The original Tandy 1000, SX, EX, TX, HX used a proprietary keyboard and keyboard layout that was not compatible with the PC/XT/AT standard then in use. The layout of the keyboard prefigured the IBM Model M keyboard introduced with the IBM PS/2 series, with function keys arranged in a row at the top, instead of on the left as in the PC and PC XT/AT. Later models of the Tandy 1000 series, such as the 1000 SL, TL and RL series used a keyboard more similar and compatible with the IBM PS/2 series keyboard connector and layout.