Atari Mega 1

A professional version of Atari's 16-bit computer was called Mega. In its typical form, it was in a small desktop ("pizza box") casing, with detachable keyboard. As 1040, the Mega 1 had 1MB of RAM, a 68000 CPU runing at 8MHz and Atari TOS system in ROM with possibility to load extensions from floppy at start-up. The primary difference, except casing, was addition of a BLITTER chip - a special co-processor for moving and copying specified areas of memory very fast. These operations are needed when processing images. Additionally, the computer had a battery-powered RTC with nonvolatime data region as well as additional time-of-day clock built in keyboard firmware. The keyboard, mouse and joystick controller was built-in detachable keyboard, connected with a 6-wired cable (serial port). There was also an internal bus for attaching expansion boards.
These computers were popular in Germany in applications which required colour graphics processing. 1MB of RAM gave it much power and disk compatibility with PC made it chosen in multi-computer environments more frequently than Amiga.

Manufacturer Atari

Origin USA
Year of unit 1989
Year of introduction 1987
End of production 199?
CPU Motorola MC 68000
Speed 8MHz
RAM 1024kB
ROM 256kB (TOS)
Colors: 512
Sound: YM2149 chip, MIDI
OS: TOS in ROM or from floppy
Display modes: Text: 40/80x25 simulated
Graphics: 320x200x16, 640x200x4,  640x400x2 (special monitor needed)
Media: Built-in 3.5" FDD (1040STf)
External 3.5" FDD
Optional external HDD.


Power supply:

Built-in switching power supply (+5V, +12V DC)

Additional Peripherals:
 - Atari SM124 monitor
 - my Universal mouse works well with these computers.
 - my PS/2 Keyboard/mouse/joystick adapter
 - Atari SH-205 hard disk (on this page)

I/O: Serial port (RS232)
Hard disk port (dedicated)
Ext. floppy disk
Keyboard, Joystick, Mouse
Cartridge expansion
MIDI ports
Possible upgrades: Memory upgrades, HW mods, commercial expansions
Software accessibility: Quite easy (TOSEC, old sites)

My unit is from Germany. Not much more can be told about it. I got it without keyboard or mouse and had to make one using PS/2 PC peripherals and Arduino chip. It was a "cost-reduced" unit, without fan and with quite poor PSU technology, but with early mainboard with 6 ROM spaces (2 chips used). Additionally RAM has been soldered not a very good way, with skewed chips.


Contents: Starting ROMs Pinouts Keyboard Hard disk Links


If you have a hard disk connected, power it on before and wait until it spins up.
Mega will also start for a quite long time with gray/white screen looking for things in a non-existing floppy disk. If the floppy drive is OK, just give it any720K floppy disk, may be a PC disk formatted to 720kB. With disk in drive it'll start more quickly.
If external hard disk is connected, it'll read a driver called AHDI from it.

Information about making system disks and starting ST in smaller Atari ST page.

If you have a hard disk, remember to park its heads it you plan to move it around. The Atari's method is to launch SHIP.PRG program from a floppy, not from disk, as returning to system would make another read from no-parked HDD to display its windows back. If you don't want parking from floppy, there is a N_SHIP program attached with my AHDI programs set, this one closes HDD windows before parking heads.


There are numerous configurations of ROMs, for 192kB TOS or 256kB. If there are 2 chips, it's usually a 256kB TOS, and it can be upgraded easily. If there are 6 circuits, they're usually 32kB ROMs for 192kB TOS. Usually ROMs from the net come with IMG files which are dumped (concatenated from all ROMs).

To restore them to "flashable" form, you have to split it. This will cover situation with TWO chips (they're 128kB each):
- Split the IMG file separating EVEN and ODD bytes to HIGH and LOW parts.
- Round resulting parts to 128kB filling them with 0xFF (faster EPROM programming).
To update TOS with EPROMs, it may be needed to make adapter for inserting an EPROM into a ROM socket. For a 256kB TOS (2x128) you need a 27C001, 011, 1001 or similar 128kx8 EPROMs. The pinouts of ROMs can be found in TOS104UP.ZIP document (see "More files" button).
I tried to run 4 TOSes: 1.04 German, 1.04 English (and they worked from 27C004!) as well as English 2.06 and 2.06 Polish, these 2.06 didn't work even when I was sure that W2/W3 jmpers were properly set (setting them to 2/3 makes pins 1 and 22 accordingly A15 and A16).
However, if you need multiple TOSes, for example, one for your language and one "original", you can make a switch using two 512kB EPROMs (e.g. 27C004).
The switch is described in document (see "More files"). However, installation of this switch requires you to have EPROM adapters I wrote about. Summing up, you need to accordingly switch address address lines to high/low states, and in both chips they should have the same state.

