SS-50 bus

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 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. The extension boards have 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.

Line	Function
	SS-50	SS-50C	SS-50 S/09
1	D0	D0	D0
2	D1	D1	D1
3	D2	D2	D2
4	D3	D3	D3
5	D4	D4	D4
6	D5	D5	D5
7	D6	D6	D6
8	D7	D7	D7
9	A15	A15	A15
10	A14	A14	A14
11	A13	A13	A13
12	A12	A12	A12
13	A11	A11	A11
14	A10	A10	A10
15	A9	A9	A9
16	A8	A8	A8
17	A7	A7	A7
18	A6	A6	A6
19	A5	A5	A5
20	A4	A4	A4
21	A3	A3	A3
22	A2	A2	A2
23	A1	A1	A1
24	A0	A0	A0
25	GND	GND	GND
26	GND	GND	GND
27	GND	GND	GND
28	+8V	+8V	+8V
29	+8V	+8V	+8V
30	+8V	+8V	+8V
31	-12V	-12V	-12V
32	+12V	+12V	+12V
33	INDEX	INDEX	INDEX
34	M RESET	MRDY	MRDY
35	NMI	BUSY	NMI/BUSY
36	IRQ	IRQ	IRQ
37	UD2	FIRQ	FIRQ
38	UD1	Q	Q
39	φ2	E	E
40	VMA	VMA	VMA
41	R/W	R/W	R/W
42	Reset	Reset	Reset
43	BA	BA	BA
44	φ1	BS	BS
45	HALT	HALT	HALT
46	110b	110b/BREQ	BREQ
47	150b	150b/9600b	A19
48	300b	300b	A18
49	600b	600b/4800b	A17
50	1200b	1200b	A16

Notes:

• 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.

References

• http://www.oldcomputers.net/swtpc-s09.html