ATARI 7800 PRO System
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(pictures copyright by M.A.Grundke)
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ATARI 7800 serial plate:
ATARI MODEL NO. 7800 PAL-B
FCC ID: EBA72R7800
ATARI CORPORATION
MADE IN CHINA
SERIAL NUMBER X2 1 6 5 056939 |
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C300633 Rev.C mainboard overview:
clearly dominating the board are a big voltage regulator's heatsink and the cartridge slot before it (both mid-rear). Left from them, the RF-modulator. Let's start from there. Below the modulator are MARIA and, still below, the CPU and a 14.187576 MHz crystal. Note that most of the machine's chips do not have 'locations' printed on the board - however, according to the schematics, MARIA should be at U1.
So going right from MARIA and CPU is another special thing: a daughterboard, soldered to the motherboard with a 28 pin socket! I cannot remove it without soldering it out... However, on the daughterboard are two 2Kx8 RAMs and a 74LS08 directly above the socket (most likely handling communication between 'mother' and 'daughter'). Below the daughterboard are the 7800 PAL OS ROM (left side), TIA (middle right) and RIOT (bottom right of daughterboard). Right from TIA, there's another crystal, this time 4.433618 Mhz (PAL color-carrier frequency). Spread across the board are also five 74xx and one CD4013 flip-flop. On the boards front you can see the four switches
(for POWER-, SELECT-, PAUSE- and RESET-buttons), the green power-LED (lower right) and the controller ports (middle), with the difficulty level switches between them. Note also that, like most of ATARI's videogames, the 7800's board is completely shielded on both sides! Must have something to do with stricter emissionary regulations for toys... Now back to the crystals mentioned earlier - there are some differences between PAL and NTSC-machines here: |
| Crystal | divided by | used for |
| 4.433618 MHz | 1 | PAL color-carrier frequency (PAL-machines only) |
| 14.187576 MHz (PAL)/ 14.32 MHz (NTSC) | 2 |
MARIA: 7.093788 (PAL)/ 7.16 MHz (NTSC) |
| 14.187576 MHz (PAL)/ 14.32 MHz (NTSC) | 4 | color burst signal: 3.546894 (PAL)/ 3.58 MHz (NTSC) |
| 14.187576 MHz (PAL)/ 14.32 MHz (NTSC) | 8 |
CPU clock 7800 mode: 1.773447 (PAL)/ 1.79 MHz (NTSC) |
| 14.187576 MHz (PAL)/ 14.32 MHz (NTSC) | 12 | CPU clock 2600 mode (*): 1.182298 (PAL)/ 1.193 MHz (NTSC) |
| (*) slow down is also required in 7800-mode, if RIOT or TIA need to be accessed |
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board silkscreening, part 1:
ATARI CORP.
C300633-001
REV C
This board is multifunctional, i.e. it can also be used for building NTSC 7800s, by just fitting some additional parts (resistors etc.) and replace some existing (crystals, MARIA). For that purpose, there are several silkscreened locations on the board marked 'NTSC' |
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board silkscreening, part 2 (lower side of the board):
C300633-001 REV C |
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daugherboard silkscreening:
© ATARI
CORPORATION
C300452 REV.C |
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| The 7800's CPU, an UMC UM6502I (aka 'SALLY'). Its an ATARI-customized 6502-CPU (also called 6502C), having been optimized to work with ATARI's customchips (in this case with MARIA). The difference to a standard-6502 is the additional HALT-line - the CPU can be halted to allow other devices (i.e. MARIA or even an additional CPU, which could come in a cartridge) full-bandwidth bus access (e.g. DMA). In all other aspects, including code compatibility, it is a fully functional 6502 (with its additional features compared to the 6507 used in the VCS-series, such as full 64K addressing range, NMI- & IRQ interrupts, SYNC signal).
By the way, SALLY was used in the ATARI 5200 SuperSystem before, and in most of the XL/XE series (although some early XL came with a standard 6502 and required additional chips to provide the HALT functionality) |
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| These two STC 8828 are the 7800's RAM (which manufacturer used that STC-prefix, by the way?). According to the schematics, they're 2Kx8 organized (6116-type). Their main purpose is to store MARIA's display list, which is used to generate the display (for that purpose, by the way, MARIA contains additional so-called 'Line-RAM'). Additionally, there also the 128x8 (128 bytes) RAM in the RIOT (aka 'SARA'). However, that RAM is only for VCS-compatibility, and cannot be accessed in 7800-mode |
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| The PAL 7800's OS ROM, C300558-01B (label hardly readable under that daughterboard, but I managed to lift it a little). Here's an interesting difference between PAL- and NTSC machines: in the original NTSC 7800, the OS ROM controls the power-up procedure (performed by MARIA) in which the inserted module is searched for a valid encryption key (showing ATARI's 'Fuji logo' while proceeding). If no valid key is found, the system is set to 2600-compatibility mode in which MARIA is disabled and only VCS2600-carts are usable. But this is not true for PAL-machines: the PAL 7800 has been redesigned here, because the 'protection circuit' (what ATARI calls it)
and the algorithm used were not allowed to be exported to outside the US (you probably know that there are strict export regulations in the states, as far as certain technologies, such as encryption, are concerned). As a result, ATARI lost that efficient way to fight cartridge piracy outside the USA - as far as I know, there's no similar protection for PAL cartridges |
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| C025718-30 'MARIA' is the 7800's graphics chip, and direct successor to the VCS-series' TIA. In NTSC 7800s, it also runs the encryption check from the ROM mentioned earlier. MARIA is disabled, however, if that check fails, and system stays in 2600 compatiblity-mode (default). With that trick ATARI wanted to prevent unauthorised games to be sold, like the 'flood' of games on the 2600. As mentioned earlier, I don't know if there's a similar encryption technique in PAL 7800s. Back to graphics: MARIA doesn't know players, playfields or missile graphics anymore. Instead, it uses a 'framebuffer-style' approach to display,
in which bitmaps are stored in a so-called 'Line RAM', which is part of MARIA, and then read to produce the output. MARIA also introduced the feature of display lists, which allows e.g. the combination of graphic modes, and was used in the later ANTIC-design as well (remember: MARIA had alread been developed in 1983!) By the way: a great advantage in comparison to TIA is MARIA's DMA-capability, in connection with the CPU's HALT option. Additionally, it's clocked 6 times faster (7.093788 MHz in PAL-, 7.16 MHz in NTSC machines), so there's much more CPU time left for the game itself (compared to the 2600, in which CPU cycles were scarce because TIA used the bus for display output) |
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| C398052-001 TIA and UM6532 RIOT, both covered by the machine's daughterboard. TIA makes the 7800's sound. Additionally, it creates the graphics when in 2600 compatibility mode (see above). Many people asked themselves why the advanced 7800-design still comes with the 2600's TIA for sound. Well, simple: there wasn't enough room in the case for an additional POKEY! But ATARI's engineers added some signals to the cartridge interface, that can be utilized to drive an external POKEY for sound. The Ballblazer-cartridge, for example, comes with its own POKEY. It's a good idea, yes, but makes cartridge production more expensive, of course, and thus wasn't applied very often.
RIOT is also a remnant of the VCS 2600. It provides the 7800 with I/O-functions (such as controller I/O). Additionally, it contains SARA (i.e. 128 Bytes RAM, 128x8) and timer functions, which can only be used in 2600 mode. While the decision to include TIA and RIOT in the 7800-design was wise for 2600 compatibility (which is almost 100%, except for some late NTSC-boards), it also has one major drawback: when accessing these chips, the machine's CPU has to slow down to 1.182298 MHz, the PAL-VCS' CPU speed (i.e. 1.19 MHz in NTSC-machines) |
See the VCS 2600-series' boards, being the 7800's forerunners!