Theory of Operation
2715 Spectrum Analyzer Service Manual 3-55
Table3 -14: Page Number/Waveform Relation
Page Number Refers to
0Waveform A
1Waveform B
2Waveform C
3Waveform D
4TextPage 1 (Readouts)
5TextPage 2
6TextPage 3
7TextPage 4
If the page to be displayed is a w aveform then the address for the waveform data is
determined by the lower two bits ofthe Page Counter and the position of the
waveform is determined by the Horizontal Counter (insideU46). This combination
of bits is placed on the boards address bus u sing U50, U51, and U52, the three 2--1
multiplexers. The waveform datais then read through a tristate buffer, U47, into the
LCA, U46, and latched. Th eC RTbeam is t hen 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 t ext then the address for the text data is determ ined by
the lower two bits of the Page Counter, the upper four bits of the CharacterR ow
Counter (inside U46), and the upper eight bits of the Character Column Counter
(inside U46). This combinat ion of bits is placed on the board’s address bus using
U50, U51, and U52. The text d ata is then placed at the inputs of the Character
ROM, U48. The character ROM outputs then provide one column of bit informa-
tion for the characterto be displayed. The character bit information is then latched
inside U46 and fed out serially andused to control the CRT beam. If a one (high) is
encountered in the characterbits, the CRT beam is turned on for five clock cycles
(2.44 uS), displaying a do t on the CRT.The text co ntinues to be placed on the CRT
using a vertical scan method. A verticalscan is used to decrease the time required
to wait for the beam to move fromthe top back down to the bottom of the CRT.
Figure 3--20 shows how a vertical scan is performed.