Allen-Bradley 1746-BAS SLC 500 Data Collection Module

The 1746-BAS​ is a programmable BASIC language module manufactured by Allen-Bradley, a brand of Rockwell Automation, for the venerable SLC 500 family of small programmable controllers. Unlike a conventional I/O or communication module, the 1746-BAS​ is an intelligent co-processor that runs user-written BASIC programs entirely independent of the SLC 500 processor’s ladder-logic scan. Seated in any available slot of a 1746 modular chassis or a 1747 fixed-style SLC, the 1746-BAS​ communicates with the processor across the 1746 I/O backplane while simultaneously managing two front-panel serial ports and a DH-485 network port. This dual role—local data exchange plus external serial communication—is what made the module uniquely valuable in an era before industrial Ethernet became ubiquitous.At the heart of the 1746-BAS​ is a 24 KB battery-backed RAM that stores both the user’s BASIC application and the data it manipulates, preserving everything through power cycles. The module’s two configurable serial channels, PRT1 and PRT2, support RS-232/423, RS-422, and RS-485 electrical standards at baud rates from 300 to 19,200 bps in full duplex. PRT2 additionally supports the DF1 protocol for modem and remote-terminal communication, while the dedicated DH-485 port integrates the module into the plant-wide DH-485 network through a 1747-AIC link coupler. A real-time clock and calendar, accurate to ±1 minute per month at 25 °C, timestamps data and schedules report generation, and eight front-panel LEDs provide immediate visual diagnostics of module activity, port traffic, faults, and battery condition.The enduring appeal of the 1746-BAS​ lies in the freedom it gave engineers to solve problems that ladder logic was never designed to handle: parsing ASCII strings from barcode readers, formatting labels on industrial printers, polling weigh scales, calculating checksums, and implementing custom or proprietary serial protocols for third-party instruments. In installations that still run today, the module often performs duties now handled by a protocol gateway—except that the 1746-BAS​ does it inside the chassis, powered by the backplane, with direct access to the controller’s data table. Rockwell Automation has since classified the 1746-BAS​ as a mature product and no longer manufactures it, but the module remains widely available through aftermarket channels and continues to operate reliably in thousands of legacy SLC 500 systems worldwide.