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1664A Theory
CPU
Located on the main circuit board, the microprocessor is a Motorola 68EC020 running at
25 MHz. The microprocessor controls all of the functions of the logic analyzer including
processing and storing data, displaying data, and configuring the acquisition ICs to obtain and
store data.
System Memory
The system memory is made up of both read-only memory (ROM) and random access memory
(RAM). A single 128Kx8 EPROM is used as a boot ROM (boot-up requires operating system
on disk loaded in the disk drive). Four 1Mx4 DRAMS are configured to provide a 1Mx16
DRAM space.
On power-up, instructions in the boot ROM command the instrument to execute its boot
routine, which loads the operating system from disk. The boot routine (on disk) includes
power-up operation verification of the instrument subsystems and entering the operating
system. The CPU searches for the operating system on disk.
The DRAM stores the instrument configuration, acquired data to be processed, and any
inverse assembler loaded in the instrument by the user.
CRT Controller and Display RAM
A Brooktree BT476KPJ66 RAMDAC color palette and a National Semiconductor LM1882CM
video frame generator control the CRT. One of the RGB outputs of the color palette provides
the eight-shade grey scale display. The video frame generator provides the horizontal and
vertical synchronization timing signals.
The display RAM is made up of two 256Kx4 DRAMS configured as 256Kx8 and stores all of
the pixel information used by the color palette. A serial address counter and an address
multiplexer control the DRAM addressing. At the conclusion of each video frame the vertical
sync signal from the video generator resets the serial address counter and a new frame is
generated.
Disk Drive Controller
The disk drive controller consists of a single floppy drive controller IC. The floppy drive
controller provides all signals to the disk drive including read and write data, read and write
signals, write gate, and step signal. The floppy drive controller also reads status signals from
the disk drive, including a track 00 signal, disk ready, and disk change signal.
Keypad and Knob Interface
The front panel keypad is scanned directly from the microprocessor address bus during the
video blanking cycle of the CRT. When a front panel key is pressed the associated address
bits are fed to the data bus through the pressed key and read by the microprocessor.
The rotary pulse generator (RPG) knob is part of the HIL circuitry. Pulses and direction of
rotation information are directed to an RPG interface IC and then to the HIL loop. The
microprocessor then reads and interprets the RPG signals and performs the desired tasks.
Theory of Operation
The 1664A Logic Analyzer
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