Operator`s manual

Chapter 2: THEORY OF OPERATION
2.1: INTRODUCTION
All printer electronic subassemblies, with the exception of the power supply, are contained
on one printed circuit board (the controller board). This chapter provides the technician
with descriptions of the various circuits contained on the controller board, ie. printhead
driver circuit, carriage drive circuit, and an overview of the microprocessor system.
2.2: MICROPROCESSOR SYSTEM
The microprocessor system consists of an Intel 8085 microprocessor, two Intel 8155 RAM/
1/0 /Timer chips, an Intel 8251 USART, EPROMS, 26K bytes of accessible RAM (see
Figure 2.1). 2K bytes of RAM are battery backed up. The RAM’s are used for input FIFO,
line buffers, dot buffers and general purpose workspace. The EPROM contains dot patterns
for printable characters and software pro, 7 rams (firmware) required to perform various
printer functions. The Intel 8155's are used for general purpose 1/0 and the parallel
interface. They also provide timers for determining baud rate and processor interrupts. The
Intel 8251 USART converts the serial data arriving at the RS-232 port into parallel form
for the processor. The processor reads this data and stores it in the FIFO buffer.
Communication with the operator is through the processor polled keypad, the 4 seven-
segment displays and the various LED’s on the front panel.
When power is turned on a resistor-capacitor network attached to the 8085 generates a
reset signal which causes the processor to begin program execution at address zero.
Figure 2.1 Functional Block Diagram
2-1