When a second key is pressed, the time taken by the operator to
release the first key and go to the second key is adequate for the
firmware to make at least one complete keyboard scan-pass, and clear
the keyboard scan-pass flag. The cleared flag allows the micro-
processor to enter the data from the second activated key.
Keyboard Lamp Illumination . After it has been entered into
memory, the microprocessor retrieves the key matrix data and inter-
prets it into row select and column select data for lighting the lamp
associated with that key (see figures FO-8 and FO-9). First, the 1-
of-4 coded row select data are written (using the row strobe) into
output interface CCA A5 (by a set of four microprocessor instructions).
Then, another set of four instructions outputs the binary column se-
lect data. The ensuing column strobe loads the matrix row data that
cause the appropriate lamp to go on.
The microprocessor selects the matrix row first by placing the ap-
propriate 1-of-4 code onto the I/O bus (see table A, figure FO-8).
For example, the lamp associated with the SQL key (matrix position 11)
requires a 0001 (1-to-4) code. Prior to placing these data on the I/O
bus the microprocessor configures the OC0 thru OC3 output lines with a
10112 code. Control CCA A7 translates this code into a true (low)
CS1l output. Then a WRR (microprocessor write) instruction places
0001 onto the I/O bus. The subsequent microprocessor OUT strobe is
combined with CS1l on CCA A5 (U19, U13F) to produce the positive row
strobe. This strobe load the 0001 code (from the I/O bus) onto row
select latch U2A of lamp driver CCA A2.
The next microprocessor instruction set outputs on OC0 thru OC3
10102 code, which is translated into a true (low) CS10 signal by CCA
A7. The microprocessor then outputs the 4-bit binary code for the
TEST TONE key lamp column position (0012). The microprocessor OUT
strobe combines with CS10 on programmer memory CCA A4 (U1-9C) to pro-
duce the negative COLUMN strobe. This strobe loads the I/O bus
column (00012) data into column select decoder U1 on CCA A2. The de-
coder responds to the I/O bus code by routing the output of the row
select latch (loaded previously by the ROW strobe) to the selected
column latch (in this example binary 00012 selects column latch U2B).
The l-of-4 code causes tile column latch to drive one of four Darling-
ton lamp drivers. Code 1 drives lamp driver Q1 low (in this example).
This low provides a current path for approximately +5 V (derived from
lamp intensity regulators on CCA A7), through the SQL lamp, causing
it to light. Tables A and B of figure FO-8 show the lamp driver CCA
A2 hardware response to micrprocessor instructions to light a select-
ed keyboard lamp.
f. Front Panel Control A/D Conversion. ADC and audio CCA A3 con-
verts analog values received from front panel controls into binary
data understood by the microprocessor (see figures 5-8 and FO-10).
The front panel conversion function is performed by a multiplexed
analog-to-digital (a/d) converter, a variable counter, and an A-quad-B
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