Allen-Bradley 1771-A2B PLC-5 Series 2-Slot I/O Chassis – 1771 Backplane for 1771 Digital & Analog Modules

Product Overview

The Allen-Bradley 1771-A2B​ is a 2-slot I/O chassis belonging to Rockwell Automation’s classic 1771 I/O™ product family, designed primarily for use with PLC-5® programmable logic controllers and SLC 500® remote I/O adapter stations. The Allen-Bradley 1771-A2B​ provides a rugged, die-cast aluminum enclosure with an integral 1771-series backplane that supplies communication, addressing, and +5 VDC logic power (when paired with a 1771 power supply such as the 1771-P6S or 1771-P7) to up to two 1771 I/O modules mounted in the chassis. One additional dedicated slot at the left end accommodates a 1771 power supply module, which energizes the backplane and powers the logic circuitry of all installed I/O modules.In a typical PLC-5 architecture, the Allen-Bradley 1771-A2B​ may serve either as a local I/O rack adjacent to a PLC-5 processor (e.g., 1771-L5 series) or as a remote I/O rack connected via a 1771-ASB/B Series adapter module plugged into one of its I/O slots. The chassis backplane automatically passes rack address, slot select, and I/O data between the adapter/processor and each module, requiring no user configuration of DIP switches for slot addressing—slot position alone determines the I/O group address within the PLC-5 data table (subject to half/full-slot addressing mode set at the processor). The Allen-Bradley 1771-A2B​ is also compatible with 1771-series intelligent modules, analog input/output cards, and high-speed counter modules, making it a versatile building block for both legacy and sustained-support control systems.Engineered for industrial durability, the Allen-Bradley 1771-A2B​ features a painted steel / aluminum housing, keyed module guides to prevent incorrect insertion, and provisions for panel or 19-inch rack mounting. It remains a widely specified spare and replacement item for plants maintaining PLC-5 infrastructure, and is fully interoperable with the extensive 1771 I/O module ecosystem that has been deployed worldwide since the 1980s.

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Description

 

Technical Specifications

Product Model 1771-A2B
Manufacturer Rockwell Automation / Allen-Bradley
Product Type 1771-Series 2-Slot I/O Chassis (with separate power supply slot)
Number of I/O Module Slots 2 (plus 1 dedicated slot for 1771 power supply at left end)
Compatible I/O Modules All Allen-Bradley 1771 digital, analog, specialty, and intelligent modules
Required Power Supply Any 1771-series power supply (e.g., 1771-P6S, 1771-P7, 1771-P10) – NOT included
Backplane Voltage (from PS) +5 VDC logic (typical 5.1 VDC ±5%), +24 VDC aux available from certain PS models
Backplane Current Capacity Dependent on installed 1771 power supply (typically 8–16 A @ 5V DC)
Slot Addressing Automatic by physical position; Half-slot (1–16 pt) or Full-slot (1–8 pt) determined by processor config
Chassis Material Die-cast aluminum with painted finish
Mounting Panel mount or 19-inch EIA rack mount (using optional 1771-MT1/-MT2 kit)
Dimensions (H × W × D) Approx. 177 mm × 267 mm × 146 mm (6.97″ × 10.5″ × 5.75″) including PS slot
Operating Temperature 0 °C to +60 °C (+32 °F to +140 °F)
Relative Humidity 5% to 95% non-condensing
Certifications UL Listed, CSA Certified, CE Marked (with appropriate modules), FM Cl. I Div. 2
Weight Approx. 2.3 kg (5.1 lb) empty

 

Main Features and Advantages

Purpose-built for the 1771 ecosystem:The Allen-Bradley 1771-A2B​ is precision-engineered with the proprietary 1771 backplane edge-connector pattern, ensuring correct signal routing for I/O data, rack addressing, and logic power distribution. Every slot in the Allen-Bradley 1771-A2B​ is mechanically keyed to accept only 1771-series modules, preventing accidental insertion of incompatible cards and protecting both the module and backplane from damage.Compact footprint for small I/O sets:With only two I/O slots plus a dedicated power supply slot, the Allen-Bradley 1771-A2B​ is ideal for remote I/O drops, small machine cells, or localized analog monitoring points where a full 8- or 16-slot chassis would be wasteful. Its modest width conserves panel space in crowded enclosures while still delivering the full feature set of the 1771 platform including interrupt-capable modules and intelligent I/O.Automatic slot-based I/O addressing:The Allen-Bradley 1771-A2B​ backplane hardwires slot position to the I/O scan sequence. The PLC-5 processor or remote I/O adapter automatically assigns data-table words based on which physical slot a module occupies and whether half-slot or full-slot addressing is selected. This eliminates manual address dials and reduces wiring/configuration errors during system commissioning or module replacement.Rugged industrial construction:The cast-aluminum frame of the Allen-Bradley 1771-A2B​ provides excellent electromagnetic shielding and physical protection in harsh plant environments. Heavy-duty module-retention latches and guide rails ensure modules seat squarely onto the backplane edge connectors and remain secure under vibration. The chassis is designed for decades of service life, which is why many Allen-Bradley 1771-A2B​ units installed in the 1990s remain in daily operation today.Legacy system continuity:For facilities committed to maintaining PLC-5 assets until a phased migration can be executed, the Allen-Bradley 1771-A2B​ offers a readily available, authentic chassis replacement. Keeping spare Allen-Bradley 1771-A2B​ chassis on the shelf is a common practice among asset managers in continuous-process industries to minimize mean-time-to-repair (MTTR) on legacy lines.

Application Field

The Allen-Bradley 1771-A2B​ is deployed wherever 1771-series I/O modules need to be housed for connection to a PLC-5 processor or a SLC 500 / ControlLogix remote I/O adapter (1771-ASB/B). Common applications include discrete manufacturing cells in automotive tier‑supplier plants, batch process areas in pharmaceutical and chemical facilities, material handling conveyor zones with remote I/O drops, and utility / auxiliary systems in power generation stations that continue to run on PLC-5 control platforms. In these environments, the Allen-Bradley 1771-A2B​ typically appears as a small local rack near a machine station—populated perhaps with a 1771-IBD (24V DC input) and a 1771-OW16 (relay output) plus a 1771-P6S power supply—linked back to the main processor via a 1771-ASB remote I/O adapter in the first I/O slot.System integrators also specify the Allen-Bradley 1771-A2B​ for brownfield expansions where adding a full-size 8- or 16-slot chassis is impractical due to panel space or cost constraints. During phased migration projects, the Allen-Bradley 1771-A2B​ continues to serve as the physical host for existing 1771 analog and specialty modules that may be retained even as the main controller is upgraded, often interfacing through a 1756-1771-A (ControlLogix 1771 I/O adapter) until full I/O replacement is completed. Its proven reliability and backwards compatibility make the Allen-Bradley 1771-A2B​ an enduring component of Rockwell Automation’s legacy support portfolio.

Contact Us WhatsApp / Wechat：+86 18150087953
Phone：+86 18150087953
Email：sales@cxplcmro.com

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