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Circuit Description
TAS 455 and TAS 465 Service Manual
3Ć5
The GPT inputs are used for measuring the main and delay sweep gates, as
well as the measurement latches. The outputs of the GPT are used for
loading or resetting various sections of the analog board.
The QSM is primarily used as a synchronous bus for setting up the internal
register bits of the GCS, HS, preamp and DACs on the analog board.
The SIM module does all of the chip selecting for all of the addressable ICs
on the processor board. U201 has three types of support circuitry. The first
type is pullĆup resistors for IRQ lines and for configuring the processor boot
up. The second type is a crystal oscillator. The crystal oscillates at 32 kHz.
The processor uses a phase lock loop to create a 16 MHz system clock from
this crystal. The third type is a powerĆon reset circuit. The reset circuit,
consisting mainly of U207, is a passive circuit that monitors VCC. When the
power supply exceeds 4.5V, U207 releases the reset line allowing the proĆ
cessor to run. If VCC drops below 4.5V a reset occurs.
ROM/RAM Subsystem A5
2
The ROM/RAM subsystem consists of 256 kb of Flash ROM, 256 kb of static
RAM, and 64 kb of nonĆvolatile static RAM. The Flash ROM has a requireĆ
ment which states that it must not have an OE during a write cycle. ConseĆ
quently, the gates of U221 in this subsystem prevent this from happening.
Readout Subsystem A5
4
The readout subsystem consists of a custom readout IC, two DACs, a 2.5 V
reference voltage, and some support resistors and capacitors for compensaĆ
tion and filtering. The readout IC has an internal RAM into which the procesĆ
sor puts ascii characters. The processor also controls the state of the readĆ
out IC for calibration, cursor positioning, and update rate. The readout IC
itself then drives the vertical and horizontal DACs as well as the blanking.
FrontĆPanel Interface Subsystem A5
3
The frontĆpanel interface subsystem consists of U215 (DUART) and its
3.686 MHz crystal. The processor communicates to the frontĆpanel procesĆ
sor through an RS232 like interface. This interface is maintained by the
DUART, running at 9600 baud. The DUART is connected to IRQ5 on the
processor. U201 talks to U215 using data lines D8 through D15.
Analog Board Interface Subsystem A5
3
The analog board interface subsystem consists of an ECL to CMOS convertĆ
er and a CMOS to ECL converter. The SGMAIN lines coming from the anaĆ
log board are ECL. These lines are converted to CMOS by using two transisĆ
tors and a couple of resistors.
The EXTTRIG lines going to the analog board need to be ECL. These lines
are converted using three resistors each.