DEC VT220 - Page 155

If the keyboard finds a key pressed on the first scan, it
continues to look for an ALL UPS condition. The keyboard sends the
corrected four-byte power-up sequence when the pressed key is
released. This avoids a fatal error condition if a key is pressed
by mistake while powering up.
The keyboard LEOs are lit during the power-up self-test. If the
self-test passes, the keyboard turns the LEOs off. If a bell is
selected on powerup, the system module can transmit a sound bell
command to the keyboard. However, this should not be done until
the system module receives the last byte of the four-byte
sequence. The request for self-test tests the serial line and
system module connection. The power-up self-test takes 70 ms or
less.
The system module can request a jump to powerup at any time. This
causes the LEOs on the keyboard to blink on and off (for the
power-up self-test) •
7.5.5
System Module to Keyboard Protocol
The
the
7-11
from
system module controls both the peripherals associated with
keyboard and the keyboard transmit characteristics. Figure
shows the protocol for command and parameter transmission
the system module to the keyboard.
COMMAND
07 06 05 04 03 02 01
o REPRESENTATION
PARAMETERS
~ DATA
~ DATA
..... ·O'1.e~
Figure 7-11 System Module
to Keyboard Protocol
7-29