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+## Apple IIe Keyboard USB
+
+Arduino project to convert Apple IIe keyboard to USB using Teensy 2.0.
+
+This is a modified version of [RetroConnector's IIe-to-USB keyboard adapter code](https://github.com/option8/RetroConnector/blob/master/IIe-USB),
+changed to work with the Teensy 2.0 (atmega32u4) instead of the Teensy++. This
+modification also requires a parallel-in serial-out shift register such as as
+the 74HC165 or 74LS165.
+
+### Differences from the original code
+
+The Apple IIe keyboard requires 24 pins, but the Teensy 2.0 only has 23 pins
+available while it's in HID USB mode.
+
+This modified code works around this limitation by using 18 pins for the X/Y
+lines, and 4 pins to drive the shift register to read CONTROL, SHIFT, CAPS LOCK,
+OPEN/CLOSED APPLE and RESET.
+
+This leaves one pin free that we give to the built-in LED to show the Caps-Lock
+state.
+
+### Wiring
+
+#### Keyboard connector to Teensy 2.0
+
+Connections from the 26-pin connector on the Apple IIe keyboard to the pins on
+the Teensy 2.0 pins. "--" denotes connection to the shift register, covered
+below.
+
+IIe Connector   Col/Row   Arduino IDE Pin   Teensy 2.0 Pin
+1               Y0        0                 B0
+2               Y1	      1                 B1
+3               +5V       +5v               +5v
+4               Y2	      2                 B2
+5               SW1/CAPL  --                --
+6               Y3	      3                 B3
+7               SW0/OAPL  --                --
+8               Y4	      24                E6
+9               CAPLOCK   --                --
+10              Y5	      4                 B7
+11              CNTL      --                --
+12              Y8	      5                 D0
+13              GND       GND               GND
+14              X0	      6                 D1
+15              RESET     --                --
+16              X2        9                 C6
+17              X7        17                F6
+18              X1	      10                C7
+19              X5        16                F7
+20              X3        23                D5
+21              X4        15                B6
+22              Y9        22                D4
+23              Y6        14                B5
+24              SHIFT     --                --
+25              Y7        13                B4
+26              X6        12                D7
+
+Wiring note: With the keyboard sitting in front of you as though you were typing
+on it, pins 25/26 will be on the end of the connector closest to you, and pins
+1/2 will be on the end of the connector farthest from you:
+
++----------------+
+|                |  +-------+
+|                |  |  1/2  |
+|    KEYBOARD    |  |   |   |
+|                |  | 25/26 |
+|                |  +-------+
++----------------+  
+
+
+#### Modifier keys to shift register
+
+Follows DIP-8 package: http://www.ti.com/lit/ds/symlink/sn74ls165a.pdf
+
+IIe Connector   Col/Row   Shift Register Input  Notes
+5               SW1/CAPL  B (pin 12)            Active High
+7               SW0/OAPL  A (pin 11)            Active High
+9               CAPLOCK   E (pin 3)             Active Low, needs pull-up
+11              CNTL      H (pin 6)             Active Low, needs pull-up
+15              RESET     G (pin 5)             Active Low, needs pull-up, active low (only active with Control)
+24              SHIFT     F (pin 4)             Active Low, needs pull-up
+
+Shift Register Pins-to-Keys:
+0: open-apple, pull-down, active high
+1: closed-apple, pull-down, active high
+2: unused
+3: unused
+4: Caps-Lock, pull-up, active low
+5: Shift, pull-up, active low
+6: Reset, pull-up, active low (only active with Control)
+7: Control, pull-up, active low
+
+#### Shift register to Teensy 2.0
+
+Teensy 2.0 Pin    Shift Register Pin
+18/F5             1:  Parallel Load / SH/LD   (active low)
+19/F4             15: Clock Enable / CLK INH  (active low)
+20/F1             2:  Clock Pulse / CLK       (low-to-high edge triggered)
+21/F0             9:  Data Pin (Q7)
++5v               16: VCC
+GND               8:  GND
+
+Follows DIP-8 package: http://www.ti.com/lit/ds/symlink/sn74ls165a.pdf
+
+#### Other Pins
+
+Leave these unconnected.
+
+Teensy 2.0  Purpose
+7/D2        USB RX
+8/D3        USB TX
+11/D6       Caps Lock LED (LED_BUILTIN)
+
+### Changes
+
+Jan 24, 2017: Initial Commit
+
+### Credits
+
+This modification is by Matthew Nielsen, and can be found at:
+https://github.com/xunker/apple_iie_keyboard_usb
+
+This is based on the original code by RetroConnector:
+https://github.com/option8/RetroConnector/blob/master/IIe-USB
+
+Shift Register code is pulled from the Arduino Playground:
+http://playground.arduino.cc/Code/ShiftRegSN74HC165N
+
+It uses the Keypad library, by Mark Stanley and Alexander Brevig:
+http://www.arduino.cc/playground/Code/Keypad
+
+Keyboard pinout information is from Apple.info:
+http://apple2.info/wiki/index.php?title=Pinouts#Apple_.2F.2Fe_Motherboard_keyboard_connector