HP 54520 Series Service Manual page 135

Chapter 8: Theory of Operation
Main Assembly Theory
e The System RAM is 4 Mbyte in eight 1M x 4-bit CMOS dynamic RAM ICs. The System RAM
uses the conventional RAS/CAS timing scheme for read/write and refresh. System RAM
stores variables, volatile waveforms, and is the waveform buffer.
e The Flash ROM is 2 Mbytes, in eight 256K x 8-bit EPROMS. System ROM is used to store
system operating code. It can be loaded from the flexible disk drive.
Interface Circuits The System Control interfaces with two main functions: the
acquisition system and the peripherals. The acquisition system includes the triggering,
time base, attenuator/preamp control, and acquisition hybrids and their associated
circuitry. The system control interfaces with the acquisition system through dedicated
address and data buses. The peripherals consist of the real-time clock, keyboard, display,
disk drive, RS-232, HP-IB, and a miscellaneous peripheral connector.
The peripherals have individual interfaces which are connected to the system control through
the peripheral interface, a subset of address bus, data bus, and control lines.
e The acquisition interface connects the system control to the acquisition system. Through a
dedicated interface, the system control provides data and address buses, control lines, and
chip selects for acquisition circuitry.
e The real time clock is a dedicated clock IC with a crystal frequency control of 32.768 kHz.
Time data is supplied to the CPU. A frequency divider network within the IC can be
programmed to provide a set of frequencies that are available on the rear panel ac CAL
output and the front panel Probe Comp test point.
The keyboard interface is a DUART IC clocked by a 3.6864-MHz oscillator. The serial
interface with the keyboard is two lines, receive data (RXD) and transmit data (TXD). The
DUART receives the keystrokes and knob turn data from the keyboard microcontroller and
transmits LED indications to it. The only other lines to the keyboard are 5 Vde power and
ground.
The DUART that drives the keyboard is also used to drive the RS-232 interface.
e The display is driven by a video frame generator and one third of an RGB video DAC. The
frame generator provides the vertical and horizontal sync and the video driver provides
eight-bit gray-scale video. The video interface also includes the pixel RAM (256K x 8-bit
DRAM) and the circuitry necessary to generate timing signals.
e The main parts of the disk drive interface are a disk drive controller IC and a 24-MHz
oscillator for clocking.
e The HP-IB interface circuitry supports communication with other instruments (such as a
printer, controller, or automated test equipment) The circuit consists of three main
components. The HP-IB controller provides an interface between the microprocessor
system and the HP-IB in accordance with JEEE 488 standards. An 8-bit data buffer and
8-bit control line buffer connect the HP-IB controller to the HP-IB bus.
The HP-IB is a 24-conductor shielded cable carrying 8 data lines, 8 control lines, 7 system
grounds, and 1 chassis ground.
Power The System Control requires +5 volts dc and +12 volts de. System control
supplies the display board with +5 volts, +12 volts, and display ground. The clicker circuit
is operated from the +12 display voltage. The remaining system control circuitry is
operated from the +5 digital voltage.
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