Product Overview

The Allen-Bradley 1771-A4B operates as a high-capacity, heavy-duty 16-slot universal input/output chassis designed for the classic Rockwell Automation PLC-5 control platform. This backbone enclosure provides the physical mounting structure, data communication pathways, and regulated power distribution lines necessary to operate complex industrial automation systems. Featuring a rugged backplane rated for up to 24 Amps of continuous current at 5.25 VDC, the 1771-A4B links central processor modules, remote I/O adapters, and high-density discrete or analog 1771 modules together. Heavy manufacturing, automotive assembly lines, and water treatment utilities utilize this physical matrix to orchestrate thousands of distributed field sensors and machinery loops from a centralized control cabinet location.

Technical Specifications

Engineering Advantages

High-Current 24A Backplane Prevents Intermittent Logic Drops

Populating a large 16-slot chassis with high-density analog modules, thermal sensors, and remote network scanners creates massive electrical loading on standard 5V supply paths. The 1771-A4B resolves this issue by incorporating a reinforced 24 Amp bus network across its entire physical length. This heavy copper backplane topology keeps supply voltages locked within strict tolerances under maximum load conditions, eliminating the thermal stress, signaling jitter, and unexpected processor drops that common low-amp legacy frames generate.

Flexible Half-Slot Addressing Multiplies Channel Population Capacity

To optimize data density without requiring expansive physical footprint re-engineering, the 1771-A4B backplane infrastructure supports 1/2-slot addressing configurations. This flexible grouping format maps any pair of physical chassis slots into a single input/output image byte, effectively allowing a single 16-slot frame to supervise up to 4 complete logical I/O racks. This configuration maximizes processing density inside existing industrial wall enclosures, cutting additional cabinet purchase costs.

Form-Enforced Plastic Keying Strips Block Incorrect Module Swapping

Field technicians executing rapid repairs under plant breakdown conditions can accidentally slide a high-voltage AC input module into a slot wired for low-voltage analog transmitters, risking catastrophic component destruction. The 1771-A4B chassis provides designated keying tracks along its backplane connector border that accept up to 36 precise plastic keying bands. These physical indicators match specialized notches on individual 1771 cards, preventing unauthorized or mismatched modules from seating into live slots.

FAQs

• Which external power supply options interface directly with the 1771-A4B chassis?

  The 1771-A4B chassis does not include an onboard power supply and requires an external source to energize the 5.25 VDC logic bus. You can insert an in-rack power supply module (such as the 1771-P4R or 1771-P7 rated for 3 to 8 Amps) directly into any slot position excluding the leftmost slot. Alternatively, for maximum slot availability, you can wire external panel-mounted power supplies rated from 6.5 to 16 Amps directly to the terminal connections located on the chassis chassis.

• How do the slot allocation rules restrict where I can mount a processor or communication adapter?

  The 1771-A4B strictly reserves the leftmost slot position for the master system coordinator. You must insert either your main PLC-5 processor card or a remote I/O adapter module (such as a 1771-ASB card linking back to a master PLC via a remote data highway) into this primary location. Peripheral components—such as standard digital inputs, analog outputs, network scanners, and in-rack power cards—can occupy all other slots from slot 1 through slot 15.

• What special ground bonding hardware comes included for electrical safety?

  To repel severe electromagnetic fields (EMI) generated by heavy manufacturing motors and high-voltage contactors, each brand new 1771-A4B shipping kit includes dedicated structural grounding hardware. The kit provides one cup washer, one star washer, four flat washers, and two #10 to 32 nuts fitted with captive lock washers. You must bond this terminal directly to the main copper grounding bus bar of your control enclosure to ensure stable backplane signaling.