DEC VT220 Technical Manual page 142

7.4.4
Keyboard Communication
This section describe the keyboard communication.
7.4.4.1 Keyboard Transmit Mode -- The keyboard codes and a few
other special codes are transmitted via a serial line output in
PORT 3 of the 8051. The transmitted signal goes from the 8051 to a
driver, through the keyboard cable, monitor, and video cable to
the
cpu.
A UART within the 8051 controls the transmission.
Transmitted characters conform to a specific format. Each
character is 10 bits long. The first bit is the start bit. It is
always a logical zero (space). The next eight bits represent the
encoded data. The last bit is the stop bit. It is always a logical
one (mark). Figure 7-10 shows the character format.
7.4.4.2 Keyboard Receive Mode -- The firmware contains features
that can be enabled by commands from the
cpu.
There are two
categories of features: one sets keyboard transmission
characteristics and the other controls keyboard peripherals. A
peripheral command covers indicator control, bell and keyc1ick
loudness, keyboard ID code, and reinstate keyboard. The commands
corne from the CPU and pass through the video cable, monitor, and
keyboard cable to the receiver and into the 8051 via PORT 3. The
commands go to the UART in the 8051.
Received characters conform to the same 10-bit format used for
transmitted characters. The eight data bits are arranged in a
specified protocol, depending on the command type.
r
CHARACTER
°1
I I' "
8 DATA BITS
"' I
MARK (11----,
,-"'-T-
T - - r - , . - T - - r - , . , .
I
I
I
I
I
1 I I I I
SPACE \01
1-_.L.-
J.
_.L _..1. _.I. _.I.
_1.
_J
i
START STOP
BIT BIT
MA,026ti"*2
Figure 7-10 Keyboard Transmit and
Receive Character Format
7-16