Allen-Bradley 1771-ASB ASCII Serial Interface Module

The Allen-Bradley 1771-ASB Remote I/O Adapter Module is a DH+/serial interface that plugs into a 1771 Universal I/O backplane to link a PLC-2/PLC-3/PLC-5 processor with remote I/O blocks. As a slave on the remote I/O network, it receives and transmits data between the I/O scanner and individual remote modules in serial form, enabling legacy AB I/O to be distributed across the plant floor. Designed for use in classic Allen-Bradley SLC- and PLC-5 ecosystems, the ASB supports ASCII-style communication with attached I/O and peripherals via the Data Highway Plus link, with a 150-ohm termination resistor typically used at the ends of the DH+ network. The module resides in the Universal I/O chassis, providing a compact, backplane-mounted solution for expanding a controller’s I/O footprint without replacing the processor. Typical applications include maintaining or upgrading PLC-2/PLC-5 lines that require remote I/O expansion, integrating legacy peripherals such as printers or bar code scanners that communicate over serial ASCII, and preserving plant topology where a dedicated remote I/O scanner manages distributed devices. In retrofit contexts, the 1771-ASB serves as the bridge between older controllers and remote devices, enabling serial data transfer within the AB ecosystem while keeping the hardware footprint minimal.