4704 Keyboard
=============
Keyboard Models:
    Model 100 6019273 50-key (grid layout)
    Model 200 6019284 62-key Alpha(60% layout)
    Model 300 6019303 77-key Expanded Alpha
    Model 400 6020218 107-key Full key

Resourse
--------
The IBM 4704: lots of pictures and info
http://kishy.dyndns.org/?p=648#more-648 

Brochure:
http://ed-thelen.org/comp-hist/IBM-ProdAnn/4700.pdf




4704 Keyboard Protocol
======================
On powering up the keyboard sends keyboard id; A3h for 6019284(62-key), for example.
After that firmware enters FC command mode and waits for parameter data from host
so that it doesn't send any scancode until you send 'FF'(End of FC command).


Keyboard to Host
----------------
Data bits are LSB first and Pairty is odd. Clock has around 60us high and 30us low part.

            ____      __   __   __   __   __   __   __   __   __   ________
    Clock       \____/  \_/  \_/  \_/  \_/  \_/  \_/  \_/  \_/  \_/
                 ____ ____ ____ ____ ____ ____ ____ ____ ____ ____    
    Data    ____/    X____X____X____X____X____X____X____X____X____X________
                Start   0    1    2    3    4    5    6    7    P  Stop

Start bit:  can be long as 300-350us.
Inhibit:    Pull Data line down to inhibit keyboard to send.
Timing:     Host reads bit while Clock is hi.
Stop bit:   Keyboard pulls down Data line to lo after 9th clock.



Host to Keyboard
----------------
Data bits are LSB first and Pairty is odd. Clock has around 60us high and 30us low part.

            ____        __   __   __   __   __   __   __   __   __   ________
    Clock       \______/  \_/  \_/  \_/  \_/  \_/  \_/  \_/  \_/  \_/
                ^   ____ ____ ____ ____ ____ ____ ____ ____ ____ ____ ___
    Data    ____|__/    X____X____X____X____X____X____X____X____X____X   \___
                |  Start   0    1    2    3    4    5    6    7    P   Stop
                Request by host

Start bit:  can be long as 300-350us.
Request:    Host pulls Clock line down to request to send a command.
Timing:     After Request keyboard pull up Data and down Clock line to low for start bit.
            After request host release Clock line once Data line becomes hi.
            Host wirtes a bit while Clock is hi and Keyboard reads while low.
Stop bit:   Host releases or pulls up Data line to hi after 9th clock and waits for keybaord pull down the line to lo.



Scancodes
---------
Keyboard doesn't send Break code for all keys except for Alt by default.

    6019284 62-key:
    ,-----------------------------------------------------------.
    |  `|  1|  2|  3|  4|  5|  6|  7|  8|  9|  0|  -|  =|???|BS |
    |-----------------------------------------------------------|
    |Tab  |  Q|  W|  E|  R|  T|  Y|  U|  I|  O|  P| ￠|  \|  PD2|
    |-----------------------------------------------------------|
    |Ctrl  |  A|  S|  D|  F|  G|  H|  J|  K|  L|  ;|  '| {}| PD3|
    |-----------------------------------------------------------|
    |Shif| <>|  Z|  X|  C|  V|  B|  N|  M|  ,|  ,|  /|???|Shift |
    |-----------------------------------------------------------|
    |Reset|blk|Alt  |         Space             |Alt  |blk|Enter|
    `-----------------------------------------------------------'
    +----------+---------------------+----------+----------+
    |`       00|PD1     04|Caps    20|LShift  30|Reset   31|
    |1       18|q       05|a       21|<>      3E|Rblank  41|
    |2       19|w       06|s       22|z       32|Alt     3F|
    |3       1A|e       13|d       23|x       33|Space   40|
    |4       10|r       14|f       24|c       34|Alt     3F|
    |5       11|t       15|g       25|v       35|Lblank  42|
    |6       12|y       16|h       26|b       36|Enter   2F|
    |7       08|u       17|j       27|n       37|          |
    |8       09|i       01|k       28|m       38|          |
    |9       0A|o       02|l       29|,       39|          |
    |0       0F|p       03|;       2A|.       3A|          |
    |-       1F|￠      1B|'       2B|/       3B|          |
    |=       0D|\       1C|{}      2C|???     3C|          |
    |???     0C|PD2     1D|PD3     2D|RShift  3D|          |
    |BS      0E|          |          |          |          |
    +----------+---------------------+----------+----------+
    Bit7 is 'press flag' which set 1 on press and 0 on release when break code is enabled.

NOTE: When break code is enabled the key sends scancode with setting 7th bit on press
and without it on release. That is, '`' sends 80h on press and 00h on release.


keyboard command
----------------
    FF  Soft Reset(0008h)
    FE  Resend(00e8h)
    FD  Buzzer stop?(00edh)
    FC  Set Key flag(00f6h)
    FB  Soft Reset(0008h)
    FA  Reset(0000h)
    

Keyboard response
-----------------
    FF  Not exist. [Outgoing buffer cannot have FFh(00h in fact)]
    FE  Overflow(key event/receive data)        at 00c5h, 0346h
    FE  Memory test error                       at 0224h
    FD  Command out of bound                    at 00d8h
        Key out of bound
    7E  Read/Parity error in receive from host  at 00bch


Set Key flag command(FC)
------------------------
After 'Power on Reset' firmware enters this command mode and waits for data from host,
so that you don't need to send 'FC' and it doesn't send any scancode until you send 'FF'.

Data sent from host:

    bit: 7   6 ... 0
         en  |     |
         |   `-----`--- scan code
         `------------- enable bit(0: enable repeat, 1: enable break)

    00-77   Enable repeat(78-7F: invalid scancode)
    80-F7   Enable break(F8-FF: invalid scancode)
    FE      Resend(011ah) no need to use
    FF      End(0114h) exits FC command mode.

Response from keyboard:
    FD      Out of bound - Invalid scancode

Examples:
    To enable break code of all keys.

    FC 80 81 ... F7 FF