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Turn on your WayBack machine. to about the dawn of time... that is, the creation of the Microprocessor.

Just after MITS Introduced its Altair, A company called SouthWest Technical products Company introduces a computer kit called, simply enough, the SWTPC (I am not sure if the PC in the product name meant "personal computer".) It features a 6800 (NOT 68000)processor and an SS-50 bus for periperhals.

SS-50 (SWTPC bus) pinout:

1
D0
Data Line 0
2
D1
Data Line 1
3
D2
Data Line 2
4
D3
Data line 3
5
D4
Data line 4
6
D5
Data line 5
7
D6
Data line 6
8
D7
Data line 7
9
A15
Address line 15
10
A14
Address line 14
11
A13
Address line 13
12
A12
Address line 12
13
A10
Address line 10*
14
A9
Address line 9
15
A8
Address line 8
16
A7
Address line 7
17
A6
Address line 6
18
A5
Address line 5
19
A4
Address line 4
20
A3
Address line 3
21
A2
Address line 2
22
A1
Address line 1
23
A0
Address line 0
24-27
GND
Ground
28-30
+8V
System Unregulated 8V power
31
-12V
-12 to -15 volts @ 1 Amp
32
+12V
+12 to +15 volts @ 1 Amp
33
INDEX
Board seating pin( no connection)
34
MRST
Manual Reset
35
NMI
Non Maskable Interrupt
36
IRQ
Interrupt ReQest(maskable)
37
UD2
User Defined
38
UD1
User Defined
39
02
Phase 2 system clock
40
VMA
Valid Memory Address flag
41
R/W
Read/Write cycle flag
42
RST
system ReSeT
43
BA
Acknowledge DMA request
44
01
Phase 1 System Clock
45
HALT
DMA request to CPU
46
110
110 Baud X 16 clock
47
150
150 Baud X 16 clock
48
300
300 Baud X 16 clock
49
600
600 Baud X 16 clock
50
1200
1200 Baud X 16 clock
*: Where is data line 11???

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