Agilent 34970A/34972A Data Acquisition / Switch Unit

Chapter 5 Theory of Operation
Switch Modules
In addition to the +5 Volt power supply (Vcc) and ground, the module
controller uses four lines for control and communication:
• RESET, from the Earth Referenced Logic A1U305. RESET is common
to all three slots. The module controller performs a reset when this
line goes high. Reset conditions vary for each plug-in.
• SRQ, to the Earth Referenced Logic A1U305. The SRQ line is a
wired-OR line that can be driven by any plug-in. Consequently, any
module that asserts SRQ (line low), asserts this line in all other slots
and at the Earth Reference Logic.
• DATA IN, from the Floating Logic A1U205 via the opto-isolator
A1U312. This line is connected in common to all three slots.
• DATA OUT, from the module controller to the Floating Logic A1U205
via the opto-isolator A1U213. This line is a wired-OR line that can be
driven by any module.
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 65) and the thermocouple reference junction
temperature corrections (see page 122). 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.
5
Module Reference
149