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C (for Common) and NC (for normally closed) are the two sides of the switch inside. At low pressure, the switch is closed.
Connect the terminals to a ground pin on the DB-25 connector, and to an appropriate digital input pin (it doesn't matter which of C or NC goes to which of ground or input).
In tyour program, the closed swith appears as a 1, changing to a 0 when the pressure gets near 115 psi. It stays open (0) until the pressure drops to is it 85 psi, then goes to 1, again.
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