Overview
The Allen-Bradley 1791-N4C2 is a compact analog I/O combination Block module designed for distributed I/O control in legacy and mid-range PLC systems. This 6-point analog module combines 4 configurable analog inputs and 2 current outputs in a single unit, making it ideal for applications requiring mixed analog signal processing. The module mounts in any CompactBlock I/O rack and communicates via the standard Remote I/O (RIO) network protocol, ensuring broad compatibility with classic Allen-Bradley PLC architectures.
Key Features
- 6 analog I/O points (4 inputs, 2 current outputs per block)
- High 0.1% absolute accuracy at 25°C with 0.05% linearity
- Configurable analog inputs: voltage or current signals
- Output: 0–20 mA current signals
- Input impedance: 10 MΩ; output impedance <1 Ω
- Onboard 24V DC source (100 mA) for loop-powered field transmitters
- Fast output update: 10 ms per output channel
- Vertical or horizontal mounting orientation
Technical Specifications
The 1791-N4C2 Block I/O provides the analog signal conditioning and conversion required for process control and data acquisition tasks. Its remote I/O connectivity eliminates the need for expensive control cabinet space, reducing installation costs and improving system flexibility.
- Power Supply: 120V AC nominal (accepts 85–132V AC)
- Analog Inputs: 4 selectable (voltage or current) with 108 ms update rate per channel
- Analog Outputs: 2 current outputs (0–20 mA) with 10 ms update rate
- Accuracy: ±0.1% full scale at 25°C
- Input Impedance: 10 MΩ
- Output Impedance: <1 Ω
- Common-Mode Rejection: −75 dB
- Operating Temperature: 0–60°C (32–140°F) in air gap below module
- Dimensions: 6.95″H × 2.71″W × 3.85″D (176.5 × 68.8 × 98 mm)
- Weight: 1.2 lbs
- Communication Protocol: Remote I/O (RIO)
- Mounting: Vertical (recommended) or horizontal in CompactBlock I/O racks
Typical Applications
The 1791-N4C2 is well-suited for distributed I/O applications where analog signal processing is required away from the main PLC enclosure. Common uses include:
- Process measurement and monitoring (temperature, pressure, flow)
- Analog output control for drives, valves, and variable-frequency devices
- Remote data acquisition in multi-zone production systems
- Legacy system retrofits and PLC-5/SLC 500 distributed I/O networks
- Compact industrial environments where space is limited
Compatibility & Replacements
Compatible With
- PLC-2, PLC-3, PLC-5 series (racks 01–03 for PLC-5/15 and PLC-5/20; racks 01–07 for PLC-5/25 and PLC-5/30; racks 01–17 for PLC-5/40; racks 01–27 for PLC-5/60; racks 00–37 for PLC-5/250)
- SLC 500 controllers (via 1747-SN Scanner module series A)
- CompactBlock I/O distributed I/O racks
- Remote I/O (RIO) communication networks
Replaces / Drop-In Replacement For
- 1791-N4C2/A (alternate part number designation)
- Direct 1:1 replacement for existing 1791-N4C2 units in legacy systems
Works With
- Allen-Bradley CompactBlock I/O system
- Remote I/O (RIO) network protocol
- Rockwell Automation legacy PLC software (PLC-5 and SLC 500 programming environments)
- SCADA and data acquisition software compatible with RIO-based I/O
- Field devices: loop-powered transmitters, analog sensors, 4–20 mA instrumentation
