Allen-Bradley 1756-DHRIO ControlLogix Communication Module

The Allen-Bradley 1756-DHRIO is a dual-channel Data Highway Plus (DH+) and Universal Remote I/O (RIO) scanner module belonging to the ControlLogix® 1756 platform. It functions as an intelligent network bridge, allowing high-speed ControlLogix processors to exchange peer-to-peer messages over legacy DH+ networks or control remote I/O chassis, blocks, and motor control centers (MCCs) over legacy RIO links.

Hardware Specifications

Note on Data Variations: While certain logistics manifests report shipping/carton weights near 5.9 kg due to bulk master-pack handling, the standalone physical module weight is 0.28 kg (0.63 lbs).

Protocol Configuration & Operation

1. Data Highway Plus (DH+) Mode

When configured for DH+, the module functions as a peer-to-peer routing terminal. It transfers programming tokens and application messaging (MSG instructions) between ControlLogix controller paths and older PLC-5®, SLC™ 500, or legacy HMI networks.

2. Universal Remote I/O (RIO) Mode

When acting as a Remote I/O scanner, the module scans discrete and analog data from remote adapters (such as a 1771-ASB or 1747-ASB) without passing through an intermediate controller. It updates local processor tags synchronously over the ControlLogix backplane.

Frequently Asked Questions

• Q: Can Channel A run on a DH+ network while Channel B is simultaneously scanning a Remote I/O network?
  • A: Yes. Both channels are structurally and electrically isolated. They execute their respective firmware stacks completely independent of each other’s protocol selection, baud rates, or node configurations.
• Q: What is the purpose of the alphanumeric display on the front face plate?
  • A: The 4-character display outputs real-time runtime diagnostics. It shows the assigned node addresses (octal format for DH+) and scrolls specific failure tracking codes (e.g., duplicate node detections, physical layer open circuits, or channel communication timeouts).

Field Installation & Wiring Guidelines

1. Set Network Addresses: Before seating the hardware into the chassis slot, use a small flathead screwdriver to adjust the rotary switches to the designated network node addresses.
2. Insert into Chassis: Align the circuit board guides with the top and bottom grooves of the 1756 chassis slot. Slide the module straight back until the locking tabs snap firmly into the chassis frame.
3. Connect Cable Trunklines: Wire the blue hose (Belden 89463 twinaxial cable) to the 3-pin phoenix connectors. Ensure matching color-coded consistency across the network trunk segment:
   • Pin 1: Blue Wire (Data Line 1)
   • Pin 2: Shield Drain Wire
   • Pin 3: Clear Wire (Data Line 2)
4. Install End-of-Line Resistors: For lines designated as the terminal endpoints of a network run, insert an external terminating resistor across Pin 1 and Pin 3. Use a 150-ohm resistor for networks running at 57.6 or 115.2 Kbaud, or an 82-ohm resistor for networks running at 230.4 Kbaud.
5. Verify Common Ground Loop: Terminate the cable shields to an industrial earth ground bus outside the chassis enclosure to keep electromagnetic noise from corrupting data frames.