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System timing is established by a crystal connected to the microprocessor. The microprocessor divides the
crystal operating frequency (running at 6.144 MHz) in half and produces the 3 MHz CLK OUT signal.
CLK OUT runs to other functional areas and produces printing and communications timing. The micro-
processor runs continuously, fetching and executing instructions from memory so long as there are no inter-
rupts. The microprocessor goes to a wait state whenever a write to, read from, or interrupt occurs. This
introduces a short time period called a wait state into the microprocessor cycle to allow more time for the
1/0 devices to respond.
When the LASO is turned on, a short initialization routine is run to verify all of the bits of the ROM and
RAM. After executing the initialization routine, the microprocessor goes to a monitor routine. The monitor
is basically a polling routine looking for status such as cover open, paper out, and any data to print or
process. Some internal status information may be too critical in nature to wait for the monitor loop to poll
for it. Hardware interrupts are used to handle this critical information or updating. Some examples are
items which must be handled in "realtime" such as processing high baud rate receiver data.
4.4.2 Communications and 1/0 Data Processing
The terminal interfaces to the host computer through a serial data port. The port includes a 82S 1 A pro-
grammable universal synchronous or asynchronous receiver-transmitter (USART) and supporting logic.
The USART translates between parallel and serial data formats, adding or removing start and stop bits as
needed.
4.4.2.1 Data Format - The LASO printer communicates using serial characters. The serial character for-
mat used must be the same character format used by the computer. Serial characters are transmitted using
a start bit, seven or eight data bits, an optional parity bit and one stop bit (Figure 4-3). The number of bits
and the polarity of the parity (even or odd) is switch selectable. Parity errors can be detected for either
polarity.
IDLE
START BITOF
STATE OF
NEW CHARACTER
D:T~RB~TS
\
SPACE+
00
~ ~1 ~ ~2 ~ ~ ~ ~
I
~ ~
;
~
07
i -
~STOP
LSB
MSB
BIT
"MARK"
t
-
..L -
..L -
..L~
-
..1. -
.L /
.l.
f
START
ONE BIT
WHEN
OPTIONAL RETURN TO IDLE
BIT
TIME=
8-BIT
PARITY
STATE OF LINE
ONE/
CHARACTERS
BIT
BAUD RATE
ARE USED
ath BIT=O
MA-7815A
Figure 4-3 Serial Character Format
4-S
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