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Theory of Operation
2715 Spectrum Analyzer Service Manual
3-55
Table 3- 14: Page Number/Waveform Relation
Page Number Refers to
0 Waveform A
1 Waveform B
2 Waveform C
3 Waveform D
4 Text Page 1 (Readouts)
5 Text Page 2
6 Text Page 3
7 Text Page 4
If the page to be displayed is a waveform then the address for the waveform data is
determined by the lower two bits of the Page Counter and the position of the
waveform is determined by the Horizontal Counter (inside U46). This combination
of bits is placed on the boards address bus using U50, U51, and U52, the three 2--1
multiplexers. The waveform data is then read through a tristate buffer, U47, into the
LCA, U46, and latched. The CRT beam is then turned on and the waveform data is
presented to the Vertical Output DAC, drawing a vector on the CRT.
If the page to be displayed is text then the address for the text data is determined by
the lower two bits of the Page Counter, the upper four bits of the Character Row
Counter (inside U46), and the upper eight bits of the Character Column Counter
(inside U46). This combination of bits is placed on the board’s address bus using
U50, U51, and U52. The text data is then placed at the inputs of the C haracter
ROM, U48. The character ROM outputs then provide one column of bit informa-
tion for the character to be displayed. The character bit information is then latched
inside U46 and fed out serially and used to control the CRT beam. If a one (high) is
encountered in the character bits, the CRT beam is turned on for five clock cycles
(2.44 uS), displaying a dot on the CRT. The text continues to be placed on the CRT
using a vertical scan method. A vertical scan is used to decrease the time required
to wait for the beam to move from the top back down to the bottom of the CRT.
Figure 3--20 shows how a vertical scan is performed.