Thermal Printer; Logic Board - HP 9020 Service Manual

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
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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.