FINCH III Rack Control Functions

 FINCH III Rack Control Functions

The FINCH III rack control functions replace the older RCM (Rack Control Module) accessory used in previous systems.

The FINCH III includes two intrinsically safe output ports:

• Thermistor Port — used for connection to a conventional 10-pin API thermistor socket.
• Optic Port — used to process signals for connection to a conventional 6-pin API optic socket.

Optic Operation and Installation

The FINCH III optic rack controller interfaces with the refinery rack control system to control the delivery pump. The system shuts down product flow when the liquid level reaches the High-High (HH) alarm condition detected by FINCH III.

System Operation

The optic system operates using an alternating signal originating from Pin 6 of the 6-pin API optic plug. This signal is processed by the FINCH III and returned to the rack control system through Pin 4 of the API plug.

Connect the API optic socket to the FINCH III according to the following pin assignments.

Optic Socket (6-Pin API)

Optic Socket to FINCH III Connections

Thermistor Operation and Installation

The FINCH III thermistor interface is designed to accommodate up to 8 sensors.
In U.S. installations, only 6 sensors are typically used because Pins 1 and 2 are not utilized.

The FINCH III drives two independent thermistor channels using a low-level AC excitation signal. The controller monitors for a sudden resistance change from the thermistor circuit. When a High-High (HH) level condition is detected, FINCH III signals the rack control system to shut down product transfer.

Thermistor Socket (10-Pin API) Channel Assignments

Channel 1

Pins 5, 3, 1, and 8

Channel 2

Pins 7, 6, 4, and 2

Ground

Pin 10 — Common ground return for all thermistor channels.

Thermistor Socket to FINCH III Connections

The FINCH III is designed to drive two thermistor channels. The API socket pins are grouped as follows:

Original Field Notes Optic Socket Field Notes

API Pin 8: PWR

API Pin 6: OUT

API Pin 4: IN

API Pin 10: GND

API Pin 9: Protective Diode

Thermistor Socket Field Notes

The FINCH III is designed to drive 2 channels, and the pins of the API are jumper wired together as follows:

API Pins / FINCH III Connectors

API Pins 5, 3, 1, and 8: Channel 1

API Pins 7, 6, 4, and 2: Channel 2

API Pin 10: GND

API Pin 9: wired to Pin 9 on Optic socket