System 18 multi - 3) Feasibility study

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    • System 18 multi - 3) Feasibility study

      Now that I've decided what hardware to work on I had to take a deeper look at the system:
      - 11 known games: Alien Storm, Bloxeed, Clutch Hitter, Desert Breaker, D.D. Crew, Hammer Away, Laser Ghost, Moonwalker, Pontoon, Shadow Dancer, Where's Wally
      - All but Pontoon have a decrypted set available (Sweet! Less work for me)
      - 4 ROM regions: main CPU (16 bit), tiles (3 * 8 bit), sprites (16 bit), sound (CPU code + samples, 8 bit)

      Then I built a spreadsheet with the regions size for each game:

      Gamemaincpu (16bit)tiles (3x8bit)sprites (16bit)sound (8bit)
      Alien Storm5127682048896
      Bloxeed256192128128
      Clutch Hitter1024153630721536
      D.D. Crew102476840961664
      Desert Breaker2048307281921792
      Hammer Away102415361536512
      Laser Ghost10247684096896
      Moonwalker5127682048896
      Pontoon512768512128
      Shadow Dancer5127682048384
      Where's Wally51276851201664
      8960117123289610496

      Not expecting much demand for this multi I wanted to follow the simplicity route once again by using big DIP EPROMs. Biggest available being the famous 27C322 (32Mbit, 4Mbyte) if using them I would need 18. That's quite a lot and probably a bad design decision. Other alternatives could have been:
      - using bigger EPROMs to reduce the number needed, that means using SMD devices, not really easy to be programmed/updated once soldered and given I can't include any copyrighted code on the multi this was clearly a problem for the end users. I also thought of using SMD chips mounted on DIP44 or DIP48 adapter boards so people could program them anyway (my TOP3000 programmer accepts to program 64Mbit or even 128Mbit SMD devices this way no problem).
      - implementing a loader with SD card reader and a small LCD screen for game selection (or stick with the dipswithes since the number of games is so small). This was clearly an other kind of dedication and time needed for something I might never sell.
      So 18 is a lot but that's the average number of ROMs original S18 games use (of lesser capacity of course). Some games like D.D. Crew, Desert Breaker and Laser Ghost even push it to 19. I convinced myself to base my future work upon this choice.

      Ok, but was there any other possible hurdles? Yes of course!
      There are 2 kown different romboards (despite MAME listing 4):
      - 171-5874: used by Alien Storm, Bloxeed, Moonwalker, Pontoon and Shadow Dancer (MAME lists it using a 171-SHADOW romboard)
      - 171-5987: used by Clutch Hitter, Desert Breaker, D.D. Crew, Hammer Away (MAME says it uses a 837-7525 romboard but that's just the reference printed on the part side, on the solder side it's a 171-5987), Laser Ghost and Where's Wally
      What's the difference you say? First romboards are quite simples with no custom chips at all but have the following limitations regarding regions size:
      - main CPU: 512Kb max
      - tiles: 3 * 512Kb max
      - sprites: 2Mb max
      - sound: 2Mb max
      So for the later games Sega upgraded their hardware with the addition on the romboard of the 315-5436 custom chip (in SDIP64 package) to expand addressing capabilities and two sockets for main CPU ROMs.
      The 315-5436 is in charge of:
      - tile banking (externalised to the custom chip where for 171-5974 romboards it's done by the mapper on the motherboard)
      - sprite banking
      - chip enable signals generation for program ROMs (since now we have two 1Mb banks)
      It doesn't provide any kind of protection and is just an ASIC chip with fairly simple logic (but we'll talk about it later).
      Size limitations with 315-5436:
      - main CPU: 2Mb max (4 times more)
      - tiles: 3 * 1Mb max (2 times more)
      - sprites: 8Mb max (4 times more)
      - sound: 2Mb max (no change)
      That's 2 different romboards to be "emulated" by the multi. That seemed doable without too much trouble.
      In other words: "Is it feasible?". Answer is yes!
      Looking for:
      - OutRun boardset even untested or not working
      - Sega Super Scaler hardware (Out Run, Model X, Model Y), even with faulty PCM chip



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      The post was edited 1 time, last by Apocalypse ().

    • -Ace- wrote:

      Awesome! So could this be considered an "official" interest page? If so, count me in. . . .
      Maybe I should start an interest thread.
      Looking for:
      - OutRun boardset even untested or not working
      - Sega Super Scaler hardware (Out Run, Model X, Model Y), even with faulty PCM chip



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    • Like i did it here, my interest is real and if you'll make a interest thread, i'm in for a kit sir ! put me on the list ! My "Clutch Hitter" will waiting for this :whistling:
      Darksoft Multi's : CPS2 / Taito F3
      Apocalypse Multi's : Irem M72
      Superguns : HAS v3.1
      Scalers : OSSC v1.6 / XRGB Mini Freimeister / RetroTINK-2X
      Scart switch : Gscartsw V5.2
      JNX: Spitfire

      My Arcade Stuff

      [nes-101] [pce] [genesis] [segacd] [neogeo] [snes] [saturn] [psx] [n64] [gamecube] [dreamcast_ss]

      The post was edited 2 times, last by AlxUnderBase ().