Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
64-character space that may be loaded with messages via
the GPIB.
Table 5-9
CONTROL PORT
Bit
Function
0 Readout on/off
1
Address/data
2
Page 1/page 2
3 Max Span dot
Bit 3 controls the frequency dot marker. This bit is set in
MAX SPAN mode to position the frequency dot with BFRD
TUNE VOLTS from the first local oscillator. When cleared,
this bit centers the frequency dot on the spectrum display.
Address/Data Port. The microcomputer loads charac
ters for crt display through the address/data port. Each
character requires four write cycles:
1 ) bit 2 in the control port is set for an address transfer;
2) the address in character RAM is sent to the
address/data port;
3) bit 2 in the control port is cleared; and
4) the data is sent to the address/data port. The bits are
defined in Table 5-10. Bits 0—5 are the lower six bits of the
character RAM address or are the ASCII code for the
character.
Table 5-10
ADDRESS/DATA PORT
Bit
Function
0—5
Address of ASCII code
6
Vertical shift
7
Blank
Bit 6 , when set, shifts upper readout characters to the
center of the crt (and lower readout characters off the bot
tom of the screen). This may be used with page 2 characters
to give prominence to a message sent to the operator.
Bit 7 is used to reduce readout display overhead. It is set
when a space is transferred to the character RAM so the
readout does not steal time from the spectrum trace to scan
a blank. When set, this bit prevents the GEN ON flip-flop
from gating R/O OFF low through U2032C.
Frequency Dot Marker
The frequency dot marker is refreshed immediately after
the last character position in the lower readout is scanned.
Normally, the marker is centered on the screen just below
the upper readout as a pointer for the center frequency
readout. When MAX SPAN is selected, however, the dot
marker moves to a point on the display that corresponds to
the center frequency value.
The negative transition of TOP triggers the marker gen
erator. A simplified diagram of the circuit and its timing is
shown in Fig. 5-23.
U2042A delays the marker dot to allow retrace while gat
ing FREQ DOT low to set up the display. FREQ DOT
affects the readout deflection outputs in the following ways.
The horizontal output is connected either to ground for a
center-screen dot or BFRD TUNE VOLTS for a max span
pointer. TUNE VOLTS is proportional to the center-frequen-
cy readout offset from the center of the frequency range.
Bits 4 and 5 at the vertical D/A input are asserted to shift
the crt beam below the upper readout (the row counter in
puts are zeros).
R/O OFF is gated low to switch the deflection amplifier
inputs for a display using the marker dot horizontal and ver
tical signals.
REV AUG 1981
5-53
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