Texas Instruments 990 Manual page 51

~~------------------
~
945424-9701
+SVDC
R38
NC 11
CONTROL
OUTPUT ~-..._-1-----------1
TBRLD-
-5VL3
1/2 NE556V
z
D
10 RESET DISCHARGE 13
----
U17
TRIG TH RES R39
12
F~i'~-
_K_B_S_IN-4-------1---ICLK
Q
BOARD
F~i'~
REPEAT-
BOARD
(A)136275A
STROBE
FLIP-FLOP
CLR
13
CLR
- - - - I D
Q
U17
-
Q
10 REPEAT
FLIP-FLOP
Figure 1-25. Model 911 VDT Repeat Logic
FROM TREMPTY
UART
CR6
TO
UART
1.4.2.6 Difference Amplifier. The difference amplifier removes the video from the composite
video waveform. The difference amplifier is biased to respond only to signals that have voltage
levels lower than approximately 0.3 Vdc. Since the video data (see figure 1-21) lies above the 0.3
Vdc reference; only the horizontal and vertical synchronization components of the incoming
composite video signal from the VDT controller are amplified by the difference amplifier.
1.4.2. 7 Dual-Gain Video Amplifier. The dual-gain video amplifier provides one of two gain paths
for the composite video from the VDT controller. One gain path provides gain for short inter-
connection cables, and the other gain path provides a higher gain to compensate for cable lengths
greater than approximately 304.8 metres (I 000 feet). Figure I
~26
shows the dual-gain video
amplifier schematic diagram.
Transistor Q2 buffers the composite video signal from the VDT controller. Switch SI in the output
circuitry of Q2 is the line compensation switch. For cable lengths less than 304.8 metres (I 000
feet), S 1 should be open to provide a divider network for the output of Q2. For cables longer than
304.8 metres, S 1 should be closed to bypass RI 5, allowing less attenuation of the output of Q2.
Transistor Q6 is biased near turnon to eliminate the horizontal sync component of the video
signal. The video component of Q6's input turns Q6 on, and Q6 amplifies that component. The
horizontal sync component is attenuated and drives Q6 toward cutoff.
Q3 provides further amplification of the video, and the signal at its output is clamped to
TTL-compatible levels by the diode network.
1-41
Digital Systems Division