Stanford Research Systems SR844, User Manual

Page: 174 / 246

6-8 Circuit Descriptions
SR844 RF Lock-In Amplifier
the fan and the RS232 port. U9 gen25V for the instrument. All the instrument
power goes out on JP4 to the motherboard 84MBD.
The power reset circuit works as follows. On power-up C7 is initially discharged, which
means
-PWR_RESET
is low when U612 has enough voltage to operate. Q3 turns on
quickly and shuts Q4 off, leaving C4 to be charged up (it takes about 200 ms) by R6. At
this point
-PWR_RESET
goes high and the power-up cycle is complete. At power-
down, C7 is initially charged, which means
-PWR_RESET
is high. When the power is
shut off,
+5V_P
falls faster than than C9 can discharge, as a result of which the base of
Q3 is pulled below 0V momentarily, shutting off Q3. But
+5V_P
is still above 4.3V,
which allows Q4 to turn on and discharges C7. This causes
-PWR_RESET
to go low
well before the voltage has dropped below the circuit’s operating point.
844S: Display Board
The front panel is assembled as a sandwich. The front panel metal is in front, then the
conductive rubber keypad with all the keys, then the 844K Keypad board with the key
contacts and shorter LEDs, and at the back the 844S Display board. 844S contains the
logic for driving the front panel LEDs (on the back of the board) and the taller LEDs (on
the front of the board).
Document Number Sheet Schematic
844S1 1 Display Drivers
844S2 2 Display LEDs
Data lines, control signals and power all come from the CPU/Power Supply board 844C
on connector JP4.
LED’s are addressed using multiplexed row/column addressing. The eight columns are
turned on in rotation by signals
STRB0-STRB7
. If row #R is on when column #C is on,
then the LED at (#R, #C) is illuminated. Each strobe lasts approximately 2 ms. At the
beginning of each strobe, data words are written for all the rows. The clocks
ODD
and
EVEN
write the rows for the 7 segment displays (
EVEN0-EVEN7
and
ODD0-ODD7
),
while clocks
LED_CLK0
and
LED_CLK1
write the rows for the individual and bar-graph
LEDs (
LED0-LED29
).
On each cycle, the current strobe word is stored in register U7. One bit is low at a time.
If
Q5
is low, transistor Q1C is turned on through N1.5. The rest of the transistors in
arrays Q1 and Q2 are kept off by the remaining high bits in U7. Q1C’s collector sources
current into all LED’s on column
STRB4
that have their row turned on.
On each cycle, the control bits for rows
LED0-LED7
are stored in register U9. If output
Q2
(U9.18) is high, the corresponding transistor in U2 is turned on through N5.2. The
collector of that transistor sinks current from line
LED1
through the LED at row #1 and
the currently active column. N9.2 is a current-limiting resistor. The other LED rows are
similar.