Theory of Operation
TDS 310, TDS 320 & TDS 350 Service Manual
3Ć11
Three control lines to multiplexers U420 and U421 select the pot input or
wiper voltage to be read. The analog voltage at the wiper of the pot selected
is applied to the frontĆpanel processor. This voltage is digitized, and the
amount and direction of change from the previously stored value is
calculated. The change information is sent to the CPU.
The frontĆpanel processor generates CALSIG. The high level is
approximately 5 V and the low level is at ground.
Switches A4Ă
4
The frontĆpanel switches and menu switches are arranged in an array of
eight rows and columns. When a switch closes, one row line connects to
one column line through an isolation diode. A complete scan of the
frontĆpanel switches consists of setting all eight row lines low, in sequence,
and performing an eightĆcolumn scan to check for a change from the state
stored in the frontĆpanel processor. Low bits in the columnĆline data tell the
frontĆpanel processor that a switch is closed.
LEDs and Power Supply A4Ă
5
The LEDs are arranged in groups of eight. They are connected between the
outputs of 8Ćbit LED latch (U202). When the CPU needs to turn a particular
LED on or off, it sends a command to the frontĆpanel processor indicating
what to do to the LED, and which one to change. The frontĆpanel processor
converts the LED identification number to the LED address within the latch.
The A20 Low Voltage Power Supply is a switching power converter. It
supplies power to all the circuitry in the oscilloscope.
The Low Voltage Power Supply does not have a main power switch. The
ON/STBY switch, located on the front panel, controls all the power to the
oscilloscope except the standby circuits in the Low Voltage Power Supply.
The A26 Monitor Assembly displays all information (waveforms, text,
graticules, and pictographs). It generates the high voltages necessary to
drive the display tube. It also contains the video amplifier, horizontal
oscillator, and the vertical and horizontal yoke driver circuitry.
The 640 by 480 pixel raster scan cathodeĆray tube (CRT) display has 60 Hz
frame and 31.5 kHz line rates. This CRT display circuitry is similar to a
television monitor.
Inputs, Video, and Vertical DeflectionĂA26Ă
1
The +12ĂV regulator is a threeĆterminal regulator (U130).
A20 Low Voltage
Power Supply
A26 Monitor
Assembly
