Allen-Bradley 1756-DHRIO Migration-Ready DH+/RIO Bridge

Overview

Allen-Bradley 1756-DHRIO Migration-Ready DH+/RIO Bridge for Legacy Control Systems

The Allen-Bradley 1756-DHRIO is a dual-port Data Highway Plus (DH+) and Remote I/O (RIO) communication bridge module designed for the ControlLogix 1756 chassis platform. As legacy PLC-5 and SLC 500 systems approach end-of-life, the 1756-DHRIO serves as the critical communication link that allows modern ControlLogix controllers to maintain backward compatibility with existing DH+ networks and RIO scanner chains — enabling phased migration without forcing a full cutover of field wiring, remote I/O racks, or HMI infrastructure.

For facilities managing aging automation assets, the 1756-DHRIO eliminates the need to immediately replace every downstream device on a DH+ segment. Engineers can retain existing PanelView terminals, legacy SLC 5/04 nodes, and remote I/O adapter racks while migrating the primary controller to a 1756-L7x or 1756-L8x ControlLogix processor. This approach dramatically reduces project scope, capital expenditure, and unplanned downtime during the transition period.

Migration Compatibility Table

Retrofit Planning for Existing Automation Systems

A successful 1756-DHRIO retrofit begins well before the module arrives on site. The first step is a full audit of the existing DH+ network topology — documenting every node address, baud rate setting, and cable run length. In most PLC-5 to ControlLogix migrations, the 1756-DHRIO slots directly into the new 1756 chassis alongside the replacement 1756-L74 or 1756-L75 processor, with the DH+ trunk cable reconnected to the module’s Channel A or Channel B terminal block.

Power budget verification is essential before commissioning. The existing 1756-PA72 or 1756-PB72 chassis power supply must have sufficient headroom to support the 1756-DHRIO alongside other installed modules such as the 1756-EN2T Ethernet/IP bridge, 1756-IB16 digital input modules, and 1756-OB16E digital output modules. Overloading the backplane power supply is a common cause of intermittent communication faults during initial startup.

For facilities that also operate Remote I/O racks — such as 1794-AENT FlexI/O adapters or legacy 1771 I/O chassis connected via RIO cable — the 1756-DHRIO’s RIO scanner port must be configured with the correct rack size, starting quarter, and baud rate to match the existing adapter settings. Changing these parameters without updating the adapter DIP switches will result in I/O communication faults that can halt production.

Where the legacy system includes a 1747-KE or 1747-KFC15 DH+/RS-232 interface for programming access, these can typically be decommissioned once the ControlLogix system is online, as direct Ethernet programming via the 1756-EN2T eliminates the need for serial programming bridges. Similarly, any 1784-PKTX or 1784-U2DHP USB-to-DH+ programming cables used during the transition should be retained until the final program download and verification is complete.

Signal isolation should also be reviewed during the retrofit. Where DH+ cable runs exceed 100 meters or pass through electrically noisy environments near VFDs or motor control centers, installing a signal repeater or verifying proper cable shielding termination at a single chassis ground point will prevent noise-induced communication errors after cutover.

Downtime Control During System Migration

Minimizing production downtime during a ControlLogix migration requires a structured parallel-run strategy. Before any physical wiring changes, the replacement ControlLogix program should be fully developed, simulated offline in Studio 5000, and validated against the original PLC-5 or SLC 500 logic using cross-reference documentation. Preserving the original program logic — including timer presets, counter accumulator values, and PID tuning parameters — is critical to maintaining process continuity after switchover.

The recommended approach is to install the new 1756 chassis, 1756-DHRIO, and associated I/O modules in parallel with the running legacy system during a scheduled maintenance window. The 1756-DHRIO can be brought online on the DH+ network as a passive node first, allowing engineers to verify communication with existing HMI terminals and remote I/O adapters before the legacy controller is taken offline. This staged approach allows the team to confirm that PanelView screen navigation, alarm setpoints, and operator interface tags are correctly mapped before committing to the final cutover.

Once the parallel system is verified, the physical cutover — transferring field I/O wiring from the legacy terminal blocks to the new 1756 chassis — can be executed during a planned shutdown. With pre-tested wiring adapters and a documented terminal-by-terminal migration checklist, experienced teams routinely complete the physical cutover within a single 8-hour maintenance shift, restoring full production control with the new ControlLogix system before the next operating cycle.

All units supplied by KNMKS undergo pre-shipment functional testing to confirm module communication, backplane recognition, and firmware integrity before dispatch, reducing the risk of receiving a non-functional module during a time-critical migration window.

Retrofit Support FAQ

Q1: Can the 1756-DHRIO directly replace a PLC-5 DH+ scanner without rewiring the field network?
Yes. The 1756-DHRIO connects to the existing DH+ trunk cable using the same screw-terminal connector style. Node addresses are reassigned in the Studio 5000 project. No field cable replacement is required provided the existing cable meets DH+ impedance and length specifications.

Q2: What firmware version should the 1756-DHRIO be running for compatibility with a 1756-L74 controller?
The module firmware must be compatible with the ControlLogix chassis firmware revision. Use ControlFLASH or Studio 5000 to verify and update the 1756-DHRIO firmware revision to match the project requirements before the first program download. KNMKS supplies modules with current production firmware; confirm the target revision with your system integrator prior to ordering.

Q3: Is the 1756-DHRIO suitable for both DH+ and RIO on the same installation?
Yes. Each of the two ports on the 1756-DHRIO can be independently configured as either a DH+ channel or an RIO scanner channel. This allows a single module to simultaneously maintain a DH+ network for HMI communication and an RIO scanner chain for legacy remote I/O racks, reducing the number of communication modules required in the chassis.

Q4: What support terms and stock availability does KNMKS provide for the 1756-DHRIO?
KNMKS maintains ready stock of the 1756-DHRIO to support urgent retrofit and breakdown replacement requirements. Every unit is pre-shipment tested for backplane communication and port functionality. A support terms confirmed by quotation is provided from the date of shipment, covering manufacturing defects and functional failures under normal operating conditions. Contact [email protected] or +86 18359268345 for lead time and volume pricing.

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Contact: [email protected] | +86 18359268345

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Allen-Bradley
Allen-Bradley

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1756-DHRIO
ControlLogix Series

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PLCs & Controllers

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