1791-8AC AC Input Module – Installation & Configuration Guide
Product Introduction
The Allen-Bradley 1791-8AC belongs to the 1791 Block I/O series. This module operates on 74-132V AC, 50/60 Hz. Each of the eight inputs is optically isolated. Moreover, the 1791-8AC includes built-in DeviceNet communication. You do not need a separate adapter. The module enclosure meets NEMA Type 12K standards. Furthermore, the operating temperature ranges from 0°C to 60°C. The 1791-8AC also features LED indicators for each channel. These lights show input status clearly. Many engineers choose this block for distributed control applications.
Core Value
This input module delivers several practical benefits. Firstly, it eliminates many individual wires. Each 1791-8AC connects to the network using a single cable. Secondly, the module saves control cabinet space. The compact design mounts directly near sensors. Thirdly, installation costs drop significantly. Meanwhile, the 1791-8AC offers robust electrical isolation. This feature protects your PLC backplane from surges. Besides that, the module performs automatic diagnostics. It detects field power loss and reports errors. Consequently, you reduce troubleshooting time. The Allen-Bradley 1791-8AC also integrates seamlessly with any DeviceNet master. This includes CompactLogix, ControlLogix, and SLC 500 scanners.
Technical Insights
Specifically, the Allen-Bradley 1791-8AC uses a 10-pin removable terminal block. Each input has two terminals: Line (L) and Neutral (N). The input impedance measures 15 kΩ typical. The minimum ON voltage is 60V AC. The maximum OFF voltage is 20V AC. Moreover, the input response time is 35 ms maximum. This suits slow-speed machinery perfectly. The module draws 160 mA from the DeviceNet network. Additionally, the 1791-8AC requires an external 120V AC field supply. Do not power inputs from the network. The module communicates using Polled or Cyclic I/O connections. The input data occupies 1 byte in the scanner's memory. Each bit corresponds to one input point.
Installation & Maintenance
Follow these steps to install the 1791-8AC correctly. Firstly, mount the module on a DIN rail or directly to a panel. Use the four mounting holes for screws. Secondly, connect the DeviceNet cable to the 5-pin header. Match the colors: black for V-, red for V+, and bare for shield. Thirdly, set the node address using the two rotary switches. The valid range is 0 to 63. Therefore, ensure no other device uses the same address. Then, apply 120V AC to the field power terminals. Wire each AC input to a sensor or push button. The common terminal connects to Neutral. For maintenance, check the LEDs regularly. A solid green network light means normal operation. A flashing light suggests a duplicate node address. Clean the module with dry compressed air only. Replace the 1791-8AC if you see burn marks on the terminal block.
Buyer’s Guide FAQ
1. What PLC systems work with the Allen-Bradley 1791-8AC?
Any DeviceNet scanner works with this module. Specifically, 1756-DNB for ControlLogix, 1769-SDN for CompactLogix, and 1747-SDN for SLC 500.
2. Can I mix the 1791-8AC with DC input blocks on the same network?
Yes, you can. DeviceNet allows mixing different I/O types. The Allen-Bradley 1791-8AC operates alongside 1791-16AC, 1791-16AD, or third-party DeviceNet slaves.
3. How do I address the eight inputs on the 1791-8AC?
The scanner maps inputs sequentially. For example, the first 1791-8AC occupies byte 0, bits 0-7. The second module occupies byte 1, bits 0-7.
4. What happens if field power drops on the 1791-8AC?
The module sends a specific fault bit to the scanner. The network LED remains green, but the module LED turns red. Check the 120V AC supply immediately.
5. Does the 1791-8AC support auto-baudrate detection?
Yes, it does. The module detects the DeviceNet baudrate automatically. You do not need manual switches. The supported rates are 125k, 250k, and 500k baud.
