Experimental 8x8 Encoded Matrix Keyboard
             ========================================
     Layout modified making the cursor keys easier to process

                     D0  D1  D2  D3  D4  D5  D6  D7
                     |   |   |   |   |   |   |   | 
                   +-+---+---+---+---+---+---+---+-+
IORQ--+---\        |                               |
 RD---+ OR >-+-----+EN         74LS245             |
PORT--+---/  |     |                               |
             |     +-+---+---+---+---+---+---+---+-+
             |       |   |   |   |   |   |   |   | 
          +--+--+    |   |   |   |   |   |   |   |     Part-processed keypress  
 A8 -->---+ EN  +-- SHF CTL                 LCK CPY    00 01          05 06 07
 A9 -->---+  7  +-- LFT RGT DWN UP  TAB RET ESC SPC    08 09 0A 0B 0C 0D 0E 0F
A10 -->---+  4  +--  0   1   2   3   4   5   6   7     30 31 32 33 34 35 36 37
A11 -->---+  L  +--  8   9   :   ;   .   -   ,   /     38 39 3A 3B 3C 3D 3E 3F
A12 -->---+  S  +--  @   A   B   C   D   E   F   G     40 41 42 43 44 45 46 47
A13 -->---+  2  +--  H   I   J   K   L   M   N   O     48 49 4A 4B 4C 4D 4E 4F
A14 -->---+  4  +--  P   Q   R   S   T   U   V   W     50 51 52 53 54 55 56 57
A15 -->---+  5  +--  X   Y   Z   [   \   ]   ^  DEL    58 59 5A 5B 5C 5D 5E 5F
          +-----+

This gives raw keypress:
        C7-C0    D7-D0
+--+--+--+--+--+--+--+--+
|  |  |  :  :  |  :  :  |  -> add &20 ASCII code &20-&5F requiring
+--+--+--+--+--+--+--+--+               some processing, eg:


; Typical keyscan code, positive logic
; ------------------------------------
LD BC,&80FD                        ; keyboard port &FD, row 7 selected
.scanLoop1
IN A,(BC):AND A:JR NZ,scanPressed  ; a key is pressed
ROR B:JR NC,scanLoop1              ; select next row, loop until '1' falls out
RET                                ; Carry set, no key pressed

; Typical keyscan code, negative logic
; ------------------------------------
LD BC,&7FFD                        ; keyboard port &FD, row 7 selected
.scanLoop1
IN A,(BC):CPL:JR NZ,scanPressed    ; a key is pressed
ROR B:JR C,scanLoop1               ; select next row, loop until '0' falls out
RET                                ; Carry clear, no key pressed

; Key pressed, convert to key number
; ----------------------------------
.scanPressed
LD C,A:LD A,&F7                    ; start at -9
.scanLoop2
INC A:LSR C:JR NC,scanLoop2        ; loop until a bit falls out of column byte
;                                  ; A is now &F8...&FF
.scanLoop3
ADD A,8:LSR B:JR NC,scanLoop3      ; loop until a bit falls out row select
;                                  ; A is now &00-&3F
LD BC,&01FD:IN B,(BC)              ; read modifier keys if positive logic
LD BC,&FEFD:IN B,(BC)              ; read modifier keys if negative logic
;
; A =raw keycode &00-&3F
; B =modifiers, b0=SHIFT, b1=CONTROL
;


             Experimental Spectrum keyboard as built
             =======================================

              Addressed with 16-port multiplexor:
      D0  D1  D2  D3  D4  D5  D6  D7         Raw keypress
       |   |   |   |   |   |   |   | 
 C0 - SHF CTL     TAB LFT RGT DWN UP     00 01    03 04 05 06 07
 C1 -             LCK CPY RET ESC SPC             0B 0C 0D 0E 0F
 C6 -  0   1   2   3   4   5   6   7     30 31 32 33 34 35 36 37
 C7 -  8   9   :   ;   .   -   ,   /     38 39 3A 3B 3C 3D 3E 3F
 C8 -  @   A   B   C   D   E   F   G     40 41 42 43 44 45 46 47
 C9 -  H   I   J   K   L   M   N   O     48 49 4A 4B 4C 4D 4E 4F
C10 -  P   Q   R   S   T   U   V   W     50 51 52 53 54 55 56 57
C11 -  X   Y   Z   [   \   ]   ^  DEL    58 59 4A 5B 5C 5D 5E 5F

Raw keypress:
      C15-C0     D7-D0
+--+--+--+--+--+--+--+--+
|  |  :  :  :  |  :  :  |  -> gives ASCII code &00-&0F and &30-&5F
+--+--+--+--+--+--+--+--+            for minor processing, eg: