The 1791 Block I/O series is a series of small I/O devices that each include a power supply, a programmable controller interface, input and output connections, and circuitry. These devices come in small and compact designs. They have been designed and distributed by Allen-Bradley. The Allen-Bradley 1791 Block I/O series devices include 2 different kinds of blocks, and these are the 1791-IOBA block that comes with 8 inputs and 8 outputs, and the 1791-IOBB block that has 10 inputs and 6 outputs. The specifications are the same for both devices. These 2 blocks are compatible with different programmable controllers from the Allen-Bradley PLC-2, PLC-3, PLC-5, and the SLC 500 families of controllers. Physically, these blocks are composed of a DH-485 port, switch assemblies, status indicators, communication and power LEDs, removable remote I/O connectors, and removable input and output connectors.

The Allen-Bradley 1791 Block I/O devices provide all the functions of different equipment in a single terminal. These include the functions of an I/O rack and module, adapter, and a power supply. Both blocks require a 24 Volts DC power supply that has current limiting capabilities. The voltage that is employed must not fall outside the range of 20.5 to 27.6 Volts DC. The Allen-Bradley 1791 Block I/O must be installed in a ventilated area. This must be done to avoid overheating and enable the blocks to have safe operations. A minimum of 2 inches must be left between different blocks and the air temperature between the blocks must not exceed 55 degrees Celsius. The blocks must be mounted horizontally.