Allen-Bradley 1756-DH485 Communication Interface Module

The Allen-Bradley 1756-DH485, also cataloged as the 1756-DH485 communication interface module, serves as the primary DH-485 network gateway interface unit utilized to execute protocol bridging between ControlLogix Logix5000 controllers and legacy DH-485 device networks. It provides deterministic backplane-to-fieldbus conversion for serial network integration within 1756 chassis architectures.

Suffix Breakdown & Model Matrix

• Base model: 1756-DH485 communication module

• No functional suffix segmentation defined

• Configuration is fixed to DH-485 protocol implementation within ControlLogix platform

Hardware Specifications

Backplane Bus Communication and DH-485 Protocol Bridging Behavior

The module executes protocol translation between ControlLogix backplane messaging structures and DH-485 serial frame formats. Data encapsulation is handled through internal buffering aligned to Logix5000 task scheduling cycles.

EtherNet/IP is not natively involved in DH-485 transport; instead, RS-485 physical layer signaling governs field communication timing. Deterministic access is maintained via token-passing behavior inherent to DH-485 topology, limiting collision conditions across up to 32 nodes.

Backplane communication is synchronized with chassis scan cycles, ensuring consistent update mapping between controller tags and legacy device registers without external protocol conversion hardware.

Frequently Asked Questions (FAQ)

Q: Does the 1756-DH485 support Ethernet/IP communication directly?
A: No. The module only provides DH-485 protocol connectivity and relies on backplane integration for Logix controllers.

Q: Can the module operate on multiple DH-485 channels simultaneously?
A: Yes. It supports two DH-485 communication channels within a single module architecture.

Q: Is hot-swap supported during active network communication?
A: Yes. The module supports hot-swapping within a ControlLogix chassis, but communication sessions will reset during replacement.

Field Installation Guidelines

Install the module into a certified 1756 ControlLogix chassis slot with confirmed backplane alignment. Ensure full connector seating to maintain stable DH-485 routing via internal interface circuits.

Use shielded twisted-pair RS-485 cable for DH-485 network connections. Maintain continuous shield termination and single-point grounding to reduce noise coupling across legacy serial segments.

Route DH-485 cabling separately from high-voltage switching and motor drive lines. Verify node addressing and baud rate consistency across all devices before initiating network communication.