Apple II
Technical Notes
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  Edited 2017 (for personal use) by: James Davis, Beagle Buddy #227.
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                                           Developer Technical Support
Apple IIe
#9:    Switch Input Changes

Revised by:    Glenn A. Baxter                           November 1988
Written by:    Earl Edwards                                   May 1988

This Technical Note describes three changes which have been made to 
the switch circuitry of Apple IIe revision C and later motherboards.

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The latest Apple IIe logic board has some changes in its switch 
circuitry.  Logic boards with part numbers 820-0087-C and later differ 
from earlier boards as follows:

  o  SW2 has been connected to the Shift keys on the keyboard by
     closing the X6 jumper.
  o  12k ohm pullup resistors have been added to SW0 and SW1.
  o  A 0.1 microfarad capacitor to ground has been added to all
     three switch inputs:  SW0 (PB0, Open-Apple, OAPL), SW1 (PB1,
     Option, Closed-Apple, CAPL), and SW2 (PB2).

Note: Differences are illustrated in Figures 1 and 2.  Schematics 
showing the differences are available in Chapter 7 of the Apple IIe 
Technical Reference, First Printing, January 1987.

The X6 jumper was closed to allow the Shift key to be read directly, 
facilitating the shift-click mouse selection feature in software 
products.  Note that this change connects SW2 to +5V through a 1k ohm 
resistor, and when a shift key is depressed, SW2 is at ground 
potential.  The 12k ohm resistors were added to ensure that the self-
diagnostic test would run when the keyboard is disconnected.  The 
resistors have negligible influence when the keyboard is connected.

The capacitors were added to reduce radiated emissions.  This 
reduction was required because of changes in the memory configuration.  
As a result of the addition, the functional bandwidth of the inputs 
has been reduced; however, the input requirements of the 74LS251 have 
not changed.  This addition may cause improper operation with 
peripheral devices that rely on high push button repetition rates.

The minimum V(IH) to the 74LS251 remains 2.0V, but for improved noise 
margin, a minimum V(IH) of 2.4V is recommended.  This requires a drive 
of about 6 ma to overcome the 470-ohm 5 percent resistor on SW0 and 
SW1.

The maximum V(IL) is 0.8V, and here again you should allow for some 
noise margin.  The low level is ensured by the 470-ohm keyboard 
pulldown resistor alone, but additional current sink will speed up the 
transition time.

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                                      +5v O---+---+
                                              |   |
                                             || ||
                                             |1| |1|
                                         R48 |2| |2| R47
    + -- -- -- +                             |K| |K|
    | J8 (DB9) |                             |_| |_|
    |          |      SW0/OAPL                |   |
    | 7 [O]--------+--------------------------|---+
    |          |   |  SW1/CAPL/OPTION         |   |
    | 1 [O]--------|-------+------------------+   |   + --- --- +
    |          |   |  SW2  |                  |   |   |         |
    | 6 [O]--------|-------|-------+------+   |   +---O         |
    |          |   |       |       |      |   |   |   |         |
    + -- -- -- +   = C96   = C95   = C94  |   +---|---O 74LS251 |
    + -- -- -- +   |.1fd  |.1fd  |.1fd |   |   |   |         |
    | J15      |   +-------+-------+      +---|---|---O         |
    | Game I/O |                   | GND  |   |   |   |         |
    |          |  SW0/OAPL        V     |   |   |   + --- --- +
    | 2 [O]-------------------------------|---|---+
    |          |  SW1/CAPL/OPTION         |   |   |
    | 3 [O]-------------------------------|---+   |   +5v
    |          |  SW2                     |   |   |   O
    | 4 [O]-------------------------------+   |   |   |
    |          |                          |   |   |  ||
    + -- -- -- +                         \|/  |   |  |1|
    + -- -- -- -- -- -- -- +  + --- +  X6 +   |   |  |K|
    |        SHIFT         |  | J17 |    /|\  |   |  |_|
    |        __|__         |  |     |     |   |   |   |
    |  +-----O   O--------------[O]-------+---|---|---+--------->
    |  |                   |  |     |         |   |   to keyboard
    | V  CAPL/OPTION     |  |     |         |   |      decoder
    | GND    __|__         |  |     |         |   |
    |     +--O   O------+-------[O]-----------+   |
    | +5v |             |  |  |     |             |
    | O---+  OAPL       |  |  |     |             |
    |     |  __|__      |  |  |     |             |
    |     +--O   O--+---|-------[O]---------------+
    |               |   |  |  |     |
    |              || || |  + --- +
    |              |4| |4| |
    |              |7| |7| |
    |              |0| |0| |   Textual Drawing:
    |              |_| |_| |   James P. Davis, April 9, 2017
    |               |   |  |
    |               +---+  |   CIRCUITRY:
    | Apple IIe     | GND  |   COPYRIGHT 1987 APPLE COMPUTER INC.
    | Keyboard     V     |
    + -- -- -- -- -- -- -- +

