ATARI 520ST+
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(pictures copyright by M.A.Grundke)
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ATARI®
ATARI 520ST+
ATARI CORPORATION
MADE IN TAIWANSERIAL NUMBER: A1 4004726 |
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board overview (with shielding removed):
beginning on the left side, there's the cartridge connector with the socketed DMA controller (U1) to its right. Below these, the six TOS ROMs (U2-U7). Right from the lowest ROM (U7), there's GLUE (U12), with a metal stripe holding it in place. Above GLUE, in DIP case, the MC68901 (U11) and, biggest chip on the board, the MC68000 (U10). Still above, the smaller DIP WD1772-PH FDC (U9), with serial driver and -receiver (U13, U14) to its right. Still right is the Yamaha YM2149F (U19) with the two MC6850 (U20, U21) below. Then, there's the RAM daughterboard, with the MMU (U15) to its left
(which is covered by a PLCC-pickaback adaptor, to take up some MMU-signals for the RAM expansion). Shifter also resides on that daughterboard (under shielding). Below these ICs, the onboard RAM bank (U16-U18, U24, U25, U28-U30, U32-U34, U38, U42-U45). Above Shifter's shielding, by the way, you can see an empty area where a RF-modulator would be (in STF-models) |
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board silkscreening:
ATARI® C070243 REV.H |
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| The Motorola 68000G8. In ST-series machines (like this one), the DIP-version was used. The later STEs used a PLCC-version instead |
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| 'GLUE' is probably the most important part of all ST-designs, even of STE, TT or STBook (apart from the CPU, of course). It provides the control logic necessary for making the machines' components work together (that's why it's called 'glue'...) |
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| The machine's memory management unit, a C025912-38. In the 520ST+, it is normally covered by a PLCC-adaptor, which takes some MMU-signals directly from its socket in order to make the additional RAM (see below) work |
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| Here's the 'pick-a-back board', that ATARI used for expansion to 1 MByte RAM. As you can see, it takes Shifter- (left) and MMU-signals (right). It contains 16 pcs. 256Kx1 RAMs (Samsung KM41256). Behind it, you can see part of the original memory bank,
also consisting of 16 pcs. 256Kx1 RAMs (NEC D41256C). In other 520ST+s (like those with REV.C boards), the additional RAMs were simply soldered onto the existing ones (which required some additional wiring). All that effort just to get rid of the surplus 520ST boards... |
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| The 520ST+'s PAL TOS 1.0, by the way exactly the same version like in my 260ST. It consists of six 32 KByte chips (256Kx1), with the ATARI part-numbers C026302-001, C026303-001, C026304-001, C026305-001, C026306-001 and C026307-001 |
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| C025914-38A 'Video Shift Register Chip' (or in short: Shifter), which creates the machines graphics. In this 520ST+, the additional RAM daughterboard resides in its socket, and Shifter upon it |
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| The Yamaha YM2149F 'Programmable Sound Generator'. ATARI claimed that producing sound with it 'places a minimal amount of processing burden on the main system' (from: Engineering Hardware Specification of the ATARI ST Computer System, ATARI 1986). This was generally true (and a great advantage especially for videogames), as long as you didn't want to process digital sound, which slowed the machine down significantly. This problem disappeared when the 'enhanced STs' were launched in 1989.
The new machines came with a PCM soundchip ('Pulse Code Modulation'), which could playback digital sound (in CD-quality) without taking much CPU performance |
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| This is the floppy-contoller used in all STs, Western Digital's WD1772-PH. It allowed the ST to have up to two 'daisy-chained' diskdrives (this was also true for machines with integrated drive, of course). It reads data at 125 KBits/s (single sided) or 250 KBits/s (double sided) |
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| The DMA-controller (here C025913-38) was mainly used for direct memory access when reading from diskdrive (at a maximum of 250 KBits/second) or harddisk-drive (with up to 10 MBit/second). In the later machines the BLiTTER was also a DMA-device |
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| The MC6850 is a cheap, 8 bit peripheral which was originally developed for use with the MC68000's ancestor, the MC6800, but works in other systems (with 8 bit-capable bus) as well. It's available as MC6850 (1 MHz), MC68A50 (1.5 MHz) and MC68B50 (2 Mhz) |
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| The 68901 was originally developed by Mostek and later adopted also by Motorola. This Motorola 68901 was especially produced for ATARI (thus the SC87898P - 'SC' means that it was especially made for a customer). It is 68000-bus compatible (having 24 programmable registers) and includes 8 parallel I/O-lines, a 16 sources interrupt-controller, four timers and a single channel, full duplex USART.
Interesting is that the 68901 is a direct descendant of the MK3801, which was made for the Z80-family, once a direct rival to the MC6800 |
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On keyboard PCB:
also made for ATARI was this SC400020P (ATARI part-no. C070122-002, a Hitachi HD6301 in later machines). As 'intelligent keyboard' (ikbd), this microprocessor doesn't just do the keyboard-scans but can also trigger mouse-movements and joystick-events (the two joystick/mouse-ports are also part of the 'ikbd'). Additionally, it contains a 'time-of-day' clock |