Application Scenarios
In a chemical plant reactor feed line, a Rosemount multi-variable flow transmitter outputs a 4-20 mA signal proportional to mass flow while also carrying HART data for static pressure and temperature. The control system needs all three values for accurate mass flow compensation, but running separate analog cables for each variable would require additional transmitters and wiring. The 1756-IF8H solves this by reading the primary 4-20 mA signal for flow control while simultaneously extracting the secondary and tertiary HART variables from the same two wires, delivering pressure and temperature to the ControlLogix processor without additional field devices. This capability is particularly valuable in brownfield upgrades where existing HART instruments are already installed but their digital data remains trapped in the field.
Parameter
| Parameter | Value/Description |
|---|---|
| Product Model | 1756-IF8H |
| Manufacturer | Allen-Bradley / Rockwell Automation |
| Product Category | ControlLogix Analog Input Module (HART-capable) |
| Input Channels | 8 differential inputs |
| Input Signals | ±10V, 0-5V, 1-5V, 0-10V, 0-20 mA, 4-20 mA |
| Isolation | 50 V continuous between inputs |
| HART Protocol | Integrated; supports PV, SV, TV, FV polling per channel |
| Backplane Current | 300 mA at 5.1V DC; 135 mA at 24V DC |
| Data Format Options | Analog only; Analog + HART PV; Analog + HART by Channel |
| Configuration Software | RSLogix 5000 / Studio 5000 (v15+ for base HART; v18+ for unicast) |
| Mounting | ControlLogix chassis, any slot |
| Country of Origin | Mexico |
Technical Principles and Innovative Values
- Dual-Signal Acquisition Without External Multiplexers: The 1756-IF8H performs simultaneous analog-to-digital conversion of the 4-20 mA loop signal and FSK demodulation of the HART digital carrier on the same pair of wires. This eliminates the need for external HART multiplexers or secondary input cards, reducing panel space and wiring complexity in installations where both control and diagnostic data are required.
- Per-Channel HART Tag Grouping: With the “Analog and HART by Channel” input data format, all data related to a single field device—the scaled analog value, PV, SV, TV, FV, and device status—is grouped in one structure. This organization simplifies programming and troubleshooting: an engineer investigating a flow transmitter reads one tag structure instead of hunting through multiple arrays for the associated HART variables.
- Broken Wire and HART Fault Detection: The module reports broken wire conditions per channel (current below expected threshold) and dedicated HART fault bits when communication with the field device degrades. These diagnostics allow maintenance teams to distinguish between a failed analog signal and a HART communication issue, enabling targeted field checks rather than blanket instrument replacement.
Reviews
There are no reviews yet.