Figure 1: Circuit for SW0 SW1, and SW2 (aka PB0, PB1, and PB2) on
          Extended Keyboard Apple IIe's

======================================================================

    + -- -- -- +
    | J8 (DB9) |
    |          |  SW0/OAPL    
    | 7 [O]---------------------------------------+
    |          |  SW1/CAPL/OPTION                 |
    | 1 [O]-----------------------------------+   |   + --- --- +
    |          |  SW2                         |   |   |         |
    | 6 [O]-------------------------------+   |   +---O         |
    |          |                          |   |   |   |         |
    + -- -- -- +                          |   +---|---O 74LS251 |
    + -- -- -- +                          |   |   |   |         |
    |   J15    |                          +---|---|---O         |
    | Game I/O |                          |   |   |   |         |
    |          |  SW0/OAPL                |   |   |   + --- --- +
    | 2 [O]-------------------------------|---|---+
    |          |  SW1/CAPL/OPTION         |   |   |
    | 3 [O]-------------------------------|---+   |   +5v
    |          |  SW2                     |   |   |   O
    | 4 [O]-------------------------------+   |   |   |
    |          |                          |   |   |  ||
    + -- -- -- +                         _|_  |   |  |1|
    + -- -- -- -- -- -- -- +  + --- +  X6     |   |  |K|
    |        SHIFT         |  | J17 |    |  |   |  |_|
    |        __|__         |  |     |     |   |   |   |
    |  +-----O   O--------------[O]-------+---|---|---+--------->
    |  |                   |  |     |         |   |   to keyboard
    | V  CAPL/OPTION     |  |     |         |   |      decoder
    | GND    __|__         |  |     |         |   |
    |     +--O   O------+-------[O]-----------+   |
    | +5v |             |  |  |     |             |
    | O---+  OAPL       |  |  |     |             |
    |     |  __|__      |  |  |     |             |
    |     +--O   O--+---|-------[O]---------------+
    |               |   |  |  |     |
    |              || || |  + --- +
    |              |4| |4| |
    |              |7| |7| |
    |              |0| |0| |   Textual Drawing:
    |              |_| |_| |   James P. Davis, April 9, 2017
    |               |   |  |
    |               +---+  |   CIRCUITRY:
    | Apple IIe     | GND  |   COPYRIGHT 1987 APPLE COMPUTER INC.
    | Keyboard     V     |
    + -- -- -- -- -- -- -- +

Figure 2: Circuit for SW0, SW1, and SW2 (aka PB0, PB1, and PB2) on
          Original and Enhanced Apple IIe's (Logic Boards: 820-0064-B
          and 820-0087-A) 

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Further Reference
  o  Apple IIe Technical Reference Manual

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Circuitry for the Apple IIc, IIc+, and IIgs is radically different 
from earlier models, but their Pushbutton/Switch functionality is the 
same as in the Extended Keyboard Apple IIe circuitry.

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