In 1987, IBM introduced the PS/2 computer line. Although the open architecture of the PC and its successors had been a great success for them, and they were the biggest single manufacturer, most of the market was buying faster and cheaper IBM-compatible machines made by other firms. The PS/2s remained software compatible, but the hardware was quite different. It introduced the technically superior Micro Channel architecture bus for higher speed communication within the system, but failed to maintain the open AT bus (later called the ISA bus), which meant that none of the millions of existing add-in cards would function. In other words, the new IBM machines were not IBM-compatible.