Radio Shack TRS-80 Service Manual page 39

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TRS-BO
®
Service Manual
In the alphanumeric mode, the data retrieved from the DRAMs
by the VDG is in the form of ASCII character codes. This
data is then used by the VDG's internal character generator,
along with the "line count" information, which is kept track
of internally, to decide which dots to display. The dot
data is loaded into two internal shift registers which are
clocked on alternate polarities of the incoming 3.58 MHz
clock. The outputs of these shift registers, along with the
vertical and horizontal sync signals, become the Y signal.
In the graphics modes of operation, the data retrieved by
the VDG contains actual pixel and color information, and is
used to generate the Phase A and Phase B signals in addition
to the Y signal. The CHB signal is a chroma reference
signal and is used to compensate for temperature drift and
other anomalies.
The GMO, GMl, GM2, A/S, A/G, INT, and CSS lines are used to
determine the display mode of the VDG. These are controlled
by one of the output ports on PIA U2. For more information,
refer to the memory map and the VDG data sheet. The HS*
output of US, the MC6847, occurs during horizontal sync time
and is used as one possible source of interrupt to PIA U7.
The FS* output occurs during vertical sync time, and is used
as another possible C60 Hz) interrupt source to PIA U7. In
addition, the FS* output is tied to the MS* input of the
VDG, disabling the address bus after the active display time
and allowing the SAM chip to supply refreshing for the
DRAMS.
The Y, Phase A, Phase B, and CHB signals from the VDG, as
well as the Sound signal and the 3.58 MHz chroma clock, are
sent to the RF Modulator (U9).
The heart of the modulator
is the MC1372 integrated circuit which takes these inputs
(except for the sound) and modulates an RF carrier with
them. The sound input is used to frequency modulate a 4.5
MHz sound carrier oscillator contained in the modulator.
This modulated sound signal then enters the MC1372 with the
chroma information, thus modulating the RF carrier.
The modulator should be considered a sealed unit and, if
defective, replaced as a unit. No attempt should be made to
repair the modulator.
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