844K: Keypad Board; 84Rto: Rear Panel And Ttl Out Board - Stanford Research Systems SR844 User Manual

U0 is a retriggerable monostable multivibrator. While triggered, the output -Q keeps the
strobe and LED row registers enabled. If no strobe comes in 6 ms, the multivibrator
returns to quiescent state, with -Q high, and all registers are disabled. With registers
disabled, all row and column transistors are off. This serves as a protection mechanism,
since neither the transistors nor LED's are rated for continuous operation. Without this
protection, an error that caused the '186 to hang could also burn out the front panel.
The front panel keys are addressed by an extension of the row/column scheme used for
LEDs. The same 8 columns are used (STRB0–STRB7) and there are 6 key rows KEY0–
KEY6. If the key at column STRB3, row KEY2 is closed (button pressed), the line KEY2
is driven high during the STRB3 cycle. In this way reading the keys once each cycle
allows the host '186 processor to determine which keys are pressed. U1 is an input latch
read once during each column strobe. The latch inputs are the KEY rows mentioned.
N13 is an array of pull-down resistors that ensures that the KEYs are read as zero when
they are not activated.
JP5 is the knob connector. The knob is an optical encoder that is read directly by the
host '186 processor. It requires no support circuitry on 844S.
Connectors J1 and J2 distribute the strobe, LED row, and Key row signals to the 844K
Keypad board.

844K: Keypad Board

This board is sandwiched between the 844S Display board and the front panel metal, as
described in the preceding section. The Keypad board contains mini-LED's and keypad
contacts that all have to be mounted close to the front panel metal. The keys and LED's
are addressed by a column/row scheme described in the preceding section. All row and
column lines come from 844S on connectors J6 and J7.

84RTO: Rear Panel and TTL Out Board

The Rear Panel Board 84RTO consists of two pieces, the larger of which is soldered
directly to the BNC connectors on the rear panel. The smaller is a detachable daughter
card that contains the drivers for the TTL output drivers.
Interconnect Board
This is the larger board mentioned above. Five of the six BNCs are wired directly from
the BNC connector to ribbon cable connector J876. The interconnect board also has
decoupling capacitors between the BNC shields and chassis ground.
For the TTL output, power and signals from J876 are wired to the daughter card attached
to J877. The TTL output from the daughter card is connected to the rear panel BNC out
J874. Note that on this board, the ground for the TTL output circuitry is not connected
Circuit Descriptions
SR844 RF Lock-In Amplifier
6-9