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The transistor circuit is intended to control signals of the keyboard and the magnetic card controller. Control is actually
undertaken by the magnetic card controller.
When the anodes of the diodes D_A and D_B are maintained at the high level, the transistors of Q_A~Q_D104 are turned ON
and the keyboard signals are led to the keyboard controller.
When the anodes are turned low, the transistors are turned OFF to separate the keyboard controller from the keyboard. The
keyboard is then connected to the magnetic card controller.
In the state of operation ready, both of the KBDDATA and KBDCK signals are maintained at the high level.
Regarding the serial data from the keyboard, one data lot is composed of 11 bits as shown below.
(1) Start bit ......................................................... 1 bit (fixed at low level)
(2) Key code ...................................................... 8 bits (LSB → MSB)
(3) Odd-numbered parity bit .............................. 1 bit
(4) Stop bit ........................................................ 1 bit (fixed at high level)
When the keyboard controller receives data from the keyboard, the clock line is once turned low as an ACK signal to the
keyboard
KB/DATA
(KBDDATA)
KB/CLK
(KBDCK
Start bit
KB/DATA
KB/CLK
Start bit
When Key "a" is pressed (Make ..... Scan code "1C")
LSB
Key code
1C
When Key "a" is released (Break ..... Scan code "F01C")
LSB
MBS
Key code
FC
2-24
MBS
Odd-numbered
Start bit
parity
Key code
1C
ACK
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