The Tandy 1000 RL/RLX/RSX series were slim-line desktop home computers. They featured a much more compact case, with at least 512 KB of memory pre-installed, smaller PS/2-style keyboard and mouse ports, and at least one ISA expansion slot. The RL-series and RSX include provisions for an internal hard disk drive, depending on the model: the RL-series featured a built-in XT-IDE hard drive interface, while the RSX featured an AT-compatible IDE interface. The keyboard connectors of the RL-series, while similar to and mechanically compatible with PS/2-style connectors, were not fully compatible with typical PS/2 keyboards, as the keyboard uses the XT keyboard protocol. The RSX, however, incorporated the AT keyboard protocol, making it the first 1000-series system to offer more complete compatibility with typical PS/2 keyboards, and AT keyboards using an adapter.