1791-8AC | Allen-Bradley Block I/O Module, 120V AC, 16 Points: 8 Inputs, 8 Outputs

The Allen-Bradley 1791-8AC is a block I/O module designed for use in distributed control systems via the Remote I/O (RIO) network. It provides a compact and integrated interface for eight 120V AC input devices and eight 120V AC output actuators. The module supports real-time data exchange with PLCs like the PLC2, PLC3, PLC5, and SLC500 (with 1747-SN scanner), enabling reliable input monitoring and output control. The RIO interface facilitates daisy-chain connections, reducing wiring complexity in large systems. This makes the 1791-8AC particularly useful in older legacy systems where central controllers are distributed across long production lines. It includes power supply, signal conditioning, and RIO interface in one device. The module enhances modularity, allowing field I/O expansion without occupying valuable rack space. Its simplicity and robustness make it ideal for heavy-duty industrial use.

The maximum output current of the 1791-8AC depends on mounting orientation and ambient temperature. For vertical mounting, it supports 300 to 600 mA per output at 30–60°C. For horizontal mounting, the current is reduced to 150 to 300 mA per output in the same temperature range. The module is capable of handling 10A surge current for 50 milliseconds, recurring every 2 seconds. This enables brief handling of high inrush loads such as relay coils or incandescent lamps. Maintaining current within the specified range ensures reliable operation and protects the output transistors. Overcurrent conditions may trigger protective circuitry or degrade the module over time. Therefore, output devices must be sized and wired appropriately. Always consider derating curves if operating near the limits of temperature or current.

Yes, the 1791-8AC module fully supports Allen-Bradley’s Remote I/O (RIO) protocol, which allows it to function as a distributed I/O adapter in legacy PLC systems. The RIO protocol uses a 3-wire Belden 9463 cable (or equivalent) to connect multiple I/O modules in a daisy chain topology. This protocol is widely used with PLC2, PLC3, and PLC5 controllers, as well as SLC500 systems using the 1747-SN scanner. The module sends input status and receives output commands through the RIO link at configurable baud rates. It also supports block transfer reads (BTR) and writes (BTW), allowing up to 32-word messages for configuration and data exchange. Integration is straightforward due to built-in signal conditioning and interface logic. This makes the 1791-8AC a reliable option for expanding I/O in Remote I/O-based architectures.

The 1791-8AC features an OFF to ON delay of 1 millisecond and an ON to OFF delay of 8.3 milliseconds at 60 Hz. These response times are typical for AC output modules and suitable for most industrial automation needs. The short delay allows the outputs to respond quickly to control commands, enabling smooth actuator operation. The slightly longer OFF delay is due to the nature of AC signal zero-crossing detection, which adds a small processing buffer. These delays are generally acceptable for controlling solenoids, contactors, and indicator lights. Applications requiring high-speed response should consider this timing characteristic in their sequence logic. The module’s response consistency enhances predictability and control stability. For critical timing, consider aligning your PLC logic scan with output reaction times.

No, the 1791-8AC is no longer in active production and is considered part of Allen-Bradley’s legacy hardware portfolio. While it is not manufactured anymore, it remains in widespread use in older installations using Remote I/O (RIO) infrastructure. Replacement units or refurbished modules may still be available through third-party suppliers or surplus equipment vendors. Rockwell Automation may offer migration paths to modern Ethernet-based I/O platforms like POINT I/O or Compact 5000. Users are encouraged to consult Rockwell’s Product Lifecycle Status portal for the most current status. For critical systems, stocking spare units may be advisable to ensure long-term operability. Modern alternatives may require protocol conversion or system redesign. Continued support for this module depends on your system integrator’s or supplier’s available inventory and repair services.