Apple IIe #7
Interfaces -- Serial, Parallel, and IEEE-488

Revised by Matt Deatherage (November 1988)
Written by Peter Baum (April 1984)

This Technical Note describes the pin configurations of three difference interface types offered on the Apple II family of computers.


Currently, Apple sells a card, called the Super Serial Card (SSC), that can be used to connect an Apple printer to an Apple II. The SSC replaces both the Communications Card and the Hi-Speed Serial Card. The SSC supports the firmware (Pascal 1.1) protocol except for the optional control and interrupt handling routines.

The SSC has a 10-pin header on it, but comes with a cable which connects the header to a female DB-25 connector. The SSC can be configured as either a modem (DCE) or as a terminal (DTE) using a jumper block (in the latter case the jumper block acts as a modem eliminator). Though the pin configuration of the DB-25 connector is well defined, there is no standard use of the handshake signals. Different printers will use the handshake lines for different functions. Table 1 shows the pin configuration for the DB-25 on the SSC. Consult your printer manual for more specific information on which signals are used.

            10-pin                                     Female DB-25
            Header Signal Name                       Terminal  Modem
              1    Frame Ground             (FRMGND)    1        1
              2    Transmit Data            (TxD)       3        2
              3    Receive Data             (RxD)       2        3
              4    Request To Send          (RTS)       8        4
              5    Clear To Send            (CTS)       8        5
              6    Data Set Ready           (DSR)      20        6
              8    Signal Ground            (SGLGND)    7        7
             10    Data Carrier Detect      (DCD)       4,5     *8
              7    Secondary Clear to Send  (SCTS)     19     **19
              9    Data Terminal Ready      (DTR)       6       20
                   *    Only if SW1-7 is closed (on) with SSC.
                   **    Only if SW2-7 is closed (on) with SSC.

                Table 1 - Pin Configuration for SSC DB-25 Connector


Apple formerly shipped a parallel card, called the Parallel Interface Card (PIC), which can be used to connect a parallel printer to an Apple II. The PIC replaced the Parallel Printer Interface Card and the Centronics Interface Card. The PIC does not support the firmware protocol, so Pascal identifies the card as a printer card (described in Pascal protocols).

Most commonly used printers operate properly if the switches on the PIC are set as in Figure 2.

                        1   2   3   4   5   6   7
                   on |   |   |   | x | x |   |   |
                  off | x | x | x |   |   | x | x |

                 Figure 2 - PIC Switch Configuration

This setting prepares the parallel interface to transfer data using a 1 microsecond strobe pulse of negative polarity when sending data, while receiving a negative acknowledge signal, with interrupts disabled.

The PIC has a 26-pin header, but it comes with a cable which connects the header to a female DB-25. The Parallel Printer Card and the Centronics Card used a 20-pin header. Most parallel printers (90%) use a "microribbon 36" as the connector. The pin configuration varies from printer to printer, but Table 2 covers most printers (Apple DMP, Epson). For other printers, refer to page 7 of the Parallel Interface Card Manual.

     PIC         Printer
     Function    Function    26-Pin    DB-25    36-Pin    20-Pin
     Ground      Ground         3         2       19         1
     Ground      Ground        22        24       16        20
     Ground      Ground         7         4
     Ground      Ground        14        20
     ACK         Acknowledge    6        16       10         2
     Strobe      Strobe         4        15        1         8
     DO 0        Data 1         9         5        2        10
     DO 1        Data 2        11         6        3        11
     DO 2        Data 3        15         8        4        12
     DO 3        Data 4        18        22        5        13
     DO 4        Data 5        20        23        6        14
     DO 5        Data 6        21        11        7        15
     DO 6        Data 7        23        12        8        16
     DO 7        Data 8 *      25        13        9        17
     DI 3        Fault         24        25       32         6
     DI 4        Busy           2        14       11         7
     DI 5        Paper out     12        19       12         9
     DI 6        Select        16        21       13         8
     DI 7        Enable        10        18       35        19
                 **                       7
     *    This may be assigned a "hard" value for some printers to 
          distinguish between graphics and normal character sets.
    **    Pin 7 is blocked on the female DB-25 connector and omitted on 
          the mail DB-25 connector to prevent the insertion of serial 
          connectors into parallel ports.


The IEEE-488 bus standard is a well defined eight-bit parallel, byte serial, asynchronous data transfer interface. The standard has been thoroughly documented with the most complete description available from the Institute of Electrical and Electronic Engineers (IEEE) in New York. Standard cables are manufactured by many companies and usually advertised as either IEEE-488, General Purpose Interface Bus (GPIB), or Hewlett-Packard Interface Bus (HPIB) cables.

IEEE-488 cards do not support Apple firmware protocols, so an assembly language driver must be used to access the cards from high level languages (see Appendix F of the IEEE-488 Interface User's Guide).

Further Reference

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