PLC I/O Modules | Allen-Bradley 1791-NDC
The Allen-Bradley 1791-NDC is a 24 VDC analog block I/O module designed for distributed automation architectures. Featuring 4 analog inputs and 2 current outputs, this unit operates as a self-contained remote I/O node complete with internal signal conditioning and programmable controller interface circuitry. The 1791-NDC connects directly to Remote I/O (RIO) scanner networks and requires an external 24 VDC nominal supply for internal logic and loop power.
System Integration and Architecture
The module interfaces with Allen-Bradley PLC-2, PLC-3, PLC-5, and SLC 500 automation platforms over standard Remote I/O communications links. To host programmable controllers or network scanners, the unit appears as a 1/4 I/O rack node, occupying 2 words of input image table memory and 2 words of output image table memory. Network baud rates and node addressing are configured via front-accessible DIP switches.
Analog Input and Output Capabilities
The 4 analog input channels support selectable single-ended or differential operation. When operating in current mode, an integrated 249 ohm precision shunt resistor converts 0-20 mA current signals for processing. Channel update timing stands at 108 ms per input channel, with common-mode rejection reaching -75 dB.
The 2 dedicated current output channels deliver 0-20 mA control signals with a 13-bit output resolution and standard internal update rates of 10 ms per channel. The analog outputs provide short-circuit protection and an absolute accuracy rating of 0.1% at 25 deg C.
Mechanical Specifications and Thermal Limits
The housing utilizes removable front wiring connectors to allow pre-wiring and rapid field replacement. Proper installation requires a minimum 2.0 inch (50.8 mm) air gap on all four sides during horizontal panel mounting to maintain adequate thermal dissipation.
| Parameter | Specification |
|---|---|
| Model | 1791-NDC |
| Brand | Allen-Bradley |
| Product Type | Analog Block I/O Module |
| Input Channels | 4 Analog (Voltage or Current via internal 249 ohm shunt) |
| Output Channels | 2 Current (0-20 mA) |
| Nominal Supply Voltage | 24 VDC (19.2-30 VDC operating range) |
| Maximum Supply Current | 600 mA |
| Power Dissipation | 11.52 W max |
| Thermal Dissipation | 39.28 BTU/hr max |
| Input Update Rate | 108 ms per channel |
| Output Update Rate | 10 ms per channel |
| Absolute Accuracy | 0.1% at 25 deg C |
| Weight | 1.00 lbs (0.45 kg) |
| Dimensions | 6.95 x 2.71 x 3.85 inches (176.5 x 68.8 x 98 mm) |
The Allen-Bradley 1791-NDC combines 4 analog inputs and 2 current outputs in a compact block form factor for remote PLC expansion.
Installation and Grounding Requirements
When mounting the 1791-NDC to an equipment panel, connect the ground strap directly to the external grounding stud on the lower left of the chassis. Tighten the equipment grounding stud nut to a maximum torque of 15 in-lbs (1.7 N-m) to prevent structural damage. Maintain signal wiring within the +10 V common-mode voltage limit relative to analog ground to ensure valid channel sampling.
Frequently Asked Questions
Q: What is the primary difference between the 1791-NDC and 1791-NDV models? A: The 1791-NDC provides 2 current outputs (0-20 mA), whereas the 1791-NDV features 2 voltage outputs. Both modules use a 24 VDC power supply and offer 4 analog inputs.
Q: How does the module map into the PLC processor image table? A: The block configures as 1/4 I/O rack on the Remote I/O link, consuming 2 input words and 2 output words in the controller image table.
Q: Can 2-wire current loop transmitters be wired directly to this block?A: Yes, terminal 25 on the block provides a +24 VDC loop power voltage source to power 2-wire current transducers.
PLC I/O Modules | Allen-Bradley 1791-NDC
PLC I/O Modules | Allen-Bradley 1791-NDC
The Allen-Bradley 1791-NDC is a 24 VDC analog block I/O module designed for distributed automation architectures. Featuring 4 analog inputs and 2 current outputs, this unit operates as a self-contained remote I/O node complete with internal signal conditioning and programmable controller interface circuitry. The 1791-NDC connects directly to Remote I/O (RIO) scanner networks and requires an external 24 VDC nominal supply for internal logic and loop power.
System Integration and Architecture
The module interfaces with Allen-Bradley PLC-2, PLC-3, PLC-5, and SLC 500 automation platforms over standard Remote I/O communications links. To host programmable controllers or network scanners, the unit appears as a 1/4 I/O rack node, occupying 2 words of input image table memory and 2 words of output image table memory. Network baud rates and node addressing are configured via front-accessible DIP switches.
Analog Input and Output Capabilities
The 4 analog input channels support selectable single-ended or differential operation. When operating in current mode, an integrated 249 ohm precision shunt resistor converts 0-20 mA current signals for processing. Channel update timing stands at 108 ms per input channel, with common-mode rejection reaching -75 dB.
The 2 dedicated current output channels deliver 0-20 mA control signals with a 13-bit output resolution and standard internal update rates of 10 ms per channel. The analog outputs provide short-circuit protection and an absolute accuracy rating of 0.1% at 25 deg C.
Mechanical Specifications and Thermal Limits
The housing utilizes removable front wiring connectors to allow pre-wiring and rapid field replacement. Proper installation requires a minimum 2.0 inch (50.8 mm) air gap on all four sides during horizontal panel mounting to maintain adequate thermal dissipation.
| Parameter | Specification |
|---|---|
| Model | 1791-NDC |
| Brand | Allen-Bradley |
| Product Type | Analog Block I/O Module |
| Input Channels | 4 Analog (Voltage or Current via internal 249 ohm shunt) |
| Output Channels | 2 Current (0-20 mA) |
| Nominal Supply Voltage | 24 VDC (19.2-30 VDC operating range) |
| Maximum Supply Current | 600 mA |
| Power Dissipation | 11.52 W max |
| Thermal Dissipation | 39.28 BTU/hr max |
| Input Update Rate | 108 ms per channel |
| Output Update Rate | 10 ms per channel |
| Absolute Accuracy | 0.1% at 25 deg C |
| Weight | 1.00 lbs (0.45 kg) |
| Dimensions | 6.95 x 2.71 x 3.85 inches (176.5 x 68.8 x 98 mm) |
The Allen-Bradley 1791-NDC combines 4 analog inputs and 2 current outputs in a compact block form factor for remote PLC expansion.
Installation and Grounding Requirements
When mounting the 1791-NDC to an equipment panel, connect the ground strap directly to the external grounding stud on the lower left of the chassis. Tighten the equipment grounding stud nut to a maximum torque of 15 in-lbs (1.7 N-m) to prevent structural damage. Maintain signal wiring within the +10 V common-mode voltage limit relative to analog ground to ensure valid channel sampling.
Frequently Asked Questions
Q: What is the primary difference between the 1791-NDC and 1791-NDV models? A: The 1791-NDC provides 2 current outputs (0-20 mA), whereas the 1791-NDV features 2 voltage outputs. Both modules use a 24 VDC power supply and offer 4 analog inputs.
Q: How does the module map into the PLC processor image table? A: The block configures as 1/4 I/O rack on the Remote I/O link, consuming 2 input words and 2 output words in the controller image table.
Q: Can 2-wire current loop transmitters be wired directly to this block?A: Yes, terminal 25 on the block provides a +24 VDC loop power voltage source to power 2-wire current transducers.
