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5.9 SALT Circuitry
The SALT chip IC8 (Supply and Level
Translator) is responsible for supply
regulation, RS-232C interface level
translation, cassette read
operations, and driving the cassette
relay, as is shown in the block
diagram in Figure 5-11.
Figure 5-12 shows the complete power
supply circuit. AC voltage is brought
into the primary of transformer Tl.
The secondary of the power
transformer provides 16.2 VAC (18.52
VAC for PAL version), center-tapped,
at AC 2.2 amps (AC 1.0 amps for PAL
version) to the Color Computer 3
circuit board. If switch SWl is
closed, this AC voltage is applied to
the cathodes of D3 and D4, and to the
anodes of Dl and D2. D3 and D4 form a
full-wave, center- tapped rectifier
with a negative output. This is
filtered by electrolytic capacitor
C31. This
UNREGULATED
+VINPUT
1
negative voltage is then applied to
pin 15 of the SALT chip, where it is
internally regulated to -5 VDC and
used for the RS-232C output drivers.
The negative voltage is not used
anywhere else in the computer.
Dl and D2 form a full-wave,
center-tapped rectifier with a
positive output which is filtered by
electrolytic capacitor C29. This
positive voltage is applied to the
collector of pass transistor Ql and
is used to power the SALT chip at pin
16. The SALT chip internally
regulates the positive voltage to +5
VDC and provides the base drive
current for Ql. The current for the
computer is drawn from the emitter of
Ql through resistor Rl9. The voltage
at this point is monitored by pin 3
of the SALT chip and the base drive
adjusted to keep the voltage at a
steady +5 VDC +5%.
,--'--f-------'
1 PASS TRANSISTOR BASE DRIVE
UNREGULATED
-V INPUT
TTL INPUT
+5 VOLT
REGULATOR
DRIVER
i . - - - - - - - t 2 PASS TRANSISTOR CURRENT SENSE
L_-.-_ __r-----13
CURRENT LIMIT
REGULATED -5 VOLTS
(INTERNAL TO SALT ONLY)
FOR RS-232C
6 1 - - - - - - - - - - . i
OUTPUT
RS-232C
INPUTS
CASSETTE DATA
!ZERO CROSSING
INPUT)
TTL
OUTPUTS
TTL OUTPUT
Figure 5-11. SALT Block Diagram (IC8)
-44-
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