Tandy 1400LT Service Manual page 67

IV-13. External Keyboard
Communication Protocol
The External keyboard shall communicate with the system using synchronus serial protocol. When no
communication is in process, the keyboard holds the data line Low and the clock line High. Transmissions consist
of a 10 bit data word.
Keyboard RTS Start B O B1 B2 B3 B4 B5 B6 B7
0 1
* *
*
* * * *
*
The system may hold the clock line low for a minimum of 12.5msec to initiate a keyboard reset. The External
keyboard shall not attempt to tramsmit data while the clock line is being held by the system. Before initiating a
transmission, the External keyboard lowers the clock line as a RTS (Request To Send). The External Keyboard then
checks the state of the data line. If the system is holding the data line low, then the External keyboard interface is
inhibited. The External keyboard shall return the keycode in the buffer, return the clock and data lines to idle state
and resume scanning until the interface is enabled. If the interface is enabled, the External keyboard shall transmit
its data. Data is valid during the falling edge of the clock. Figure 4-21 shows the External keyboard interface circuit
and timing diagram.
24u 24u
KBD DATA
KED CLK
KCONi
GATE4
KP
i
KCON2
KCON2G
BRST
I
36u 154u
ioou
t
R65
47K
EXTERNAL KEYBOARD C O N N E C T O R
R66
p . &
4.7K
74LSi25
i
KBD
CLK
2
KBD DATA
3 -
74LSi25 R68
4.7K
R69
$
4c78p7
4.7K
1.2
6ND
*
Figure 4-21. Keyboard-5 (The External Keyboard Interface Circuit and Timing Diagram)
4-30