Theory
of
Operation
2-41
Thermal Printer
The thermal printer
is
an
assembly composed of printhead, motor, switches, chip modules,
and
four boards. Figure 2-23
is
a block diagram of the printer. The boards in the printer are as follows:
• Logic Board
• Motor Drive Board
•
Paper
Sensor
Board
• Printhead Interconnect Board
The printer
is
a modular assembly that can
be
removed from the computer by loosening two
captive fasteners, disconnecting the safety ground wire,
and
lifting the printer from the computer.
The printer receives power
and
110
signals via two 50-pin connectors
mounted
in the computer
base. Two guide pins with these connectors enable alignment of the printer as
it
is
installed in the
base. When the printer
is
mechanically seated
in
the computer,
it
is
also electrically connected.
Two character sets are available
in
the printer:
1)
Roman Extended,
and
2) Katakana. The set
is
selected by
command
from the processor as determined by the keyboard language resistors con-
figuration. Refer to
Chapter
4 for additional information
on
keyboard language configurations.
A stream of
ASCI[ bytes enters the printer through the
110
interface. The bytes are then separated
into normal
ASCII characters which can
be
buffered or ASCII control codes which must
be
processed immediately. Normal ASCII characters are entered into
an
80-byte print buffer which
is
printed
upon
receipt of the 81st byte or upon receipt of
an
appropriate control code.
The printer responds to a subset of
ASCII lower
and
upper control codes as soon as
it
receives
them. Undefined control codes are ignored
and
do not contribute to the character count in the
internal print buffer.
Escape sequences are printer commands that are
embedded
within the normal print data stream.
An escape
sequence
begins with the Escape character code (code 27)
and
normally
ends
with a
capital letter. Escape sequences are not unique to the printer, but are used to control a variety of
peripheral devices. The escape sequences not applicable to the printer are ignored.
The following paragraphs describe printer components
and
their main functional blocks as shown
in
Figure 2-16. Critical signal lines
and
interfaces are described.
Logic
Board
The
logic
board
is
the printer's control board, containing
all
the critical digital logic. This board
contains the
lOP interface, the microprocessor,
and
RAM
and
ROM.
This
board
controls
all
printer
functions. It
is
suspended
between the side plates of the printer so that when the printer
is
installed
the two
50-pin connectors
on
the logic board mate with the connectors
mounted
in the computer
base. Connector
J3
on
the logic board mates with the connector
on
the lOP cable. Logic
board
connector
J5
mates with the power bus connector mounted
in
the base.
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