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Table 4-1 Printer Interface Signals
RS-232
Function
Direction
Pin
Mnemonic
Receive Data
To printer
3
BB
Send data
From printer
2
BA
Terminal ready
From printer
20
CD
Request to send
From printer
4
CA
Busy or ready
From printer
11
Protective ground
From printer
1
Signal ground
Common
7
AB
4.4.3 Printhead Positioning and Printing Process
This paragraph describes how data stored in RAM buffers is processed and how characters are formed on
paper. It also describes how the system controls the printhead position and forms a character in the correct
location.
The LASO is an impact printer that uses a 9-element solenoid printhead. The printhead is driven horizontal-
ly and prints characters in a 7
X
9 dot matrix.
The printer can print double-width characters in 5, 6, and 8.25 character per inch modes. Therefore, the
maximum number of characters per line is 40, 48, and 66 respectively. Text or graphics mode printing is
selected by the control and escape sequences.
While processing characters in text mode, characters are printed as they are received.
In
graphics mode,
each character received defines a specific set of dots to be printed. A one-to-one relationship does not occur
between the sequence of received characters and the activity of printing. Each character selects 1 of 64
possible 1
X
6 dot combinations. This is done by using the top six elements of the nine-element head. The
ratio of horizontal to vertical dots is variable by changing the number of horizontal dots per inch with a
communication switch setting. The number of vertical dots is kept constant at
72
dots per inch. The ratio is
either 2: 1 or 2.5:
1.
Figure 4-6 shows a functional overview of the printing/printhead positioning process. The microprocessor/
control system issues a series of commands to the 8155 and printhead logic which contain instructions to
print a character. This includes character storage, carriage drive speed/direction, and a start print com-
mand. Speed depends on the selection of enhanced printing. The print direction is left to right in the graph-
ics mode. In the text mode, printing is left to right as long as less than one received line remains unprinted.
If there is more than one line to be printed, the printer automatically goes to bidirectional printing. The
printhead position is determined by count from the start of print.
Carriage drive information, from the microprocessor, flows through the 8155
1/0
buffer to the motor
drivers and motor. Drive information and print information are coordinated by the 815 5 and microproces-
sor control logic. When a request is made to print a character, the ASCII-coded representation of the char-
acter stored in the RAM is transferred by the microprocessor to the LS374 register in the form of "dot
data" bytes. Each dot data byte has an encoded pattern of dot bits that fire specific head solenoids to create
the desired character on paper. If the printer is operating in graphics mode, a different type translation is
performed. Each graphic character sent by the host is processed and printed as one vertical pattern of dots.
When enabled by the control logic and print timing, the LS374 register fires the solenoids for the first
vertical dot pattern representation of the character. Immediately more dot data shifts into the register, the
carriage moves, and the printhead fires. The process continues until the character is completely printed at
which time the carriage and printhead wait for the next microprocessor command.
4-9
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Troubleshooting
