5-3
3. MODE DISCRIMINATING CIRCUIT OPERATION
The circuit which discriminates three modes of VGA, is as follows.
ICH01
M52346SP
ICH02
TC4028BP
BCDA
FREQUENCY
DISCRIMINATION
fH > 28KHz/Hig
VGA350
VGA400
VGA480
DH31
Hor. sync
Vert. sync
2
19 18
6 8 4 6 7
13 12 11 10
Fig. 2
Mode discrimination of VGA is done by the circuit in Fig. 2. ICH01 M52346SP also performs process of sync signal, and the
logic output is shown in Table-3. ICH02 is Decoder IC TC4028BP, and the truth table is shown in Table-4.
Input at pin 6
H. COMP.
Input at pin 8
V.
Output pin
1 2 18 19
H. COMP. (POS.)
H. COMP. (POS.)
H. COMP. (POS.)
NON
V. (POS.)
V. (NEG.)
H
H
H
L
H
H
L
L
L
L
L
H
H. COMP. (NEG.)
H. COMP. (NEG.)
H. COMP. (NEG.)
NON
V. (POS.)
V. (NEG.)
H
H
H
L
H
H
H
H
H
L
L
H
NON
NON
NON
NON
V. (POS.)
V. (NEG.)
L
L
L
L
H
H
L
L
L
L
L
H
Table-3. Logic output of M52346SP Table-4. Truth table of TC4028BP
When Macintosh signal is input in form of negative composite sync, diode DH31 prevents confusion between Macintosh signal
and VGA400.
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