Theory of Operation 5
Keysight 34970A/34972A Service Guide 163
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 RP102. The ferroelectric RAM U102 provides data
storage of the calibration constants for the analog output channels. Data in and
out of U102 is serial.
8 data lines, connected to U101 port 1, are used by the digital input and output
ports. One of the data lines is used to send serial data to the analog output
channels. The totalizer edge count is controlled by U101 P0.6 and read at U101
P3.4.
U101 enters a low-power idle mode when inactive. U101 responds when a
command is received or when a scheduled alarm scan is needed.
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