The diagram below shows one of the simplest method to build an interface to hook an external modem to a C64. I guess you can find the IC 7404 easily. This schematics can handle 9600 bps in C128, but for a stock C64 it requires special tricks to exceed the limiting speed of 2400 bps.

The schematics originally belongs to Walter Wickersham.

user port                   RS232 DB25   DB9   description
---------                   ----------   ---   -----------

  A & N---------------+--------1 & 7      5      ground
               _____  I 
  B & C-------[2   7]-+      
              [     ]    
    M---------[3   1]----------3          2      rx
              [     ]
    2---------[14  4]----------2          3      tx
              [_____]

Part List

• 7404 IC (you can easily find it)
• DB25 male connector (or DB9 male)
• User Port connector
• 2m. 2 line shielded cable. (you don't need this if your modem has the cable)

Connector Views (looking at the front faces)

      User Port                  
                       
                  1 1 1
1 2 3 4 5 6 7 8 9 0 1 2
- - - - - - - - - - - -
- - - - - - - - - - - -
A B C D E F H J K L M N


             DB25                       DB9
                     1 1 1 1       
   1 2 3 4 5 6 7 8 9 0 1 2 3         1 2 3 4 5
\-----------------------------/   \-------------/
 \ . . . . . . . . . . . . . /     \ . . . . . /
  \ . . . . . . . . . . . . /       \ . . . . /
   -------------------------         ---------
    1 1 1 1 1 1 2 2 2 2 2 2           6 7 8 9
    4 5 6 7 8 9 0 1 2 3 4 5

İlker Fıçıcılar
01.06.1996

Keywords: Commodore, RS232, hardware