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Apple II
Technical Notes
_____________________________________________________________________________
                                                  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.
_____________________________________________________________________________

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:  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.


Further Reference
    o    Apple IIe Technical Reference Manual

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