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Service Guide ML320/ML321
Chapter 2 Principles of Operation
2.2.03 Control Board
Microprocessor
The microprocessor is the nucleus of the control circuitry. All peripheral circuits operate under
the control of the microprocessor.
For the Rev A, B, and C printers, the microprocessor is Q7 - 80C154.
For the Rev D printers, the microprocessor is Q1 - 80C154.
Program Read Only Memory (ROM)
The printer's control program and resident character fonts are stored in the Program ROM. The
microprocessor operates under the control of this program.
For the Rev A, B, and C printers, the Program ROM is Q13 - 128 kilobytes.
For the Rev D printers, the Program ROM is Q4 - 64 kilobytes.
Random Access Memory (RAM )
The RAM stores the following information.
Data to be printed (Receive Buffer)
All character fonts
Q12 has a battery backup. Data is retained even after power is removed.
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