41791-8AR Remote I/O Module Installation & Maintenance
Product Introduction
The Allen-Bradley 1791-8AR belongs to the 1791 Block I/O family. Rockwell Automation designed this module for remote and distributed I/O architectures. The module combines eight 120V AC input channels with eight electromechanical relay output contacts. This integrated design reduces panel space and simplifies wiring for automation systems. The module communicates seamlessly with Allen-Bradley controllers including PLC-2, PLC-3, PLC-5, and SLC500 systems through a 1747-SN Remote I/O scanner module. The Allen-Bradley 1791-8AR operates on an external supply of 85-132 VAC at 150 mA.
Core Value
The Allen-Bradley 1791-8AR delivers significant value for industrial automation systems. Firstly, the module eliminates the need for external interposing relays in most applications. The built-in relay contacts provide direct load switching capability. Moreover, the 1791-8AR supports RIO network distances up to 10,000 feet at 57.6K baud with proper termination. This feature enables flexible system layouts across large facilities. Furthermore, the module provides robust isolation between I/O groups and control logic. Specifically, the isolation rating reaches 1250VAC between I/O groups and logic circuits. This isolation ensures stable operation even in electrically noisy industrial environments. The power dissipation of the Allen-Bradley 1791-8AR reaches a maximum of 12.9 Watts, with thermal dissipation at 56.1 BTU/hour.
Technical Insights
The Allen-Bradley 1791-8AR features eight isolated AC input channels with an on-state voltage range of 79 to 132 VAC. The input circuits maintain a low off-state voltage maximum of 35V, which ensures clear signal definition from field devices. Meanwhile, the relay outputs support multiple load types. Specifically, the output ratings include 125 VDC at 1A, 30 VDC at 1A, and 125 VAC at 0.2A for resistive loads.
For network communication, the 1791-8AR utilizes the Allen-Bradley Remote I/O protocol. The module supports three baud rate options: 57.6K, 115.2K, and 230.4K. Network distances depend on the selected baud rate. Specifically, 10,000 feet at 57.6K, 5,000 feet at 115.2K, and 2,500 feet at 230.4K are achievable. The network requires 82-ohm or 150-ohm terminating resistors at each end of the daisy chain for proper signal integrity.
Additionally, the Allen-Bradley 1791-8AR incorporates switch assembly and status indicators for easy diagnostics. The removable terminal strip simplifies wiring connections and facilitates maintenance activities. The module also supports asynchronous data transfer for efficient communication with connected controllers.
Installation & Maintenance
Proper installation of the Allen-Bradley 1791-8AR ensures reliable long-term operation. Mount the blocks vertically with a minimum of 2 inches of clearance around the module. This spacing allows adequate airflow for heat dissipation. Use shielded twisted-pair cables for the RIO network connections. Always ground the shield at only one point to prevent ground loops. Maintain a separation of at least 6 inches between I/O wiring and AC power lines. This practice minimizes electrical interference and improves signal integrity.
For routine maintenance, inspect all terminal screw connections periodically. Loose connections can cause intermittent faults and unreliable operation. Check the status LEDs on the front panel regularly. A steady green indicator confirms normal module operation. Blinking or red indicators signal communication errors or power supply issues. Clean the module using low-pressure compressed air to remove dust accumulation. Avoid using liquid cleaners or solvents on the module surface. Always power down the system before removing or inserting the 1791-8AR module. This precaution prevents damage to both the module and the backplane.
When replacing a failed module, record the existing switch settings first. Set the new module's switches to match the original configuration. This step ensures the new module maintains identical network node addressing and data mapping. Test all input and output channels after replacement. Use a multimeter to verify correct voltage levels on each input point. Actuate each output relay individually to confirm proper switching action. Document all maintenance activities in your plant's log system for future reference.
Buyer’s Guide FAQ
Q1: What communication protocol does the Allen-Bradley 1791-8AR use?
A1: The Allen-Bradley 1791-8AR uses the Allen-Bradley Remote I/O (RIO) protocol. This protocol supports daisy-chain network topologies and integrates with PLC-2, PLC-3, PLC-5, and SLC500 controllers through a 1747-SN scanner module.
Q2: What are the input voltage specifications for the 1791-8AR?
A2: The input channels operate at 120V AC with an on-state voltage range of 79 to 132 VAC. The off-state voltage maximum is 35V. Each input draws approximately 5 mA at nominal voltage.
Q3: Can the 1791-8AR outputs switch DC loads?
A3: Yes, the relay outputs support both AC and DC loads. The ratings include 125 VDC at 1A, 30 VDC at 1A, 125 VAC at 0.2A, and 240 VAC at 0.2A for resistive loads. Always verify load compatibility before connecting field devices.
Q4: How do I troubleshoot a non-responsive 1791-8AR module?
A4: Firstly, verify the power supply voltage is within the 85-132 VAC range. Check the status LEDs for error indicators. Ensure the terminating resistors are properly installed. Verify the node address switches match the scanner configuration. Inspect the network cables for damage or loose connections. Replace the module if these steps do not restore operation.
