3. THE KEYBOARD MATRIX
As explained earlier, the 45 keys of the VZ-200 keyboard are
arranged in an 8 x 6 matrix scanned by the eight least
significant address lines. This means that each row of the
keyboard matrix effectively occupies a specific memory location
(or more accurately, a series of memory locations, due to the
simplified decoding). The individual keys are mapped into the
corresponding bits of that memory location, according to the
column they occupy in the matrix.
The keyboard is scanned by the software taking each of the eight
address lines down to logic low level in turn. If a key is
pressed, it effectively "pulls down" the bit line at the address
for its row. For example if the "2" key is pressed, this causes
bit 1 to drop to
0
at address 68F7 (also 69F7, 6AF7, 6BF7, 6CF7,
6DF7, 6EF7 and 6FF7). Providing no other keys are pressed in
that row, the data retrieved by reading that address will be 3D
hex (binary 111101).
The keyboard matrix and its (lowest) row addresses in hex are
shown below. Note that each key causes a logic
0
to appear at
the bit position shown, when its row address is read.
5
4
3
2
1
0
BIT POSITION
ROW ADDRESS
68FE
68FD
68FB
68F7
68EF
68DF
68BF
687F
R Q E
W T
F A D CTRL S G
✓
Z
C SHFT X B
4
1
3
2
5
M SPC ,
. N
7
0
8
-
9
6
U
P I RETN 0 Y
J
;
K
:
L
H
4. I/O MAPPING:
The Z-80A microprocessor used in the VZ-200 can address 256
ports in I/O space. Tentatively the following I/O address ranges
have been set aside for expansion peripherals:
I/O ADDRESS (hex)
DEVICE
00 - OF
Printer
10 - 1F
Floppy disk controller
20 -
2F
Joystick interface
30 - 3F
Communications MODEM
70 - 7F
Memory bank switch
- 6 -
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