The physical space is quite limited between chips. If you cram two pull-up resistors in adapters, it will require only 3 wires to switches: One from ground and two from resistor-EPROM pins connections. The resistors are only to have high on pins if nothing is connected there, so the switches can be open-close switches instead of pin toggle ones. You need two switches to switch TOS versions. How to install them without damaging the casing? Use vent holes!
To prepare ROMs for such module, just concatenate all 128kB HIGH ROMs to one file and LOW to another in the same order, then flash them into chips.





See Small Atari ST page.





This is a different topic. The keyboard, connected with RS232 at TTL levels, has quite significant intelligence inside. Currently (2017) it may be hard to get keyboard, so a PC keyboard and mouse may be used with help of a microcontroller translating protocols and emulating non-existing devices.
There is a probably complete solution called Eiffel, to connect PS2 keyboard and mouse, as well as joysticks, to ST but it requires microcontroller not easily available today. The PIC16F876A is accessible more easily, but will NOT work as its flash memory procedures are different.
As a quick solution I tried to implement another PS/2 - ikbd translator using Arduino platform with ATMega328 chip which is, and for a few years probably will be, better accessible. It is on its own hacks page. The problem is that it is not tested extensively - default, power-on mouse, keyboard and joystick settings seem to work.


Hard disk

The SH205 hard disk is an unit connected with ACSI - a reduced SCSI mutation used by Atari. It requires 19-pin connectors (DB19) connected 1:1 (computer to HDD's IN connector!), if they're not accessible, you can make one by removing 6 pins from 25-pin connectors, remove them on one side and it may even connect without grinding down part of casing.
The ACSI is converted by Adaptec ACB4000 module to typical MFM/ST506 standard. Usually WD, Tandon or Seagate drives were mounted.
Before you power Atari Mega on, turn the HDD on and wait until it spins up.


The SW1 triple DIP switch is used to enable secondary data channel (by turning them OFF) on MFM interface, so having a connector installed in you can theoretically (or practically - see this in German) install TWO drives. I have no idea how they may fit well without getting too warm. Usually this switch is protected some way, it may be covered with transparent tape seal or filed flat not to make user accidentally switch it.
The row of jumpers above ROM (J7) or near CPU is software-defined, controlled by 8156 by a pulled-up port.

The jumper J8 has the following positions: T,S,R and PU and is repsonsible for write precompensation signal generation. It is possible to connect:
- S to R
- T to R
- PU to R.
The R is a control of write precompensation enable line in AIC-250 chip (see its documentation). It is usually tied to Write Gate. Selecting T makes it AND-dependent on a CPU READY signal (usually is high). S makes it AND-dependent on Reduced write current signal generated by 8156. PU totally forces the line on LOW. According to notes on the SH204 schematic, The R-S position (dependent on RWC) goes when ST225 drive is used and R-PU (force line low) when Tandon TM262. See schematic , page 12.

Excerpt from technical notes, but I think not all ROM revisions may have it (my unit doesn't go with it and is 100% OK!):


These self-diagnostics are for troubleshooting on a system-level only. They are not intended for repair of the board.

The ACB-4000 Series Controller has built-in diagnostics. These are performed at power-on time when the O-P jumper is installed. DO NOT connect the cables. If the board is functioning properly the LED will flash conitnuously with duration of 0.5 - 1.0 secondd per flash. If there is a problem with the controller, the LED will stay on for 6 seconds, flash once for one second (notating the start of diagnostics), and then flash in 0.5-second bursts. This will then be repeated as long as the O-P jumpoer is installed. The number of 0.5 second bursts is the error code. The error codes are detailed in below table:

ERROR CODE (number of 0.5-second bursts) - PROBABLE PROBLEM AREA
1 - 8156 RAM
6 - AIC-010 BUS

When a known good drive is correctly connected to the controller, it sill seek and read. Error codes are invalid in this mode.

ACSI Pinout:

1 - D0
2 - D1
3 - D2
4 - D3
5 - D4
6 - D5
7 - D6
8 - D7
9 - /CS
10 - IRQ
11 - GND
12 - /RST
13 - GND
14 - ACK
15 - GND
16 - A1
17 - GND
18 - R/W
19 - REQ

The 19-pin male connectors are hard to get nowadays. You can use 25-pin connectors. It is possible to re-make this connector by removing 6 pins, in 2 rows of 3. To cover the mounting screw in one side only, remove these pins on a single side of connector. For Atari Mega casing, remove the ones which will cover the rightmost mounting hole, not to cover FDD connector too much (verify it practically!).

Links - Various Mega ST boards - Mega ST Schematic - AHDI versions history

For software and mods, go to small Atari ST links page.