DEC VT220 - 7.5 Keyboard Programming

7.4.5
Reset Signal for 8051 Microprocessor
Whenever the system is turned on, the 8051 microprocessor in the
keyboard must be reset. This allows the 8051 to start operating.
The reset signal generator is active only during powerup. The
input is +5 V. The output is connected to the RESET input of the
8051. When power is turned on, the +5 voltage starts to rise from
zero. The reset signal circuit output follows it and drops off
when a steady state of +5 V is reached. This circuit holds the
8051 RESET input high (+3.5 V to +5 V) long enough to enable the
reset action in the 8051. This action occurs only during powerup.
7.4.6
Hardware Keyboard Identification (IO)
At powerup, the keyboard performs a self-test and sends the
results to the CPU. One piece of information to be sent is the
keyboard hardware 10, which is read from hardwired jumpers.
There are six jumpers. Each jumper line goes from an input in PORT
3 of the 8051 to ground. All jumpers are installed so the keyboard
hardware 10 is zero.
7.4.7 Voltage Supplies
The
-10
V.
only voltage sent to the keyboard is +12 V. However, +5 V and
V are also required. These voltages are derived from the +12
There is a +5 V supply that handles most of the requirements for
this voltage. The four keyboard LEOs have their own +5 V supply. A
-10 V supply provides voltage for the driver in the serial out
line.
7.S KEYBOARD PROGRAMMING
This section describes the functions that the keyboard performs
under system central processor control. This section also
describes keyboard programming machine language, but does not
describe high level user programming.
7.5.1
Keyboard Layout and Key Identification
Each keyboard key has a ~nique location. Each location is scanned,
and when closure or release is detected, the location is verified.
This information is then decoded to an 8-bit keycode. Figure 7-7
shows the keyswitch locations. Table 7-2 shows the 14 functional
divisions of the keyboard. Table 7-3 shows the divisions, keycaps,
and keycodes.
7-17