Editing Atari Mega ST

{{Infobox computer
| name = Atari Mega ST
| image = [[File:Atari-Mega-ST.jpg|260px|frameless]]
| caption = Mega ST with an SM124 monochrome monitor, a Megafile hard disk and the detached keyboard
| manufacturer = Atari Corporation (Atari Taiwan Manufacturing)
| type = Desktop computer
| release_date = June 1987 (Mega ST2, West Germany)<ref name="acm-megast" />
| discontinued = 1991<ref name="acm-megast" />
| 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 at 8 MHz
| memory = 1, 2 or 4 MB RAM; 192 KB ROM<ref name="om-spec" />
| storage = Internal 3.5-inch double-sided floppy drive, 720 KB
| graphics = Atari Shifter; Blitter
| sound = Yamaha YM2149 programmable sound generator, three voices
| os = TOS 1.02 (1987), TOS 1.04 (1989)
| 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]]
}}

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" />

== 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.<ref name="acm-megast" />

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" />

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" />

== Specifications ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Mega ST specifications (Atari)'''<ref name="om-spec" />
! 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 ==

[[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)]]

=== Main board ===

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 main integrated circuits, with Atari's part numbers and locations:<ref name="sm-parts" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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.<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 ===

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" />

=== 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.<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>

{| class="wikitable styled-table" style="width:60%; text-align:center;"
|+'''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
|-
| 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.<ref name="sm-theory" />

=== 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.<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]].

=== Expansion connector ===

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" />

=== Power supply ===

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" />

== Operating system ==

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" />

== 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.<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]].

Fault diagnosis is covered in [[Atari Mega ST Troubleshooting Guide]].

== Documentation ==

* [[: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.

== Related pages ==

* [[Atari Mega ST General Maintenance]]
* [[Atari Mega ST Troubleshooting Guide]]
* [[Atari Mega ST Capacitor Replacement Guide]]
* [[Atari Mega STE]]
* [[Atari 1040ST]]

== 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>

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