FINCH III Rack Control Functions
Built-in rack control, no separate RCM required
The FINCH III rack control functions replace the older RCM (Rack Control Module) accessory used in previous systems, with two intrinsically safe output ports built in.
- 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)
| API Pin | Signal | Description |
|---|---|---|
| Pin 4 | IN | Processed alternating level signal from FINCH III |
| Pin 6 | OUT | Status/command signal from rack controller |
| Pin 8 | PWR | Power supplied from rack to optic sensor |
| Pin 10 | GND | Ground reference common return |
| Pin 9 | Protective Diode | Protective diode connection |
Optic Socket to FINCH III Connections
| API Pin | FINCH III Connection |
|---|---|
| 8 | PWR |
| 6 | OUT |
| 4 | IN |
| 10 | GND |
| 9 | Protective Diode |
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 | Pins |
|---|---|
| Channel 1 | 5, 3, 1, 8 |
| Channel 2 | 7, 6, 4, 2 |
| Ground | 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:
| API Pins | FINCH III Connection |
|---|---|
| 5, 3, 1, 8 | Channel 1 |
| 7, 6, 4, 2 | Channel 2 |
| 10 | GND |