Allen-Bradley 1756-DHRIO Replacement for ControlLogix

Product Overview

Allen-Bradley 1756-DHRIO Replacement for ControlLogix: Seamless DH+/RIO Migration & Upgrade

The Allen-Bradley 1756-DHRIO is a ControlLogix-platform DH+/RIO Bridge Scanner Module designed to connect modern ControlLogix controllers to legacy Data Highway Plus (DH+) networks and Remote I/O (RIO) links. As Rockwell Automation progressively phases out older SLC 500, PLC-5, and 1771 I/O architectures, the 1756-DHRIO has become the critical bridge module enabling engineers to retain existing field wiring, remote I/O drops, and DH+ peer-to-peer communication while migrating the controller backbone to the current ControlLogix platform. This page provides complete compatibility, replacement, and upgrade guidance for industrial facilities planning a smooth transition with minimal production downtime.

Compatibility Comparison Table

Seamless Upgrade Solutions

A successful migration from a PLC-5 or SLC 500 system to ControlLogix using the 1756-DHRIO typically involves coordinating several interconnected components. The 1756-DHRIO module itself occupies a single slot in a 1756-A7 or 1756-A10 chassis, sharing the backplane with the primary 1756-L73 or 1756-L74 ControlLogix controller. Engineers replacing a legacy 1785-KA5 DH+ interface card will find that the 1756-DHRIO replicates both DH+ channels and RIO scanner functionality in a single module, eliminating the need for separate bridge hardware.

On the power side, the existing 1756-PA72 or 1756-PB72 power supply module continues to serve the chassis without modification, as the 1756-DHRIO draws standard ControlLogix backplane current. For facilities retaining legacy 1771 Remote I/O drops — such as 1771-OBD discrete output modules or 1771-IFE analog input modules — the 1756-DHRIO’s RIO scanner port connects directly to the existing Belden 9463 blue-hose cable, preserving field wiring investments entirely.

Where the legacy system used a 1747-KE DH+/RS-232 interface on an SLC 500 rack for SCADA polling, the 1756-DHRIO assumes that role within the ControlLogix chassis, allowing the SCADA host to continue communicating over the same DH+ segment without reconfiguration. For EtherNet/IP connectivity to the plant network, a companion 1756-EN2T or 1756-EN2TR Ethernet module is typically added to the same chassis, enabling dual-network access — DH+ legacy and EtherNet/IP modern — simultaneously.

During commissioning, engineers commonly use a 1784-U2DHP USB-to-DH+ programming cable to verify DH+ node addresses and confirm that all legacy PLC-5 nodes on the network are visible from Studio 5000. For facilities with DeviceNet or ControlNet segments adjacent to the DH+ network, a 1756-DNB DeviceNet Bridge or 1756-CNB ControlNet Bridge module can be added to the same 1756 chassis, creating a unified multi-network gateway without additional hardware enclosures. Signal isolation between the DH+ segment and sensitive analog I/O circuits is handled by inline signal isolators mounted on DIN rail within the same control cabinet, maintaining EMC compliance throughout the retrofit.

Retrofit Guidance FAQ

Q: Can I reuse existing DH+ trunk cable when replacing a 1785-KA5 with the 1756-DHRIO?
A: Yes. The 1756-DHRIO uses the same Belden 9463 shielded twisted-pair cable and the same DB-9 or Phoenix connector pinout as legacy DH+ interfaces. No cable replacement is required; simply reconnect the existing trunk to the 1756-DHRIO’s Channel A or Channel B port and verify node addressing in Studio 5000.

Q: Do I need to rewrite my PLC-5 ladder logic for the ControlLogix platform?
A: Rockwell’s Migration Assistant tool within Studio 5000 can import RSLogix 5 programs and convert the majority of standard ladder rungs automatically. DH+ messaging instructions (MSG blocks) require remapping to the 1756-DHRIO’s module path, but the underlying logic structure is preserved. Plan for a validation phase of 1–3 days depending on program complexity.

Q: What RIO drop count does the 1756-DHRIO support?
A: Each RIO channel supports up to 32 logical rack addresses (drops 0–31), matching the capacity of legacy 1771-ASB adapters. Existing 1771 I/O racks with 1771-ASB or 1771-ASBD adapters connect without modification.

Q: Is the 1756-DHRIO compatible with both 1756-A4 and 1756-A17 chassis?
A: Yes. The module is slot-independent and compatible with all standard ControlLogix chassis sizes (1756-A4, A7, A10, A13, A17) as well as the redundancy-capable 1756-A17R chassis. Slot placement does not affect DH+ or RIO performance.

Q: How do I set DH+ baud rate on the 1756-DHRIO?
A: Baud rate (57.6 kbps, 115.2 kbps, or 230.4 kbps) is configured via the module properties dialog in Studio 5000 — no hardware DIP switches are required. Ensure all nodes on the DH+ segment share the same baud rate before going live.

Q: Can the 1756-DHRIO coexist with EtherNet/IP modules in the same chassis?
A: Yes. The 1756-DHRIO occupies one backplane slot and operates independently of EtherNet/IP modules such as the 1756-EN2T. Both networks are managed simultaneously by the ControlLogix controller through the backplane, enabling hybrid legacy/modern network architectures.

Warranty Assurance

Every Allen-Bradley 1756-DHRIO module supplied by TOPNLMS is covered by a 12-month replacement warranty from the date of shipment. Prior to dispatch, each unit undergoes functional verification including backplane communication testing, DH+ channel continuity checks, and firmware version confirmation. Units are shipped in anti-static packaging with full documentation.

In the event of a confirmed module fault within the warranty period, TOPNLMS will arrange a replacement unit shipment within 3 business days. Our technical support team is available to assist with installation verification, Studio 5000 configuration guidance, and compatibility confirmation before and after purchase. All modules are sourced through verified supply channels and are accompanied by a test report upon request.

For volume procurement, spare-parts stocking programs, or expedited delivery requirements, contact our sales team directly. We maintain ready stock of the 1756-DHRIO and related ControlLogix migration components to support planned shutdowns and emergency replacement scenarios.