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> You can still get motherboards with a pair of PS/2 ports --- and they're often the high-end gaming/enthusiast-oriented ones too. Part of the reason is becaus

> You can still get motherboards with a pair of PS/2 ports --- and they're often the high-end gaming/enthusiast-oriented ones too. Part of the reason is because USB HID supports 6-key rollover only (whereas PS/2 enables proper NKRO), as cheap keyboards wouldn't include a diode with each switch to properly prevent ghosting. Apparently it does get in the way of some games, but the only case that comes to mind is local multiplayer with the same keyboard, which isn't exactly a common or high-end thing - I imagine most people enjoying local multiplayer on a big screen from a PC would get at least one controller anyway. So most likely, something to do with latency.

hakfoo 4y ago
Actually, you can start to see rollover issues with fewer than 6 keys down. Consider if the keys are (from the perspective of the controller) wired in a 3x3 grid: A B C D E F G H I Most controllers work by running a signal down the "rows" and reading back on the "columns" (or vice versa) to determine which switches are closed. If you hold down A and D, and then press either "B" or "E", without diodes, it looks like both B and E are pressed. Controllers try to mitigate this problem by locking out troublesome combinations, so you end up with worst-case of 2-key rollover. The logical key matrix may end up designed in a way very different from the physical layout, to avoid putting commonly-used-together keys in the same row or column. This does have non-gaming implications: commands like shift-control-alt-S (IIRC, Photoshop's "save as non-PSD" command) are blocked on some keyboards.
- rollcat 4y ago
  Yep, most "consumer grade" peripherals are a race to the bottom. Diodes are cheap, but manufacturers are cheaper. Unfortunately the protocols follow.
AshamedCaptain 4y ago
> USB HID supports 6-key rollover only I don't understand why. In USB HID the report size is practically unlimited. Is there a limit on popular OSes that would be the reason for this ?

hakfoo 4y ago

Actually, you can start to see rollover issues with fewer than 6 keys down. Consider if the keys are (from the perspective of the controller) wired in a 3x3 grid: A B C D E F G H I Most controllers work by running a signal down the "rows" and reading back on the "columns" (or vice versa) to determine which switches are closed. If you hold down A and D, and then press either "B" or "E", without diodes, it looks like both B and E are pressed. Controllers try to mitigate this problem by locking out troublesome combinations, so you end up with worst-case of 2-key rollover. The logical key matrix may end up designed in a way very different from the physical layout, to avoid putting commonly-used-together keys in the same row or column. This does have non-gaming implications: commands like shift-control-alt-S (IIRC, Photoshop's "save as non-PSD" command) are blocked on some keyboards.

rollcat 4y ago
Yep, most "consumer grade" peripherals are a race to the bottom. Diodes are cheap, but manufacturers are cheaper. Unfortunately the protocols follow.

AshamedCaptain 4y ago

> USB HID supports 6-key rollover only I don't understand why. In USB HID the report size is practically unlimited. Is there a limit on popular OSes that would be the reason for this ?