"Having found success in the home console market with the NES, Nintendo developed arcade hardware to run its most popular NES games inside an arcade cabinet. Dubbed the PlayChoice-10, the machine was compatible with the NES, but with some key differences. An extra CPU controlled the gameplay timer, game select, and displayed hints for the current game on a separate monitor (on single-monitor systems, a button would switch between gameplay and the hint screen). Most normal NES cartridges could not be used; rather, the PlayChoice used special expansion cards containing NES games along with an extra 8KB ROM to display hints. Because the PlayChoice-10 outputs RGB video using a slightly different palette, games did not look exactly the same as they did on the NES. It is in fact possible to replace the NES PPU with the PlayChoice-10 PPU, allowing it to output RGB natively.
Each machine had a different mix of games in it. Instead of a player getting to play one game until it was finished, the player got a fixed time limit to play as many PlayChoice games as they wanted to. Nintendo created several variations of the hardware, including a standalone PlayChoice unit which only had a single available game. By the late 1980s, other companies developed similar systems. Sega's Mega-Tech and Mega Play hardware was capable of running certain Sega Master System and/or Sega Mega Drive/Genesis games. SNK's Neo Geo was another cartridge-based system that was made available for both arcades and home consoles."