Allen-Bradley 1746-IN16 PLC Input Module – SLC 500

Allen-Bradley 1746-IN16: 16-Point 24V AC/DC Discrete Input Module for SLC 500 Control Architecture

The Allen-Bradley 1746-IN16 is a 16-channel discrete input module engineered for the SLC 500 programmable controller platform. Operating across a unified 24V AC/DC input range, this module consolidates mixed-signal field wiring into a single backplane slot — a design decision that directly reduces panel footprint and simplifies I/O mapping in ladder logic programs. Its optical isolation barrier, rated at 500V AC, decouples the field-side wiring from the SLC 500 backplane bus, protecting the processor from transient surges common in motor-driven and solenoid-heavy environments.

In a discrete control loop, the 1746-IN16 occupies the signal acquisition layer: it conditions raw field-device states — proximity switches, limit switches, push buttons, float switches, and pressure contacts — into logic-level signals that the SLC 5/01 through 5/05 processors can act upon within a single scan cycle. With a backplane current draw of 75 mA at 5V DC, the module imposes minimal load on the chassis power supply, making it suitable for high-density rack configurations where power budgeting is a constraint.

The module’s dual AC/DC input architecture is not a marketing simplification. Internally, the input circuit uses a full-wave rectifier bridge ahead of the optical coupler, allowing the same input terminal to accept either a 24V AC sinusoidal signal or a 24V DC level signal without hardware reconfiguration. This is particularly valuable in retrofit projects where legacy AC field wiring coexists with newer DC sensor networks — both can terminate on the same 1746-IN16 without rewiring the field cabinet.

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Technical Parameters

Hardware Logical Analysis

The 1746-IN16’s input circuit architecture follows a three-stage signal conditioning path. At the field terminal, an RC filter network attenuates high-frequency noise and contact bounce — a critical function in environments where inductive loads switch frequently on shared ground rails. The filtered signal then passes through a full-wave bridge rectifier, which is the mechanism enabling the module’s dual AC/DC compatibility. The rectified DC output drives the anode of an optocoupler LED; the phototransistor on the logic side generates the binary state that the SLC 500 processor reads via the backplane.

The optical isolation stage is not merely a safety feature — it is an EMC design element. By breaking the galvanic path between field wiring and the backplane, the module prevents common-mode noise from coupling into the 5V logic bus. In installations near variable-frequency drives (VFDs) or large contactors, this isolation is the primary defense against spurious input transitions that would otherwise cause false logic states in the control program.

The module’s ON-state threshold of 14V (for both AC and DC) provides a 10V margin below the nominal 24V supply, accommodating voltage drop across long cable runs — a practical consideration in large machine tools or distributed panel layouts where field wiring may span 50–100 meters. The OFF-state maximum of 5V ensures that leakage current from solid-state sensors (which can present residual voltage even when nominally off) does not cause false ON states.

From a backplane protocol perspective, the 1746-IN16 communicates with the SLC 500 processor via the proprietary SLC backplane bus. The processor reads the 16-bit input image word from the module’s I/O image table address (I:x.0, where x is the slot number) on every scan. The module does not support block-transfer operations — all 16 points are mapped directly to the input image file, which means scan-cycle latency is deterministic and bounded by the processor’s scan time rather than any asynchronous transfer mechanism.

System Integration Benefits

• Deterministic scan-cycle input capture: All 16 input states are latched into the I/O image table synchronously at the start of each scan, ensuring that the logic program always operates on a consistent snapshot of field conditions — no mid-scan state changes that could cause race conditions in sequential control logic.
• Mixed AC/DC field wiring on a single module: Eliminates the need for separate AC and DC input modules in retrofit projects, reducing slot count and simplifying the I/O address map in RSLogix 500.
• 500V AC optical isolation: Provides a defined isolation barrier that meets IEC 61131-2 requirements for discrete input modules, supporting compliance documentation for machinery safety assessments.
• Low backplane power consumption (75 mA @ 5V DC): Allows high-density rack configurations — a fully populated 13-slot 1746-A13 chassis can accommodate multiple 1746-IN16 modules without exceeding the 1746-P4 power supply’s 5V output capacity.
• Direct RSLogix 500 I/O configuration: The module is natively recognized by RSLogix 500 and RSLogix Micro without requiring custom add-on profiles or EDS files, reducing commissioning time.
• Diagnostic transparency via input image file: The 16-bit input word is directly readable in the processor’s data file, enabling straightforward fault diagnostics using forced I/O and online monitoring without additional diagnostic software.
• Wide operating temperature range (0–60°C): Suitable for installation in non-climate-controlled enclosures in light industrial environments, reducing HVAC requirements for the control panel.
• Standardized SLC 500 form factor: Single-slot module with standard 1746 terminal block wiring, compatible with existing panel layouts and spare-parts inventories in facilities already operating SLC 500 systems.
• Long field deployment history: The 1746 I/O family has been in continuous industrial service for over two decades, with extensive application documentation, known failure modes, and a large base of trained maintenance personnel — reducing mean time to repair (MTTR) in production environments.
• Supports both sinking and sourcing field devices: The AC/DC input circuit is compatible with NPN (sinking) and PNP (sourcing) 24V DC sensors, as well as dry-contact and wet-contact AC field devices, without requiring jumper reconfiguration.

Quality Assurance & Global Logistics

Every 1746-IN16 unit supplied by our team is sourced from verified Allen-Bradley / Rockwell Automation authorized distribution channels or from traceable surplus inventory with documented provenance. Before shipment, each module undergoes a functional power-on verification procedure: input terminals are exercised across the full 16-point range, optical isolation integrity is confirmed, and backplane connector pins are inspected for mechanical damage and oxidation.

Units are shipped from our warehouse in Xiamen, China, with access to major international freight lanes via Xiamen Gaoqi International Airport and Xiamen Port. Standard international shipments are dispatched within 1–3 business days of order confirmation. Expedited DHL Express and FedEx International Priority services are available for time-critical maintenance situations, with transit times of 3–5 business days to most destinations in Europe, North America, Southeast Asia, and the Middle East.

All shipments include commercial invoice, packing list, and certificate of origin documentation. For projects requiring export compliance documentation, ECCN classification records are available upon request. A 12-month warranty covers all units against manufacturing defects and functional failure under normal operating conditions. Extended warranty terms are negotiable for volume procurement contracts.

Contact Information

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