Front-Panel Controls; Front Panel Switches; Fre>Nt Panel Potentiometers - Tektronix 2212 Service Manual

Front
Panel
Controls
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and0)
2212 Service Manual
Digital Circuitry
The front panel is the operator's interface for controlling the user selectable
oscilloscope functions. Along with the crt, it provides visual feedback to the
user about the present operating state of the instrument.
The front panel micro controller U1401 and the main processor on the storage
board communicate using a three wire interface;
Clock signal (FP _CLCK)
Data input (into the front panel) line (FP _SDI)
Data output line (FP _SDO)
The 68HC05P7 micro controller has a synchronous serial in- and output port
(SIOP), therefore communication needs a minimum software overhead.
The clock signal is supplied by the 68070 main processor on the processor
board (A15), with a maximum of 500 kHz, so micro controller U1401 is operated
in slave mode. fo eight clock pulses one byte is transferred to and from the front
panel micro controller. Every two milli seconds the main processor polls the
communication channel. This means that within 2ms U1401 must be ready to
put data into the SIOP. Therefore in the main program of U1401 tasks are split
up in parts. Between each part a synchronisation byte is send so there is always
communication between the processors.
Front Panel Switches
The front-panel switches S1400 to S1465 (38pcs) are arranged in a seven-row by
eight-column matrix (see diagram 14).
When scanning, micro controller U1401 sequentially sets each row line in the
LO state via U1402. When a switch is closed, it pulls down a column line,
which will be read by the micro controller. The intersection of the selected row
and the pulled down column uniquely identifies the switch that is closed.
Since switches S1400, S1401 and S1402 are not momentary switches they are
isolated from the column lines by diodes CR1400 to CR1411.
Front Panel Potentiometers
The position of the wipers of the seven Front-Panel potentiometers R1410
through R1415B are digitized by the 10-bit aid converter U1401, which has a
serial interface with the micro controller. Beside these potentiometers also the
CH1 and CH 2 probe coding circuits (diagram 1) and the plus and minus peak
detectors (diagram 4) are read. Hold-off potentiometer R1416 is not digitized,
but is directly connected to the circuits involved.
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