Stanford Research Systems SR844, User Manual

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6-6 Circuit Descriptions
SR844 RF Lock-In Amplifier
Circuit Descriptions
A description of the circuitry on each board follows. The parts lists and schematics for
each board follow the circuit descriptions.
844C: CPU/Power Supply Board
This board is located at the rear of the instrument. The two sections of this board are (a)
the CPU and (b) the Power Supply.
Document Number Sheet Schematic
CPU–1 1 CPU
CPU–2 2 Address and Data Buffers
CPU–3 3 System ROM
CPU–4 4 System RAM
CPU–5 5 System Decode
CPU–6 6 System I/O
CPU–7 7 System Interfaces
CPU–8 8 System Interconnect
CPU–9 9 Unregulated Supplies
CPU–10 10 Power Supplies
CPU–1... CPU–8: CPU System
The host processor in the SR844 is an 80C186 microprocessor (U101) running at 12
MHz. This processor runs the front panel interfaces (keypad, display and knob), controls
the instrument settings, and runs the remote interfaces (GPIB and RS-232). In addition
the ‘186 performs numerous instrument calculations, such as computing calibration
values that are then downloaded to the DSP, doing offset and phase calculations, and
also computing miscellaneous functions such as Xnoise and Ynoise.
U201–U203 are latches for the ‘186 address lines. U204–U205 are bidirectional latches
for the ‘186 data lines. In addition U210 and U209 are gated latches for address and data
respectively that are only enabled for communication to the motherboard, ie when
-PCS4
is low. These separate gated outputs
XA1–XA6
and
XD0–XD7
reduce the
microprocessor noise on the instrument circuit boards. U207, U208 and U211 perform
basic gating so as to generate read/write/select strobes for memory (
-LO_WR,
-HI_WR
), for the motherboard (
-XWR, -XRD
) and for other interfaces (
-SYS_WR,
-SYS_RD, -SYS_DS
). The
-XDT/R
line controls the direction of data to/from the
motherboard.
U303 and U304 are the system boot ROMs, which contain the ‘186 program code and all
the instrument calibration data. The boards are built for normal ROM based operation,
however JP303 allows conversion of the board to emulator operation. U401 and U402
are the system RAM, battery backed up by BT701. When power is on, Q701 is on and
D701 is reverse–biased, which disconnects the battery from the memory chips, leaving