- Terence J. Boldt's Apple II Site

Hardware Projects
Software Projects
Info and Pictures
  Apple TIL
    Apple IIc
    Apple IIe
      DMA Protocol
      Double Hi-Res
      RDY Line
      /INH Line
      Paddle Circuits
      Enhanced ROMs
      Switch Inputs
      LC Card
    Apple IIgs
    Memory Card
    UniDisk 3.5
Web Ring


Apple II
Technical Notes
                                                  Developer Technical Support

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 

            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) 

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
    o    Apple IIe Technical Reference Manual
    o    Parallel Interface Card Manual
    o    IEEE-488 Card Manual

All content Copyright ©1984-2003, Terence J. Boldt, unless noted otherwise.