THEORY OF OPERATION Model 360E Instruction Manual
All four analog outputs are connected back to the A/D converter through a Loop-back circuit. This permits the
voltage outputs to be calibrated by the CPU without need for any additional tools or fixtures.

10.3.6.5. Internal Digital I/O

This channel is used to communicate digital status and control signals about the operation of key components of
the Optical Bench. The CPU sends signals to the synch/demod board that initiate the ELECTRICAL TEST and
DARK CALIBRATION procedures. Likewise, the synch/demod board uses this interface to send the SYNC
warning signal to the CPU (See Sections 6.9.5, 6.9.6 and 11.1.1).

10.3.6.6. External Digital I/O

This External Digital I/O performs two functions.
Status Outputs
Logic-Level voltages are output through an optically isolated 8-pin connector located on the rear panel of the
analyzer. These outputs convey good/bad and on/off information about certain analyzer conditions. They can
be used to interface with certain types of programmable devices (See Section 6.13.1.1).
Control Inputs
By applying +5VDC power supplied from an external source such as a PLC or Data logger (See Section
6.13.1.2), Zero and Span calibrations can be initiated by contact closures on the rear panel.
10.3.7. I2C Data Bus
An I2C data bus is used to communicate data and commands between the CPU and the keyboard/display
interface and the relay board. I2C is a two-wire, clocked, digital serial I/O bus that is used widely in commercial
and consumer electronic systems. A transceiver on the motherboard converts data and control signals from the
PC-104 bus to I2C. The data is then fed to the keyboard/display interface and finally onto the relay board.
Interface circuits on the keyboard/display interface and relay boards convert the i2c data to parallel inputs and
outputs. An additional, interrupt line from the keyboard to the motherboard allows the CPU to recognize and
service key presses on the keyboard.
Power up Circuit
This circuit monitors the +5V power supply during start-up and sets the Analog outputs, external digital I/O ports,
and I2C circuitry to specific values until the CPU boots and the instrument software can establish control.
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