Tektronix 1740A Series, User Manual

Page: 80 / 368

Circuit Theory
3–16
1740A/1750A/1760 – Series Service Manual
Output signals from the Microprocessor are timed out to analog switches and
DACs by the Line Rate Controller clocks synchronous latches. Each latch is
clocked by its own individual output from the Line Rate Controller.
Diagram
8
Readout
The Readout Control state machine interprets the readout instructions from the
Microprocessor and loads digital data into the DAC. Analog output of the DAC
drives the stroke generators to create the readout segments that are displayed on
the CRT.
The Readout Control is a programmable logic device, U27, configured as a state
machine. It uses 13 buffered addresses and 8 buffered data bits to produce an
8-bit data word (R0 – R7) that is converted by an 8-bit DAC (U37). Device
clock is the buffered 16 MHz from the Microprocessor. When Readout Control
is off U36, a RAM, can be written to directly by the Microprocessor, through its
13 bit address port. A screen draw requires 13 bits.
U37 is a dual DAC, that decodes the Microprocessor instructions. The A output
drives the Vertical Readout Stroke Generator and the B half performs the same
function for the Horizontal Readout Stroke Generator. Calibration constants are
provided by the serial digital to analog DACs shown on Diagram 9.
The Readout Stroke Generator consists of two identical circuits. Each generator
has an inverting buffer amplifier, U48A or U48B, whose gain is unity. The
output of the buffer amplifier drives a sample-and-hold, U54A or U54C. Timing
for the sample-and-holds is identical and determined by a single RC circuit
(R376 and C76). The output of the sample-and-hold drives an integrator, U48C
or U48D, whose output is a negative-going sawtooth waveform. Charging
current is controlled by the Shape adjustment (R134 or R135). The output of the
Readout Stroke Generator drives the Vertical and Horizontal deflection circuits
when readout is enabled.
Diagram
9
DACS & Serial
U24 is a 4-line to 8-line decoder that outputs chip select and enable signals for
the non-synchronous switching control lines. U9 is an 8-bit parallel load serial
output shift register. Status of the A, B, C, and D input lines identify the Main
circuit board revision level. SW1 is included for troubleshooting purposes. R28
is a set of pull-up resistors for 8 parallel inputs and 3 of the Microprocessor
(Diagram 6) control lines.
Synchronous Outputs
Readout Control
Readout Stroke Generator
Serial Interface & Serial
Static Outputs
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