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According to the Computer Archeology page on Space Invaders, port 1 presents "player 1's shoot/left/right" input to the CPU, whereas port 2 presents player 2. Playing around the code in an emulator, this tracks: if I start a 2-player game of Space Invaders, then when player 1 is on, only the inputs mapped to player 1 work in moving the spaceship/shooting; then when player 2 is on, only the second set of inputs work.

However, looking at photos and descriptions of the original arcade machine's control panel, there are only five buttons visible: left/right, fire, and 1-player start/2-player start:

Space Invaders control panel

So what's going on here? Is it simply that the same move left/move right/fire buttons were connected both to the player 1 and the player 2 inputs (by physically splitting the wires)? Was the internal hardware designed so that it can be used in other arcade games where there is simultaneous two-player input, and on those machines there really is two set of inputs?

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    My guess that that control buttons ares connected to both input ports and the separate ports only exist for a "cocktail table" version which would have two sets of controls, one for each end.
    – user722
    Dec 20, 2020 at 5:52
  • that looks like a good answer to me. Space invaders has a cocktail mode (it's even in source comments)
    – Jean-François Fabre
    Dec 21, 2020 at 20:12

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Many arcade games came in upright and cocktail versions, using the same PCB assembly for both versions. They usually had one dip-switch position on the circuit board to designate the cabinet type, while some had a software configuration menu where you could change the cabinet type, and some others (for example, I think Space Invaders) had a different mainboard part number for each cabinet type, with the only difference being the ROMs installed or a solder jumper installed.

Depending on the setting of this parameter, the ROM would either map both players to the "Player One" set of controls, or map each player to its respective physical control set, while flipping the video on the monitor depending on which player was playing.

Source: 20+ years ago I repaired arcade machines for a living and was given a malfunctioning original black and white "Trimline" upright Space Invaders. I spent many hours poring over its service documentation before I got it fully working again. Browsing through some of the online documentation to refresh my memory, it appears as if an 8-bit input is dedicated for the player input switches, with three bits dedicated to left, right, and fire inputs for each of both players, and the last two bits dedicated to the one and two player game select buttons on the mainboards of both cabinet types. This characteristic appears to be the same, regardless of whether the machine is of Taito or Midway manufacture.

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  • @peterduniiho - Thanks for the edit, although I'm unsure of the "8-bit" part lol. I've always prided myself on my grammar, and never even realized I had been using the contraction form of "it is" all my life(I had thought it was used the same in both situations) to refer to an object in its possessive form. It had always just occurred to me that anything possessive that ends with an "s" would require an apostrophe, but I had neglected cases such as "your" or "my", and it seem that no one had ever previously corrected me. Thanks for that, and I'll never do it again! :)
    – Hitek
    Jan 3, 2021 at 1:10

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