9:28 AM 12/29/98
Paul T. Barton
CoCo Parallel Port for OS9
==========================

CoCo-Bus
============                68B21                        DB25-female
 1 - -12v                   PIA                          PC type
 2 - +12v                                                Printer Port
 3 - /halt
 4 - /nmi              ====================              ============
 5 - /reset ---------- 34   /RESET      PA0 ------------  1 /strobe
 6 - E --------------- 25   E           PA1 ------------ 13 slctin
 7 - Q                                  PA2 ------------ 16 /init
 8 - /cart ----------- 37   /IRQB       PA3 ------------ 14 /auto
 9 - +5vdc ----------- 20   VCC         PA4 ------------ 12 pe
10 - d0 -------------- 33   D0          PA5 ------------ 15 /error
11 - d1 -------------- 32   D1          PA6 -
12 - d2 -------------- 31   D2          PA7 ------------ 11 busy
13 - d3 -------------- 30   D3          PB0 ------------  2 data0
14 - d4 -------------- 29   D4          PB1 ------------  3 data1
15 - d5 -------------- 28   D5          PB2 ------------  4 data2
16 - d6 -------------- 27   D6          PB3 ------------  5 data3
17 - d7 -------------- 26   D7          PB4 ------------  6 data4
18 - rw -------------- 21   RW          PB5 ------------  7 data5
19 - a0 -------------- 35   RS1 Yes!    PB6 ------------  8 data6
20 - a1 -------------- 36   RS0 Yes!    PB7 ------------  9 data7
21 - a2  /------------  1   GND         CA1
22 - a3  !          /- 24   CS1         CA2 ------------ 17 slct
23 - a4  !  +5vdc -*-- 25   CS0         CB1 ------------ 10 /ack
24 - a5  !        /--- 23   /CS2        CB2
25 - a6  !        !    ====================              18 to 25 GND
26 - a7  \ -\     !                                      ============
27 - a8     !     !
28 - a9     !     !
29 - a10    !     !
30 - a11    !     *-----------------------------------------------\
31 - a12    !     !                                               !
32 - /cts   !     !                                               !
33 - gnd ---*     !                                               !
34 - gnd --/      !                                               !
35 - sound        !                                               !
36 - /scs         !                                               !
37 - a13          !                                               !
38 - a14          !                                               !
39 - a15          !                          VCC - 16             !
40 - /slenb ------/                            E - 1              !
                                              a2 - 2              !
---------------------       a4 - 1            a3 - 3              !
A0  A1     Ports            a5 - 2 74F27 12 ------ 4              !
---------------------       a7 - 13  3nor     a6 - 5              !
0   0      A Data          VCC - 14           a8 - 6              !
0   1      B Data          GND - 8            a9 - 7  74F133   9 -/
1   0      A Control                         a10 - 10  13nand
1   1      B Control                         a11 - 11
---------------------                        a12 - 12
                                             a13 - 13
                                             a14 - 14
                                             a15 - 15
                                             GND - 8


A15 - A8, A7 A6 A5 A4   A3 A2
   FF     0  1  0  0    1  1 = $FF4C - $FF4F


Yes, RS0 and RS1 are tied backwards.
This preferred when 16-bits are being transfered to/from external devices.
Decoding is at $FF4C to $FF4F. This can be changed by using the other two gates
in the 74F27 NOR package. Package B: 3,4,5 in, 6 out; Package C: 9,10,11 in, 8 out.
The signal /SLENB is needed to disable the floppy disk circuit.

To make a Dummy-Plug, use a DB25-male and tie pins
1 to 10, 15 to 17, 11 to 23, 12 to 25.
Using the Dummy-Plug allows software to be tested without a printer attached.
