Measurement Processor (Diagram 8); Readout System (Diagram 9) - Tektronix 2246 1Y Service Manual

or probe code to determine if a change has
occurred and, if so, the amount and direction of the
change. The Measurement Processor uses that
information to generate new control voltages to the
circuitry affected by the change.
The Probe Adjust circuit
circuitry) produces a square-wave signal which is
output to the front panel PROBE ADJUST jack for
compensating voltage probes and checking the
vertical deflection system of the oscilloscope.
The Volts Cal circuit (U931 and an associated pre­
cision voltage divider) provides the accurate dc
voltage levels used during vertical SELF CAL to
check the gain and offset of the measurement
channels.
The Scale Illumination circuit is made up of three
incandescent graticule lamps and current-source
transistors. The SCALE ILLUM potentiometer sets
the bias level on the transistors' emitters.

MEASUREMENT PROCESSOR (Diagram 8)

Many of the oscilloscope circuitry functions are
directed by the Measurement Processor (U2501).
The Measurement Processor, under firmware con­
trol, monitors the front-panel controls and sets up
the circuitry under its control according to the set­
tings made and the instructions contained in the
System ROM.
The Measurement Processor communicates directly
with the devices on its eight-bit data bus. The
Measurement Processor
transfer data to or from by placing the address of
the device on the Measurement Processor Address
Bus. That address is decoded to produce a strobe
that enables the bus device corresponding to the
address. Writing to or reading from the enabled
device is controlled by write or read (WR and RD)
pulses from the Measurement Processor. Com­
munication on the data bus is usually limited to high
speed data transfer only (to and from the System
RAM and from the System ROM) and not direct
control of any circuit functions.
For controlling most of the circuit operating states,
the Measurement Processor places serial bits on the
bidirectional MB DATA line. Appropriate enabling
strobes and clocks are generated either in its
address decoding circuitry or by the Processor Inter­
face circuitry (Diagram 4) to load the control data
into 24-bit or 32-bit shift registers. The outputs of
these registers control such things as attenuator set­
tings, preamplifier gains, sweep timing, and trigger
Theory of Operation—2246 1Y and 2246 Mod A Service
(U930 and associated
selects the device to
operating modes; all circuit operating functions that
either change with front panel settings only or at a
slow rate.
Scanning of the front panel controls and lighting of
the front-panel LEDs that back light the buttons is
under control of the Measurement Processor. These
events occur at long intervals compared to the
operating speed of the Measurement Processor.
The front-panel switch closures are read by the
Measurement Processor
munication line (SW BD DATA).

READOUT SYSTEM (Diagram 9)

Readout System
The Readout Processor (U2400) controls the display
of text and cursor readouts as directed by the
Measurement Processor. The ASCII code of each
character (blanks included)
readout (one field) is loaded into the appropriate
memory location of the
(U2406) by the Measurement Processor. It is then
up to the Readout Processor to control the display
process.
When the Readout Processor addresses the Charac­
ter Code RAM for display of the loaded characters,
the address of a memory location dictates the place
that the addressed character will appear on the face
of the crt. The ASCII code found at the addressed
location in the Character Code RAM then accesses
the character to be displayed from the Character
Dot Position ROM (U2408). The screen position of an
individual dot within an addressed character is
directed by the character data obtained from the
Character Dot Position ROM.
The data bits specifying the character position on
screen and the dot position within a given character
are converted to analog vertical and horizontal
position signals by the readout DACs (U2412 for
vertical and U2413 for horizontal). For cursors and
cursor related text, voltages representing the cursor
positions are added in the output mixer circuitry
(U2414, U2415, and U2416) to place the readout
correctly on screen. Vertical position information
needed for the measurement-tracking cursors and
readouts is added in the Vertical Position Switching
circuit (Diagram 2).
The dots are continually refreshed to maintain a
flicker-free readout. When the readout data needs
changing, the Measurement Processor halts the
refreshing and loads the new screen of data into the
Character Codes RAM.
over a serial com­
in a full screen of
Character Code RAM
3-5