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Theory of Operation
HARDWARE IMPLEMENTATION
Firmware
All operations of the Model 1140A are controlled by firmware flashed onto the digital board. The
firmware performs the following tasks:
•
Manages the user interface including the front-panel controls and display.
•
Monitors the temperature of the instrument and each front-panel terminal.
•
Monitors battery state.
•
Controls the output of the digital-to-analog converter (DAC) on the analog board.
•
Manages instrument alignment.
•
Controls the meter-mode feedback loop.
•
Logs meter-mode readings for later retrieval.
•
Manages the remote interface.
The firmware communicates directly with the remote interface plugged into the digital board.
The presence of the interface board is detected at power up and the appropriate configuration
made at that time. The remote interface is periodically checked for traffic and the appropriate
response made.
The battery state is monitored via the battery charge controller on the power supply board. All
other tasks are performed via the analog board.
Front-panel Assembly
The front-panel assembly contains the output binding posts, thermocouple connector, display,
KEYPAD, and ENCODER. Its function is to provide mechanical support for these components. It
also contains the faceplate and front-panel legend.
The keypad board contains the keypad controller, which scans the KEYPAD for key presses or
ENCODER rotations and sends that information to the digital board firmware via a ribbon cable.
Analog Assembly
In response to firmware commands, the analog assembly produces output voltages, measures the
temperature of the analog board, measures the temperature of each of the binding posts and
thermocouple connector pins, measures the meter-mode input, measures the alignment system
output, and stores alignment data in nonvolatile memory.
System firmware commands are received through a serial link that passes through isolators on
the power-supply board. The firmware also retrieves readings from the analog board via this
same link. An absent or unplugged analog assembly is detected by the firmware and an error
message displayed at startup.
Storage of alignment data is done in EEPROM memory located on the analog assembly.
Communication with this memory is done via 6-pin ribbon cable J10 directly from the digital
assembly. If this memory malfunctions or fails to be detected at power-up, an error is displayed.
Source Mode
In the Model 1140A, source-mode operation is accomplished using a voltage reference, digital-
to-analog converter (DAC), and output buffer (see Figure 7-2).
7-4
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