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Figure 26 2-input change over circuit and separate synchronizing signal

1.9.3 2-input change over circuit and synchronizing signal circuit

There are two systems; SINGAL-A and SIGNAL-B, as well as the video input terminal, in syn-

chronizing signal input terminal, and both has a D-SUB connector. Since input terminals and

circuit operation are different in each synchronizing signal (separate, composite, picture com-

posite), each synchronizing signal is explained in this section.

[Separate synchronizing signal] (Separate Sync)

Horizontal synchronizing signal which has been input from SINGAL-A is input from pin 13 of D-

SUB connector J215 to pin 15 of analog switch IC216, and the vertical synchronizing signal is

input from pin 14 of D-SUB connector J215 to pin 16 of analog switch IC216. Besides, the

horizontal synchronizing signal which has been input from SINGAL-B is input from pin 13 of D-

SUB connector J216 to pin 7 of analog switch IC216, and the vertical synchronizing signal is

input from pin 14 of D-SUB connector J216 to pin 8 of analog switch IC216. (Refer to A point.)

The analog switch IC216 selects a signal (2-input change over) when SIGNAL-A and SIGNAL-

B are simultaneously input, as well as the video signal.

As for the method of selecting a signal, like the video signal, according to SELECT signal of

pin 53 of microcomputer IC101, input signal of SIGNAL-A is selected when pin 13 of analog

switch IC216 (SELECE SW) is HIGH, and input signal SIGNAL-B is selected (refer to B point)

when it is LOW. They are output from pin 19 (horizontal synchronizing signal) and from pin 18

(vertical synchronizing signal) of analog switch IC216. (Refer to C point.)

The horizontal synchronizing signal and vertical synchronizing signal, which are output from

analog switch IC216, are supplied to pin 30 (H-SYNC) and to pin 20 (V-SYNC) of microcom-

puter IC101 on PWB-MAIN via the flat cable, respectively.

As for the polarity of the separate synchronizing signal, there are the positive polarity (POS)

and the negative polarity (NEG). The following Fig. 26 shows the case that the positive polar-

ity (POS) is input.

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Compaq P1220 Wartungshandbuch Seite 59