The Digital Board - HP M2350A Service Manual

Understanding the Recorder Module
The Digital Board
The Digital Board
The digital board of the thermal recorder contains a microcontroller that controls the
input/output operations and the motor and print format functions.
The microcontroller receives and responds to commands from the system, senses the state of
the control keys and switches, and converts waveforms and annotation into a format suitable
for driving the printhead. The serial data link between the system and the thermal recorder
operates asynchronously on two separate lines, providing full duplex communication at a
500 kbaud rate. The microcontroller interprets messages from the system and sends back
identifying and status messages via the power supply pcb.
The RUNKONT and STOP switches, the DOOR OPEN switch, and the PAPER OUT switch are
mounted directly on the digital pcb, near the front of the recorder. When requested, the I/O
microcontroller reports information about the state of the control keys and switches in status
messages.
The paper out sensor is an optical device which aims a pulsed infrared beam at the paper as
it passes around the paper drive roller. When paper is present, the infrared light is strongly
reflected back to a phototransistor which is connected to the I/O microcontroller. The M1116B
has an infrared sensing circuit to determine if the paper-loading door is closed. If the door is
closed, a tab on the door interrupts the light beam. Software routines pulse-on the infra-red
beam for the optical interrupter as needed.
Waveforms and annotation data from the system must be translated into row and column
format and merged into a matrix before being transmitted to the printhead. The I/O
microcontroller writes row and column data into a dual port RAM, so that the matrix is a
snapshot of the dot pattern that is to be printed over the next few tenths of a second, RAM is
maintained
as a rotating buffer so that memory locations are reloaded with future printing data
as soon as the oldest column of dots has been printed.
The motor and print format circuits regulate motor speed and print darkness. The recorder
mechanism provides information to the microcontroller about each dot (such as average
wattage consumed and which dots have been on recently) as well as the speed of the motor
and the temperature of the prlnthead. The motor and print format circuits use this information
to maintain uniform print contrast by modulating the strobe width applied to the dots for
printing.
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