Allen-Bradley 1746-NO8I Analog Output Module – SLC 500

Allen-Bradley 1746-NO8I: 8-Channel Isolated Analog Output Module for SLC 500 Backplane Architecture

The Allen-Bradley 1746-NO8I is a single-slot, 8-channel isolated analog output module engineered for the SLC 500 modular programmable controller platform. Each output channel operates with full galvanic isolation from adjacent channels and from the backplane bus, a design decision that directly eliminates ground-loop interference in multi-loop process installations. The module supports both current (4–20 mA, 0–20 mA) and voltage (0–10 VDC, ±10 VDC) output modes, selectable per channel via hardware jumper configuration, giving process engineers the flexibility to interface with a broad range of field actuators — proportional control valves, variable-frequency drives, electro-pneumatic positioners, and analog-input transmitters — without external signal conditioning hardware.

At 12-bit resolution across a ±10 VDC or 20 mA full-scale span, the 1746-NO8I delivers a theoretical step size of approximately 4.88 mV or 4.88 µA per count. In practice, this resolution is sufficient for Class 1 process control loops where valve positioning accuracy of ±0.1% full scale is required. The module draws 250 mA from the 5 VDC backplane rail and 75 mA from the 24 VDC rail, placing it within the power budget of all standard SLC 500 chassis configurations from the 4-slot 1746-A4 through the 13-slot 1746-A13.

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Technical Parameters

Hardware Logical Analysis

Per-Channel Galvanic Isolation Architecture
The 1746-NO8I implements isolation at the individual channel level rather than at the module boundary. Each of the eight output channels incorporates a dedicated DC/DC converter and optocoupler stage. The DC/DC converter generates a floating supply rail for the channel’s DAC and output driver, while the optocoupler transfers the digital control word across the isolation barrier without a conductive path. This topology means that a fault condition — including a wiring short to earth or a high-potential transient on one field loop — cannot propagate to adjacent channels or back to the SLC 500 backplane. In installations where multiple field devices share a common panel earth but originate from different process areas with differing ground potentials, this architecture prevents circulating currents that would otherwise introduce a DC offset error into the analog signal.

12-Bit DAC and Output Stage
The digital-to-analog conversion is performed by a 12-bit DAC per channel. The DAC output feeds a precision voltage-to-current converter for current-mode operation, or a unity-gain buffer amplifier for voltage-mode operation. The output stage is short-circuit protected on the current output; if the load impedance drops below the minimum threshold, the driver limits current rather than entering a latch-off state, which preserves loop continuity during transient wiring faults. The voltage output stage includes slew-rate limiting to suppress high-frequency noise injection into sensitive field wiring.

EMC Design and Backplane Signal Integrity
The module’s PCB layout routes the isolated channel supplies and signal paths away from the backplane data bus traces. Decoupling capacitors are placed at each power entry point to suppress conducted emissions from the switching DC/DC converters. The module housing is grounded to the chassis rail, providing a Faraday shield effect against radiated emissions from adjacent high-power modules. In environments with significant variable-frequency drive switching noise (typically 2–20 kHz PWM harmonics), the isolated channel architecture attenuates common-mode interference by a factor consistent with the isolation barrier’s common-mode rejection specification.

Backplane Communication Protocol
The 1746-NO8I communicates with the SLC 500 processor via the SLC backplane parallel bus. The processor writes output data to the module’s output image table (O-file) during the output scan phase of the program cycle. The module latches the new data word and initiates a DAC update within one backplane cycle. For a typical SLC 5/04 processor running a 10 ms scan time, the worst-case output update latency is one scan period plus the DAC settling time (typically <1 ms to 0.1% of full scale), yielding a total control loop delay well within the requirements of most process control applications.

System Integration Benefits

• Zero ground-loop error in multi-loop installations: Per-channel isolation eliminates the DC offset errors caused by ground potential differences between field devices, a common failure mode in large panel installations where cable runs exceed 50 m.
• Single-slot density for 8 analog outputs: The 1746-NO8I occupies one chassis slot, compared to discrete relay or signal conditioner solutions that require dedicated DIN rail space and additional wiring terminations. This reduces panel footprint and wiring labor cost.
• Deterministic output update timing: Output data is refreshed every processor scan cycle via the SLC backplane bus. There is no asynchronous communication overhead or token-passing delay, which ensures that PID loop output commands reach the field device within a bounded, predictable time window.
• Dual-mode output flexibility without external hardware: Current and voltage output modes are selected by on-board jumpers per channel. Changing output mode requires no additional signal conditioner, isolator, or converter module, reducing BOM cost and panel complexity.
• Diagnostic transparency via processor fault file: The SLC 500 processor’s I/O fault file (S:5) captures module communication errors and out-of-range conditions. Maintenance personnel can read fault codes directly from the RSLogix 500 online monitor without physical access to the panel.
• Wide thermal operating range for harsh environments: The 0–60 °C operating range covers the majority of industrial enclosure environments without forced cooling. Storage to −40 °C supports cold-climate logistics and spare-parts storage in unheated facilities.
• Compatibility across the full SLC 500 processor family: The module is compatible with all SLC 500 fixed and modular processors from the SLC 5/01 through SLC 5/05, including the SLC 5/04 with DH+ and the SLC 5/05 with EtherNet/IP, without firmware modification or adapter hardware.
• Short-circuit protected current outputs for loop continuity: The current output driver limits rather than shuts down under short-circuit conditions, maintaining loop power and allowing the processor to detect the fault condition via the output image table rather than losing communication with the field device entirely.
• Reduced commissioning time with RSLogix 500 auto-configuration: When the module is inserted into a configured chassis, RSLogix 500 automatically populates the I/O configuration tree with the correct module type, channel count, and data file mapping, eliminating manual address assignment errors during commissioning.

Quality Assurance & Global Logistics

Every Allen-Bradley 1746-NO8I supplied by siemensplc.com is sourced through verified industrial distribution channels and undergoes a structured pre-shipment inspection protocol. Physical inspection covers housing integrity, label authenticity, connector pin condition, and firmware revision marking against OEM reference documentation. Functional verification confirms that the module powers up correctly and that backplane communication is established before packaging.

Units are packed in anti-static ESD shielding bags, placed in foam-lined cartons, and sealed with tamper-evident tape. Each shipment includes a siemensplc.com inspection record with the unit serial number, inspection date, and inspector ID. A 12-month operational warranty covers manufacturing defects from the date of shipment.

Logistics operations are based in Xiamen, China. In-stock units ship within 1–3 business days of payment confirmation via DHL Express, FedEx International Priority, or UPS Worldwide Expedited. Export documentation — commercial invoice, packing list, certificate of origin, and ESD compliance certificate — is prepared as standard for all international shipments. HS code classification assistance and export licensing guidance are available on request for regulated-destination shipments. Bulk orders of 5 or more units qualify for consolidated freight pricing and priority allocation from reserve stock.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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