5 Theory of Operation
148 Keysight 34970A/34972A Service Guide
The DATA IN and DATA OUT lines are optically isolated from the floating logic
controller. These lines communicate with the Floating Logic using an
asynchronous serial bit stream.
The serial communications use an 11 bit protocol; a start bit, 8 data bits, an
attention bit, and a stop bit. The attention bit is 1 if the 8 data bits are an address/
command, or 0 if the 8 data bits modify or provide data for the previously sent
command.
The module controller uses the hardwired slot-ID bits to decode the serial bit
stream address. When the address/command message address matches the
slot-ID, the plug-in is selected and responds to the following commands. All other
plug-ins will ignore the commands until a new address/command message is
received.
A 12 MHz crystal, Y101 is the clock for the module controller. The module ID is a
four bit pattern set through R106, R107, R108, and R109. The ferroelectric RAM
U150 provides data storage of the relay usage on the module (see page 58) and
the thermocouple reference junction temperature corrections (see page 119).
Data in and out of U150 is serial.
The 34901A, 34902A, and 34908A each have two onboard, solid state
temperature sensors, U151 and U152, physically located near the isothermal
block at the input connections. The temperature sensors are used as the
thermocouple temperature reference.
U101 controls the relays on the module using an 8-bit data bus and three control
lines. The data lines are latched and applied to the relay drivers.
U101 enters a low-power idle mode when inactive. U101 responds when a
command is received or when a scheduled reference junction temperature
measurement is taken.
The relays use a buffered +5 Volt power supply. U101 supplies two drive enable
lines (DR_EN and +5NL_EN) that connect Vcc from the digital bus with the relay
drive lines through Q101 (+5R or +5NL). To minimize the current through DGND
caused by static discharge, the ground return (ZGND) is isolated from the
backplane ground through a bead L102.
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