The SS-50 bus was created in 1975 by Southwest Technical Products Corp. (SWTPC) of San Antonio, Texas, for their 6800-based SWTPC 6800 computer kit. Other manufacturers started producing boards for this bus too. Later SWTPC modified the bus to for their 6809 version of the SWTPC computer. This variant is sometimes, but not always, referred to as SS-50C. And there is also a version with 20 address lines instead of the 16 in the original. It was designed for the STWPC S/09, a paged-memory 6809 system.
The bus is made by a sequence of Molex connectors, with pins on the motherboard and the extension boards had sockets which fit over the pins. This is different from the much more common edge connector style of e.g. the S-100 bus.
The following table describes the signals of the various SS-50 versions. Underscore (e.g. D0) is used to indicate a complemented or inverted signal, as the more common way of using a horisontal line above the signal doesn't seem to be available in mediawiki.
- Verification is still needed for some of these signals!
- D0 - D7: Data Bus: The SS-50 data bus is the complement of the CPU data bus lines D0-D7. These lines are bidirectional.
- R/W: Read/Write: The read/write line of the processor. When it is high, D0-D7 are inputs to the CPU, when low D0-D7 are outputs from the CPU.
- A0-A15: Address bus: Address bus of the CPU.
- A16-A20: Address bus extension (S/09 only): Used by bank switching or paged memory systems to address up to 1MB of memory
- VMA: Valid Memory Address: The complemented 6800/6809 VMA line. Goes low when a valid address has been placed on the address bus.
- M RESET: Manual Reset: This pin is the input to a one-shot multivibrator. When pulled low by e.g. pressing a reset button, the one-shot pulses the Reset line which resets the system
- Reset: Reset: The reset line is the output of the one-shot triggered either by M RESET or automatically on system power up. It is connected to the reset input of the processor, and to other resettable pheripherals.
- NMI: Non-maskable interrupt: Active low line connected to the processor's NMI input.
- IRQ: Interrupt Request: Active low line connected to the processor's IRQ input.
- HALT: Halt: Active low line connected to the processor's HALT input. The 6800 halts the execution and floats the address and data buses and the R/W line. External devices can then access memory for DMA, for example
- BA: Bus Available: This is the BA output of the processor. Goes high in response to a Halt input, to signal that buses are available (floating)
- BS: Bus Status: This is the BS output of the 6809 processor. Goes high with BA on e.g. BREQ (-C and -S/09 only)
- φ1: The Phase 1 output from the two-phase non-overlapping clock of the processor
- φ2: The inverted Phase 2 output from the two-phase non-overlapping clock of the processor, used to indicate the presence of valid data on the data bus
- UD1, UD2: User Defined: User defined lines are not assigned pre-defined functions.
- 110b,150b,300b,600b,1200b: Serial clock: Outputs of a crystal-controlled baud rate generator, may be used by a serial interface peripheral
- 4800b,9600b: Serial clock: (-C only)
- GND: Ground lines
- +8V: Power: Unregulated +8V (labelled as '7-8 VDC UNREGULATED' on SWTPC 6800 motherboard), may be used by on-board regulators to create +5V, for example
- -12V: Power: Regulated -12V
- +12V: Power: Regulated +12V
- INDEX: No pin: Physically plugged to prevent incorrect insertion of a board
- MRDY: Memory Ready (6809 only): When low, E and Q may be stretched in intervals of 1/4 bus cycles (-C and -S/09)
- BUSY: Busy (-C and -S/09 only)
- FIRQ: Fast Interrupt Request: Active low line connected to the 6809's FIRQ input (-C and -S/09)
- BREQ: Bus Request: Active low line connected to the 6809's DMA/BREQ input(-C and -S/09 only)
- Q: Quadrature Clock Signal (6809 only) (-C and -S/09)
- E: Clock Signal similar to the 6800 φ2 (6809 only) (-C and -S/09>
The table entries with two options, e.g. 110b/BREQ are configurable with switches on the motherboard.