DEC VT220 - Page 172

8.2.2.2 Horizontal Driver Circuit The horizontal driver
circuit converts the square wave input from the horizontal
processor to a bias potential to the horizontal output circuit.
The horizontal driver circuit (Figure 8-6) consists of the
following components/circuits.
4' Driver (Q231)
• Base bias resistor (R212)
• Power source filter (C210)
• Output network (T201, R213, R214, and C2l1)
A high input is applied to the base of Q201 for horizontal retrace
and first half of trace, a low for second half of trace. A high
input forward biases Q201, placing 10 volts across the primary of
T201, with the resulting secondary potential turning the
horizontal output circuit off. When a low input turns Q201 off,
current is reversed at the secondary of T201, turning on the
horizontal output circuit.
R213 limits the current through Q201, and R214 and C211 limit
voltage across this transistor.
FROM
+12V
FILTER
C211
.1
100V
R214
68
Ii2W
I
~2~
T201 TO
~ 3 HORIZ
I 4 ~11~=:2=======:-_· ~L~b:;~T
I
,
0201
I MPS ~-...,
, uos
i
)---J.. __ .l..-_--l. __ -l-_ GND 8
R2l2
470
FRO~ .. 1
HORIZ
PROCESSOR -N..--!--H
(E201
PIN 1;
Figure 8-6 Horizontal Driver Circuit Diagram
8-6