12.5.  How do I build a 2400/9600 bps RS-232 interface cable?

+ A new programming technique makes it possible to acheive 9600 bps on an
+ unexpected Commodore 64 without the use of a Swiftlink cartridge.
+ Daniel Dallmann of Denmark is the originator of this technique and is
+ currently writing modem drivers for some of the popular C64 terminal
  emulation programs to take advantage of this new technique.

+ The interface described below is a little more complex than the one
+ in Section 12.4, but has many advantages:

+ * The interface generates proper EIA232 (RS-232) voltage levels.
+   The one in Section 12.4 relies on the ability of most newer RS232
+   equipment to handle lower voltages.

+ * The following interface can be used as a direct replacement for the
+   interface in Section 12.4, and will work will all existing
+   telecommunications programs at up to 2400 bps, while providing the
+   necessary interfacing to allow suitably equipped applications to
+   operate at up to 9600 bps.

+ * if you currently own a normal RS232 interface, that interface can be
+   modified to incorporate the new interfacing for 9600 bps operation.
+   (in some cases one line of the old interface has to be disconnected,
+    see note **1 )

+ 9600 bps is the maximum limit this type of technique due to the timing
+ limitations of the C64.  The delay between the activation of the NMI-handler
+ and the falling edge of rxd is too long.  The handler must be called in
+ less than 100 cycles.  That's no problem at 9600 bps, but for 19200 baud
+ (the next higher baud rate), you have only 50 cycles, and that's impossoble
+ (because of VIC-DMA ).  On a C128 in fast mode 38400 baud may be possible,
+ but I don't have a C128, so someone else has to try it out.


+             Schematic of a very simple RS232-Interface:


+   userport                       MAX232         rs232   9pin  25pin
+                               +----------+
+                               !          !
+ (C)    pb0--+                 !          !
+             !                 !    /!    !
+ (B)  flag2--*--------------------O< !---------  rxd     2      3
+             !                9!    \!    !8
+ *7*    sp2--+                 !          !
+                               !          !
+ *6*   cnt2--+                 !          !
+             !                 !          !
+ *L*    pb7--+ <NOTE **1!>     !          !
+                               !  !\      !
+ (M)    pa2--*--------------------! >O---------  txd     3      2
+             !               11!  !/      !14
+ *5*    sp1--+                 !          !
+                 74ls00        !          !
+                 +-----+       !          !
+                 !     !       !  !\      !
+ (D)    pb1-*------>O-------------! >O---------  rts     7      4
+            !   1!     !3    10!  !/      !7
+            +----!     !       !          !
+                2!     !       !    /!    !
+ (K)    pb6--------O<-----*-------O< !---------  cts     8      5
+                6!     !4 !  11!    \!    !13
+                 !     !--+    !          !
+ (1)    gnd------!     !5      !          !---   gnd     5      7
+                7!     !       !          !15
+ (2)    +5V------!     !       !          !     (dsr     6      6)
+               14+-----+       !          !
+                               !          !     (dcd     1      8)
+                               !          !
+                               !          !     (dtr     4     20)
+                       ! !+    !          !
+ (1)    gnd------------! !-----!          !     (ri      9     22)
+                       ! !    2!          !
+                               !          !
+                      +! !     !          !
+ (1)    gnd------------! !-----!          !
+                       ! !    6!          !
+                               !          !
+                       ! !+    !          !
+                     +-! !-----!          !
+                     ! ! !    1!          !
+                     !         !          !
+                     +---------!          !
+                              3!          !
+                       ! !+    !          !
+                     +-! !-----!          !
+                     ! ! !    4!          !
+                     !         !          !
+                     +---------!          !
+                              5!          !
+                               !          !
+ (1)    gnd--------------------!          !
+                             15!          !
+                               !          !
+ (2)    +5V--------------------!          !
+                             16+----------+

+ (x) - connections required for a normal RS232-Interface.
+ *x* - additional connections to go up to 9600 baud.

+ remark: The MAX232 needs 4 capacitors (22uF each)
+         to generate +/- 10volts signals.

+ notes :

+    **1 Some other interfaces use this pin.  In that case you have to cut
+        the connection and change it to new way.  PB7 is normally used to
+        receive the DSR (Data Set Ready) signal from the modem, but this
+        signal isn't supported by all terminal programs.  Because of the new
+        wiring DSR will seem to be active to all 'old' programs, so that
+        there won't be any troubles.


+    Top view of the used ICs:

+         74 LS 00                    MAX232
+         +------+                   +------+
+    inA -!1 \/14!- vcc       +cap1 -!1 \/16!- Vcc
+    inA -!      !-  inC      +cap2 -!      !- GND
+   outA -!      !-  inC      -cap1 -!      !- RS232/txd
+    inB -!      !- outC      +cap3 -!      !- RS232/cts
+    inB -!      !-  inD      -cap3 -!      !- TTL/^cts
+   outB -!      !-  inD      -cap4 -!      !- TTL/txd
+    gnd -!7    8!- outD  RS232/rts -!      !- TTL/^rts
+         +------+        RS232/rxd -!8    9!- TTL/rxd
+                                    +------+
+         4 X NAND
+                                    2 X RS232 transmitter
+                                    2 X RS232 receiver

+       user port (view on C64s backside)

+      1  2  3  4  5  6  7  8  9 10 11 12   (top)
+     ------------------------------------
+      A  B  C  D  E  F  H  J  K  L  M  N   (bottom)

+         1 - GND       A - GND
+         2 - +5V       B - ^flag2
+         3 - ^reset    C - pb0
+         4 - cnt1      D - pb1
+         5 - sp1       E - pb2
+         6 - cnt2      F - pb3
+         7 - sp2       H - pb4
+         8 - ^pc2      J - pb5
+         9 - atn in    K - pb6
+         10- 9V AC     L - pb7
+         11- 9V AC     M - pa2
+         12- GND       N - GND