SS-50 Specification
Bob Applegate, Corsham Technology

https://www.corshamtech.com/ss-50-faqs/ss-50-specification/
copied June 2023 from corshamtech.com by Herb Johnson

SS-50 Specification

This isnít really any definitive definition of the SS-50 bus, as boards were simply produced that worked with other SS-50 boards. When SWTPC devised the bus, they just made things work and other manufacturers made compatible boards. Years ago I found two sites that gave a clear description of the pins for both the SS-50 and SS-50C versions, and since they havenít been touched in decades, I decided to copy the information here just in case the original pages ever go away.

So, the two pages I started with are:
[two pages from a SWPTC site by Yakownek now unavailable]
www.cs.unc.edu/~yakowenk/swtpc/ss50.html
www.cs.unc.edu/~yakowenk/swtpc/ss50b.html

Since taking those pages I've added some things, changed formatting, etc.

The SS-50 bus

The SS-50 was the main backplane in 6800 based SWTPC machines, and connected the CPU board with memory, disk controllers, and so on. As was common in that era, a board usually did just one thing, and did it well, so a system would have a CPU board, one or more memory boards, a serial board connecting to a terminal, and either a cassette I/O or disk controller board for program/data storage. Physically, the SS-50 bus consisted of rows of male Molex connectors, spaced 0.156 inches apart. The boards that plugged into it had the corresponding female connectors along one edge, rather than the more modern (and cheaper) printed-circuit contacts. The SS-50 bus had all signals from the processor.

For I/O, see the next section below about the SS-30 bus.

All signal symbols starting with backslash are active low. Descriptions as double-quote, means to refer to details from the previous signal description.

  1. /D0 – (complement) data bus line 0
  2. /D1 “
  3. /D2 “
  4. /D3 “
  5. /D4 “
  6. /D5 “
  7. /D6 “
  8. /D7 – (complement) data bus line 7
  9. A15 – address bus line 15
  10. A14 “
  11. A13 “
  12. A12 “
  13. A11 “
  14. A10 “
  15. A9 “
  16. A8 “
  17. A7 “
  18. A6 “
  19. A5 “
  20. A4 “
  21. A3 “
  22. A2 “
  23. A1 “
  24. A0 – address bus line 0
  25. GND – ground
  26. GND – ground
  27. GND – ground
  28. +8V – power line
  29. +8V – power line
  30. +8V – power line
  31. -12V – power line
  32. +12V – power line
  33. INDEX – no pin – prevents backwards insertion
  34. /M.RST – (complement) manual reset
  35. /NMI – (complement) non-maskable interrupt
  36. /IRQ – (complement) interrupt request
  37. UD – user-defined
  38. UD – user-defined
  39. /Phase 2 – (complement) processor clock 2
  40. /VMA – (complement) Valid Memory Address
  41. R/W – Read / (complement) Write
  42. /RESET – (complement) Reset or power-up
  43. BA – Bus Available for DMA
  44. /Phase 1 – (complement) processor clock 1
  45. /Halt – (complement) halts the processor
  46. 110b – 110 baud clock signal
  47. 150b – 150 baud clock signal
  48. 300b – 300 baud clock signal
  49. 600b – 600 baud clock signal
  50. 1200b – 1200 baud clock signal

The SS-30 Bus

I/O was handled on a distinct 30-pin bus (the SS-30), which was generally similar to the SS-50 but had a "board select" signal instead of the address bus. The logic to select individual I/O boards in the SS-30 was hardwired to memory-map them into four-byte slots starting with board 0 at address $8000. Very few address lines from the SS-50 bus were present, usually only A0 and A1 (called RS0 and RS1) but the two UD (user defined) pins could have A2 and A3 connected to them.

  1. UD – user-defined
  2. UD – user-defined
  3. -12V – power line
  4. +12V – power line
  5. GND – ground
  6. GND – ground
  7. INDEX – no pin – prevents backwards insertion
  8. /NMI – (complement) non-maskable interrupt
  9. /IRQ – (complement) interrupt request
  10. RS0 – register select – like A0
  11. RS1 – register select – like A1
  12. D0 – data bus line 0
  13. D1 “
  14. D2 “
  15. D3 “
  16. D4 “
  17. D5 “
  18. D6 “
  19. D7 – data bus line 7
  20. /Phase 2 – (complement) processor clock 2
  21. R/W – Read / (complement) Write
  22. +8V – power line
  23. +8V – power line
  24. 1200 baud clock signal x 16
  25. 600 baud clock signal x 16
  26. 300 baud clock signal x 16
  27. 150 baud clock signal x 16
  28. 110 baud clock signal x 16
  29. /RESET – (complement) Reset or power-up
  30. /Board Select