The 520 ST featured same hardware basis and same amount of memory as the 260 ST.
The main difference between them was the built-in ROM TOS operating system and GEM Graphics Interface. In fact, the Atari 520ST originaly came with the OS on floppy as the OS was not completly finished. Very shortly afterward they came with the OS on 6 ROM chips (TOS 1.0).
It was first sold in Germany where it met a great success then released in the United States about six monts later. Colour and monochrome version were available. Sadly, users of the colour version couldn't expect programs written for the monochrome version to work until someone wrote an emulator allowing the software written for the monochrome version to run on a colour monitor.
Atari was the first company to offer built-in MIDI ports. This made the computer very popular with musicians. One game even used the MIDI conectors (Midi Maze) to connect up to 16 computers together in a MIDI network.
The 520 ST+ offered 1 MB of RAM instead of 512 KB. As the main board was designed to provide space for only 512 KB of RAM (16 x 41256 type chips), the second 512 KB bank chips were soldered on top of the original chips.
Apparently, 1 Mb versions were sold as Atari 1040s in Australia...
The model 520 STM came with a built-in TV modulator allowing a direct connection with the TV-set.
The original Atari 520 ST came with an external 360K single sided 3.5 floppy drive, the SF354. Most dealers either bundled it with separate power supply, or a much neater Cumana or Triangle branded drive at lower cost.
More information from Dan Osers:
Actually, there never was a colour vs. monochrome version. All ST models featured at least 2 colour and 1 monochrome setting. They were not interchangable, as the highest resolution was only available in monochrome.
Hence a program has to have different versions (or fat binary) to work on either - but the system supported both. Many games would only run in colour mode, many pro apps only in hi-res mono. Colour monitors generally could not display the mono-hi-res and vice versa, though this was more of a limitation of TV / Monitor technology of the time.
About fat binary apps, Ari Feldman adds:
Atari ST apps were NOT "fat binaries" - e.g. two types of executables embedded in the same executables in the same way that apps written for the old MacOS were.
Rather, most apps supported the ST's color and monochrome resolutions by using different Resource files (i.e. dialog, menu and alert box definitions). These were external files that resided on disc and were loaded in according to the screen resolution being used.
Some games supported Monochrome directly by adding subroutines to detect what screen resolution was running and adjust themselves accordingly.
520 ST / ST+ / STM
Full-stroke keyboard with numeric and editing keypads
'Shifter' and 'Glue' custom chips
512 KB (520 ST/STM), 1 MB (520 ST+)
40 or 80 columns x 25 lines
320 x 200 / 640 x 200 / 640 x 400 dots
16 among 512 (320 x 200) / 4 among 512 (640 x 200) / monochrome (640 x 400) this last mode needs a special monitor.
3 voices, 8 octaves
SIZE / WEIGHT
47 (W) x 24 (D) x 6 (H) cm
RGB, TV modulator (520STm), Cardridge, Midi (in/out), Centronics, RS232c, Hard Disk, Floppy Disk, Joystick, Mouse