MKS Granville-Phillips 358 series, Instruction Manual

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Theory of Operation
Series 358 Micro-Ion Controller Instruction Manual - 358013 - Rev. B
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7.3 Microcontrollers and Bus
Structure
The electrometer module in the controller has a dedicated microcontroller
with internal ROM, RAM, timing, and interrupt management functions. This
architecture provides high performance at low cost with greater reliability
and noise immunity than more complicated microprocessor systems using
external buses and memory hardware.
The microcontroller is equipped with a watchdog timer, which
automatically generates a reset if the processor fails to fulfill timing
“checkpoints” within its code. Interprocessor communication is
accomplished via the display bus. These lines carry BCD-format pressure
data that is used to generate the controller display.
7.4 Capacitance Manometer
Theory of Operation
Within the capacitance manometer, a diaphragm is distorted by the
pressure of the gas in the system under measurement. This diaphragm forms
part of a capacitor, and its deflection causes changes in capacitance. Thus,
the electrically measured capacitance is a measure of pressure. The device
is very sensitive to the elastic properties of the metal of the diaphragm. For
this reason, large pressure excursions, such as occur when the system is
raised to atmospheric pressure, can cause offsets to the pressure reading.
The diaphragm is also extremely sensitive to temperature effects, and
although it may be held in a temperature controlled chamber, this
temperature control is never perfect, resulting in further perturbations to the
devices theoretical accuracy.
Note that these perturbations are inherent in the capacitance manometer
design and are not a property of the electronic module used to operate the
transducer.
Capacitance manometers are capable of exceptional accuracy, and read
pressure independent of gas type, but are also subject to zero point drift,
and must be calibrated at vacuum frequently if high accuracy is to be
obtained. Refer to the manual for your transducer for instructions.
7.5 Process Control Theory of
Operation
The process control module contains a dedicated microcontroller and a
non-volatile memory chip for storage of the setpoints. The microcontroller
compares the setpoints with the pressure display data on the display bus
and makes a decision as to whether or not to activate a channel's relay.