April 1989).[1] The C100167 board has six 28-pin ROM sockets, U3, U4, U6, U7, U9 and U10, marked HI-0 to HI-2 and LO-0 to LO-2; a two-chip ROM set uses U9 (HI-0) and U10 (LO-0).[4][5] Atari's diagnostic ROM test prints six checksums whichever set is fitted; with two ROMs, an error in L0–L2 means the low ROM and one in H0–H2 the high ROM.[10]

Known faults

The exxos forum's list of recommended fixes starts with recapping the power supply on every ST, and names the Mega ST for four more: a 10 kΩ pull-up network on the DMA data bus, pull-up resistors on the WD1772 outputs when a drive without its own pull-ups is fitted, the reset-circuit fix for machines that need the reset button to start, and replacing the 10 kΩ bus resistors with 2.2 kΩ.[11] The supply is covered in Atari Mega ST Capacitor Replacement Guide. Take the AA cells out of the clock battery housing if the machine is to be stored; see Atari Mega ST General Maintenance.

Fault diagnosis is covered in Atari Mega ST Troubleshooting Guide.

Documentation

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 "Desktop-Modelle: Der Atari Mega ST", atari-computermuseum.de, last edited 13 July 2024, https://www.atari-computermuseum.de/megast.htm. Secondary source for the development names, announcement, show and release dates, prices, the late Blitter and its free retrofit, the Mega 1 and the renamed Mega 2 and Mega 4, the end of production, the unreleased expansion plans, the board numbers, the alternative chip part numbers, the ASPF34 supply variants and the factory TOS versions.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Atari Corporation, Computer MEGA ST Owner's Manual, 1987, appendix C, "MEGA ST Computer Specifications", pp. 109–110. Hosted as File:Atari Mega ST Owner's Manual (1987).pdf.
  3. ↑ Atari Corporation, Mega Service Manual (Mega 2 and Mega 4), undated, section 1, "Introduction", pp. 1.1–1.5. Hosted as File:Atari Mega ST Service Manual.pdf.
  4. ↑ 4.0 4.1 4.2 Atari Corporation, Mega Service Manual, section 8, "Schematics and silkscreen": main-board silkscreen (PCB C100167 Rev 5.0, assembly CA200019), power supply ASP34-1 schematic and wiring diagram. Hosted as File:Atari Mega ST Service Manual.pdf (PDF pp. 71–81).
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Atari Corporation, Mega Service Manual, section 7, "Parts list and assembly drawings" (PDF pp. 61–68). Source for the assembly numbers, the IC part numbers and locations, the crystals, the connectors and sockets, and the capacitor values. Hosted as File:Atari Mega ST Service Manual.pdf.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Atari Corporation, Mega Service Manual, section 2, "Theory of operation", pp. 2.1–2.15, including main memory (p. 2.3), the real-time clock (p. 2.7), the monitor port (p. 2.8) and system start-up (p. 2.15). Hosted as File:Atari Mega ST Service Manual.pdf.
  7. ↑ Atari Corporation, Mega Service Manual, section 9, "Glossary of part names and terms", p. 9.3. Hosted as File:Atari Mega ST Service Manual.pdf.
  8. ↑ Atari Corporation, Computer MEGA ST Owner's Manual, 1987, "Clock Batteries", pp. 6–7, and "The Computer's Top Panel", p. 18. Hosted as File:Atari Mega ST Owner's Manual (1987).pdf.
  9. ↑ Atari Corporation, Mega Service Manual, section 4, "Disassembly/assembly", pp. 4.1–4.3. Hosted as File:Atari Mega ST Service Manual.pdf.
  10. ↑ 10.0 10.1 Atari Corporation, Mega Service Manual, section 3, "Testing", pp. 3.1–3.19: the dead-unit procedure (pp. 3.2–3.3), the diagnostic cartridge revision 4.0, its tests and error codes, and the expansion connector test (pp. 3.16–3.17). Hosted as File:Atari Mega ST Service Manual.pdf.
  11. ↑ exxos, "MANDATORY FIXES", exxos's Atari, Amiga & retro forum, 31 January 2019. Community source; item 1 covers power supplies on all machines, items 3, 4, 11 and 12 name the Mega ST.


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