Overview
The Allen-Bradley 1791-N4C2 is a 6-point analog block I/O module designed for legacy remote I/O distribution on classic Allen-Bradley programmable controller networks. This compact block operates on 120V AC and delivers 4 configurable analog inputs and 2 current outputs in a single self-contained package with integrated power supply, signal conditioning, and RIO network interface. The module is discontinued by the manufacturer but remains in service across maintenance and retrofit applications.
Key Features
- 4 Configurable Analog Inputs: Each input can be set as voltage (single-ended or differential) or current input to accommodate diverse transducer types
- 2 Current Outputs: 0–20 mA rated outputs with update rate of 10 milliseconds per channel
- High Accuracy: 0.1% absolute accuracy at 25°C with 0.05% linearity
- Loop-Powered Support: Integrated 24V DC current-limited source for loop-powered (2-wire) transmitters
- Remote I/O Communication: Block transfer or discrete transfer via Allen-Bradley I/O networks; compatible with PLC-2, PLC-3, PLC-5 block attachment, and SLC 500 via 1747SN remote I/O scanner
Technical Specifications
- Input Impedance: 10 megohms
- Output Impedance: Less than 1 ohm
- Input Accuracy: 0.1% at 25°C; linearity 0.05%
- Common-Mode Rejection: −75 dB
- Input Update Rate: 108 milliseconds per channel
- Output Update Rate: 10 milliseconds per channel
- Operating Temperature: Air gap below module must not exceed 60°C (140°F)
- Dimensions: 6.95″H × 2.71″W × 3.85″D (176.5 mm × 68.8 mm × 98 mm)
- Weight: 1.20 lbs
Typical Applications
- Analog signal acquisition and output on legacy distributed I/O networks
- Temperature, pressure, and flow measurement interfaces
- Process control loop feedback and control signal distribution
- Retrofit expansion of analog I/O on PLC-2/PLC-3/PLC-5 systems
Compatibility & Replacements
Compatible With
- PLC-2 family (requires 1771SN sub-I/O scanner or 1772SD/SD2 remote I/O scanner)
- PLC-3 family (requires 1771SN sub-I/O scanner or 1772SD/SD2 remote I/O scanner)
- PLC-5/250 and PLC-5 family (block attaches directly to controller)
- SLC 500 / SLC 5/02 and later (requires 1747SN remote I/O scanner)
- Remote I/O networks using block transfer or discrete transfer protocols
Replaces / Drop-In Replacement For
- 1791-N4C2 (exact replacement for existing installations)
- Other 1791 series analog blocks where 4-input/2-current-output configuration is required
Works With
- Allen-Bradley Remote I/O (RIO) network protocols
- Block transfer communication method
- Discrete transfer communication for SLC controllers
- Classic Allen-Bradley PLC programming software (RSLogix 5000 for documentation/commissioning)
