Tektronix 2212 Service Manual page 81

2212 Service Manual
Digital Circuitry
The data needed for writing the data to the screen is all directly read from the
68070 microprocessor memory using DMA (Direct Memory Access). To
perform this task, the highest priority DMA channel of the internal DMA
controller is used (in burst mode). To get a constant flow of display data, the
display system is coupled to the microprocessor bus through a first-in/first-
out buffer (FIFO) with a depth of 16 words (U1900 - U1905).
The 16 bits of data from the databus are directed to the DAC's U1922 and
Ul 923, to generate analog signals. In this way readout positioning can be
performed with a resolution of 256 x 256 pixels. To get the full performance
needed to get all the features generating the 2212 display, the display system
is made "programmable" by adding a readout control byte (RC0 - RC7 from
Ul 906). This byte is fed into the FIFO's synchronously with the display data
and can be set by the microprocessor to select the type of display function
needed.
The main functionality of the display system is determined by the 22V10
PAL U1925 in combination with counter U1932.
The PAL, also controlled by the readout controlbyte, determines the timing of
the display process and generates the digital signals needed for the analog
mainboard (clocked by the CLK5M timing signal).
In combination with the 74HC592 counter U1932, longer timing periods can
be generated. In this way magnification of waveforms can be performed by
stretching the display time of one acquisition-dot, times ten or times fifty.
The following analog circuitry is added to get more resolution than the 256
positions provided by the 8 bit DAC's:
- An analog sweep generator for displaying the 4096 position waveforms
horizontally (U1600A and C1903). Displaying readout data will
automatically reset the sweep generator.
A "dwell" control is provided to stop the waveform during display when
a trigger point is written or a store mode time cursor.
Two 12 bits daculator outputs REFCUR and DELTACUR for the
positioning of the voltage and time cursors in analog mode and the voltage
cursors in store mode.
The analog multiplexers U1930 and U1931 (also controlled by the readout
controlbyte), determine which signal is routed to the analog outputs ROHOR
andROVERT.
Readout control bytes are used as stated in Table 3-2.
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