Stanford Research Systems SR844 User Manual page 173

Circuit Descriptions 6-7
them powered through Q701. Actually the battery isn't completely disconnected, the
reverse leakage current through D701 trickle charges the battery when the unit is on !
Port addresses are decoded by U501 (display writes), U213A(keypad and knob reads),
U212C (GPIB) and U207C (RS232).
The data bus is connected to the front panel connector JP602 through bi-directional
latches U614 and U615. The data bus is 'read' only for the keypad, it is 'write' for all
the display control. (Reading the knob doesn't require the data bus).
The knob is read as follows. The knob has 4 phases, with combinations of (KNOBA,
KNOBB) = (0,0), (0,1), (1,1), (1,0) for the 4 phases. By reading the state of the
knob every time the knob changes phase, the '186 can keep track of the knob position
and the direction of travel. U610A clocks falling edges of KNOBA, while U610B clocks
rising edges. Either event causes the corresponding -Q to go low, which generates a
knob interrupt (-KNOB_INT = low) via U602A and U901F. Similarly a rising or falling
edge on KNOBB generates a knob interrupt. The processor responds to the knob interrupt
by reading the knob state, which also clears the flip-flops U610 and U611.
The knob interrupt and GPIB interrupt are gated together by U208D and U901A to form
a system interrupt SYS_INT. By issuing a status read -STAT_RD, the processor can
determine the source of the interrupt and at the same time read the knobs.
The speaker is controlled by signals TIMER1 from the '186, which is a square wave that
sets the frequency of sound to be produced, and SPKR_EN, which turns the speaker on
and off. These signals are combined in U602B; if SPKR_EN is high, the output of
U602B turns switch Q705 on/off at the frequency of TIMER1. The speaker LS701 is a
piezoelectric element which forms the load on this switch.
U902 is a GPIB controller, which is connected to the GPIB connector JP902 through line
driver chips U903 and U904. U905 is a UART, it is connected to the RS232 connector
JP903 through buffers U705 and U906. These ports have a clock separate from the
processor clock; the oscillator circuit uses a 3.6864 MHz crystal X902 and an inverter
U701D. JP1000 is the expansion connector for communication between the '186 and the
instrument motherboard.
CPU–9 & CPU–10: Power Supply
The power transformer T1 has two primary coils and three secondaries. The primaries
can be connected in different ways for different AC line voltages, selection is done by a
small card in the power entry module.
The secondaries are hooked up in full-wave rectifier bridges to generate +34V, ±21V,
+11V, ±10V, all unregulated. There are seven regulators attached to the heatsink at the
center of the board. U3 generates +5V for the CPU ('186) circuitry and for the front
panel display. U4 generates +5V for the instrument, ie the motherboard and boards
mounted on the motherboard. U10 generates +8V for the instrument. U5 generates
-7.7V for the instrument (primarily the ECL circuitry). U6 and U8 generate ±12V for
SR844 RF Lock-In Amplifier