The SS-50 bus was an early computer bus designed as a part of the SWTPC 6800 Computer System that used the Motorola 6800 CPU. The SS-50 motherboard would have around seven 50-pin connectors for CPU and memory boards plus eight 30-pin connectors for I/O boards. The I/O section was sometimes called the SS-30 bus.

Southwest Technical Products Corporation introduced this bus in November 1975 and soon other companies were selling add-in boards. Some of the early boards were floppy disk systems from Midwest Scientific Instruments, Smoke Signal Broadcasting, and Percom Data; an EPROM programmer from the Micro Works; video display boards from Gimix; and memory boards from Seals. By 1978 there were a dozen SS-50 board suppliers and several compatible SS-50 computers.

In 1979 SWTPC modified the SS-50 bus to support the new Motorola MC6809 processor. These changes were compatible with most existing boards, and this upgrade gave the SS-50 bus a long life. SS-50-based computers were made until the late 1980s.

The SS-50C bus, the S/09 version of the SS-50 bus, extended the address by four address lines to 20 address lines to allow up to a megabyte of memory in a system.

Boards for the SS-50 bus were typically 9 inches wide and 5.5 inches high. The board had Molex 0.156-inch connectors, while the motherboard had the pins. This arrangement made for low-cost printed circuit boards that did not need gold-plated edge connectors. The tin-plated Molex connectors were only rated for a few insertions and were sometimes a problem in hobbyist systems where the boards were being swapped often. Later systems would often come with gold-plated Molex connectors.

The SS-30 I/O bus had the address decoding on the motherboard. Each slot was allocated four addresses (the later MC6809 version increased this to sixteen addresses). This made for very simple I/O boards, as the Motorola peripheral chips connected directly to this bus. Cards designed using the SS-30 bus often had their external connectors mounted such that they were accessible outside the computer chassis when installed in SWTPC motherboards.