Description
Product Introduction
The Allen-Bradley 1756-OF6CI is a high-accuracy, 6-channel analog current output card designed for ControlLogix chassis. It drives field devices that require precise 0 to 20mA or 4 to 20mA control signals, such as control valve positioners, variable frequency drive speed references, and current-to-pressure (I/P) transducers.
Unlike standard non-isolated analog cards like the 1756-OF8, where channels share a common ground loop, the 1756-OF6CI features full channel-to-channel individual isolation. Every output circuit is electrically isolated from the backplane and every neighboring channel up to 250V continuous, preventing ground loops and stray noise from destroying delicate field instrumentation in harsh industrial environments.
Key Technical Specifications
| Parameter | Value / Specification |
| Outputs | 6 Individually Isolated Current Channels |
| Output Current Ranges | 0 to 20 mA | 4 to 20 mA |
| Resolution | 13 Bits across 21 mA (approx. 2.7 µA/bit) |
| Channel-to-Channel Isolation | 250V Continuous (1350V AC for 1s factory test) |
| Drive Capability | 0 to 750 Ω load resistance per channel |
| Backplane Current (5.1V DC) | 250 mA |
| Backplane Current (24V DC) | 170 mA |
| Power Dissipation | 4.5 W max @ 60°C |
| Thermal Dissipation | 15.3 BTU/hr |
| Module Error Rate | 0.1% of full scale at 25°C |
| Open-Circuit Detection | Yes (Flagged in Studio 5000 status tags) |
| Removable Terminal Block | 1756-TBNH (20-Pin NEMA) or 1756-TBSH (Spring Clamp) |
- 1756-OF6CI
- 1756-OF6CI
Application Scenarios & The “Trench” Experience
A water treatment plant’s dosing system suffers from erratic chemical pump speeds. The 4-20mA signal line keeps drifting, swinging pump output randomly by 15%. Downstream pH levels spike out of spec. You measure the loop with a meter and find a 1.2V DC ground potential difference between the main PLC rack and the chemical skid motor control center 200 feet away. Installing a non-isolated analog module caused a ground loop across the common wire. Swapping in a 1756-OF6CI breaks that ground path with 250V channel-to-channel isolation, restoring steady 4-20mA control instantly.
Target Application Scenarios
- Chemical & Petrochemical Processing: Drives critical control valve actuators across hazardous or distant plant skids with ground potential variances.
- Wastewater Treatment Skids: Manages chemical feed pump speed references and variable frequency drives over long cable runs.
- Power Generation Facilities: Controls turbine governor valves and damper positioners where high electrical EMI/RFI noise is present.
- Paper & Pulp Converting Lines: Provides isolated speed commands to multi-section drive systems where common grounds cause signal crosstalk.
Field Case Study
An oil refinery experienced intermittent 4-20mA signal degradation on a critical steam header control valve. Diagnostic checks revealed that high-frequency noise from a neighboring 480V VFD was coupling into the unshielded analog loop. The installed 1756-OF8 shared a common return line, transmitting that noise to adjacent channels. We supplied a bench-tested 1756-OF6CI module. Once landed on the 1756-TBNH terminal block, the individual channel galvanic isolation blocked the noise coupling, stabilizing the steam valve position and preventing unit shutdown.
Transparency SOP: Quality Assurance & Testing
We don’t just ship boxes; we test them on actual Allen-Bradley racks. Every 1756- module moving through our shop goes through a strict 5-step verification process:
- Inbound Physical Inspection: Examining edge connectors, front plastic latch assemblies, side vents, and checking the internal PCB for shorted component traces.
- Live Rig Chassis Test: Inserting the card into an active 1756 chassis running on 24V DC auxiliary power to verify backplane boot sequence and status LED operation.
- Multi-Point Current Precision Calibration: Wiring each of the 6 isolated channels to a calibrated digital multimeter and 500 Ω precision load. We command 4mA, 12mA, and 20mA outputs via Studio 5000 to verify DAC linearity and loop precision.
- Channel Isolation Verification: Applying test potential across adjacent channel grounds to confirm zero voltage bleed or crosstalk between isolated channels.
- ESD Isolation & Final Packaging: Neutralizing static risks at an ESD-grounded bench before vacuum-sealing the module in anti-static shielding bags with fresh desiccant packs.
The Veteran’s Tech Trap Guide (Crucial Value-Add)
⚠️ Current Only—No Voltage Output: Remember that the “C” in 1756-OF6CI stands for Current. This card ONLY outputs 0-20mA or 4-20mA signals. If you need a 0-10V or ±10V DC analog output signal, you need a 1756-OF6VI (Voltage Isolated) or a 1756-OF8 module. Do not order this expecting a voltage output!
❗ 24V DC Field Power Requirements: The 1756- requires 24V DC supplied through the backplane (170 mA load) or an external supply depending on the chassis configuration. If your chassis power supply (like an older 1756-PA72) is overloaded on its 24V rail, the card will throw an analog backplane power fault. Verify your rack power budget!
⚠️ Watch Out for Open-Loop Faults: If you configure a channel for 4-20mA in Studio 5000 but leave the channel terminals un-wired or open-circuited, the module will light the
OKLED red/flashing and set anOpenWirefault tag. If you aren’t using a channel, disable it in the module properties window to avoid nuisance fault alarms.
Frequently Asked Questions (FAQ for Conversion)
Q: What is the main difference between the 1756-OF8 and the 1756-?
A: The 1756-OF8 provides 8 channels with a single common return (non-isolated between channels), whereas the 1756- provides 6 channels with individual channel-to-channel isolation. The 1756- is designed for harsh, noisy environments where ground loops exist between field devices and the PLC.
Q: Can I output 0 to 10 Volts from a 1756-?
A: No. The 1756- is strictly a current-loop module (0-20mA / 4-20mA). If your field device requires a 0-10V DC reference signal, you must purchase the 1756-OF6VI module or install an external current-to-voltage resistor network.
Q: Is the 1756- hot-swappable in a live chassis?
A: Yes, it supports RIUP (Removal and Insertion Under Power). You can pull and replace the card without interrupting the rest of the ControlLogix backplane or adjacent I/O modules. Always ensure field loop power is safe before disconnecting the terminal block.
Q: What terminal blocks fit the 1756- card?
A: The 1756- uses a standard 20-pin Removable Terminal Block (RTB). You can choose between the 1756-TBNH (screw clamp) or the 1756-TBSH (spring clamp). Terminal blocks are sold separately from the module.
Q: What warranty coverage comes with this module?
A: Every New Surplus and Refurbished 1756- analog current output card carries our full 12-month operational warranty. If the unit experiences hardware failure under normal operating conditions within one year, we issue an immediate replacement or full refund.
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