The keyboard circuits generate an 8-bit character code, whenever a key on the key-board is depressed, and supply this code ready for writing into the keykey-board input character register in the RAM. The keyboard circuits are physically located in two places. The keyboard encoder, an oscillator that generates the character strobe pulses, and supporting logic are all located within the keyboard enclosure. A key-board input register, character strobe synchronization circuitry, and automatic character repeat circuitry are all located in the display enclosure, on the video/
RAM card. A cable between the keyboard enclosure and the video/RAM card interconnects the two physical locations. Figure 4-15 illustrates all keyboard cir-cuits and Table 4-2 lists all codes output by the keyboard.
Whenever a key is depressed, the closed key switch contacts are sensed by the key-board encoder. The keykey-board encoder continuously scans all key switch locations and when it senses a closed contact, responds by outputting a 10-bit code, related to that particular key location. The scanning rate is determined by the oscillator frequency and the code output is synchronous with a single pulse character strobe, also derived from the oscillator signal. In addition to the 10-bit code and the char-acter strobe pulse, the keyboard circuits also output a key down logic level signal, which is used by the keyboard interface circuit as described below. The RESET key on the keyboard does not generate a code, it simply acts as a ground return switch on the reset signal line.
The 10-bit code output by the keyboard encoder consists of an 8-bit word supplied to the RAM, and two control bits used by the keyboard and keyboard interface circuits. The 8-bit word includes a 7-bit ASCII code and one additional bit, as explained in paragraph 4-10. The ninth bit is a logic 0 for all letters of the alphabet and is used to gate the input from the ALL CAPS key on the keyboard. The tenth bit is asserted with all codes that represent a repeatable key. This bit is used in the keyboard interface circuit to perform the repeated writing of a code into the RAM, as described below. Bits 6, 7, and 8 are always routed through the control and caps logic and are modified if either the CTRL key or the ALL CAPS key is depressed. If the CTRL key is depressed, bits 6, 7, and 8 are forced to 00 I in all codes in which bit 7 is output as a logic I level from the keyboard encoder. If the ALL CAPS key is depressed, bit 6 is forced to a logic 0 in all codes that represent letters of the alphabet (those in which bit 9 is a logic 0).
On the video/RAM card the character code lines are connected directly to the key-board input register, which is loaded asynchronously with the character strobe.
The character strobe is also delayed and stored in a flip-flop, whose output becomes one input to an AND gate. The other AND gate input is a signal called write key-board gate. This signal comes from the video signal generating circuitry and deter-mines when during any given video field the keyboard character code is written into the RAM. (As described in paragraph 4-27, the writing occurs as a part of the video refresh cycle, once every video scan field.)
OSCILLATOR KEYBOARD • DISPLAY CONSOLE
Figure 4-15. Block Diagram of Keyboard and Keyboard Interface Circuits
4-34
I ~k~s I
RESET IAUTO BACK BACK TAB TAB AUTO TAB
ESC
CTRL
TAB
I START I ENTER I
~~~~R
I EOS EOL IDELETEISHIFT
1 12 13
@
"
PAGE
t
PAGE
~
~
INSERT REPLACE
MODE
t
HOME
+
4 5
SCROL
t 7 8 9
SCROL
~
4 5 6
~
1 2 3
FORM
EDIT
(Z) •
Bit No.
SOFTWARE TREATS CTRL KEYS (COLUMN 8 & 9)
4 5 6 7
Table 4-3, Character Generator Chart
21 765 000 001 010 all 100 101 110 111
0000 0
-
0 @ P 1 P0001 .1
r
! 1 A Q a q0010 T
J
" 2 B R b r0011 ~ ~ # 3 C S c s
0100 w .. $ 4 0 T d t
0101 0 0 % 5 E U e u
0110 {; - & 6 F V f v
0111 p a , 7 G W 9 w
1000
..-
~ ( 8 H X h X1001 t::. "V ) 9 I Y i y
1010
t • *
J Z j z1011
L
A + , K [ k{
1100 C. , < L \ I I I
r---.
}
1101 f X - = M 1 m
1110
f n •
> N A n-1111 C :J I ? 0 - 0
•
Whenever the write keyboard gate signal occurs, the RAM data input selector routes the keyboard character code to the RAM input lines, and a write command is also supplied to the RAM. The write address is supplied from the RAM address counter in the video signal generating circuitry.
Another feature that is a part of the keyboard circuitry is the repeated writing of a character code into the RAM. This occurs when a key is held depressed for more than one half of a second, except if the key, by the nature of its usage, is specifically excluded. Whether or not a key is incl,uded as a part of the repeating set, is deter-mined by the code it generates.
Whenever a key is held depressed, the key down signal triggers a timer. After 0.5 second the timer outputs a pulse to an AND gate. The other input to the same AND gate is bit 10, which determines whether the code stored in the keyboard input register is a repeatable one. If it is, a true input is provided to the AND gate and its output sets the character strobe flip-flop. Thereafter, the next occurrence of the write keyboard gate signal causes the character code to be written into the RAM. If the operator holds the key depressed, the write cycle will be repeated once every video scan field, when the write keyboard gate signal is true.