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Description
Application Scenarios
A plant operates a mixed-vintage production line: a ControlLogix 1756-L8x processor controls new servo equipment, while a PLC-5/40 still manages the original 1771 I/O racks on the filling section. The two systems must exchange start/stop interlocks and production counts. The 1756-DHRIO solves this by configuring Channel A as a DH+ port at 115.2 Kbps for peer-to-peer messaging with the PLC-5, while Channel B serves as an RIO scanner for a 1794 Flex I/O drop added during a recent expansion. Without the 1756-DHRIO, the plant would face either a full PLC-5 replacement or a costly network gateway retrofit; with it, the legacy and modern systems share data natively within the ControlLogix chassis.
Parameter
| Parameter | Value/Description |
|---|---|
| Product Model | 1756-DHRIO |
| Manufacturer | Allen-Bradley / Rockwell Automation |
| Product Category | ControlLogix DH+/Remote I/O Communication Interface Module |
| Channels | 2 (Channel A and Channel B, independently configurable) |
| DH+ Baud Rates | Channel A: 57.6 / 115.2 / 230.4 Kbps; Channel B: 57.6 / 115.2 Kbps |
| RIO Scanner Capacity | 32 logical rack connections or 16 block transfer connections per channel |
| Backplane Current | 850 mA at 5 VDC; 1.7 mA at 24 VDC |
| Power Dissipation | 4.5 W maximum |
| Isolation | 30 V continuous (basic insulation; type tested at 877 VDC for 60 s) |
| Node Address Range | 00–77 octal (DH+ mode only) |
| Programming Terminal | 8-pin mini DIN (parallel to Channel A) |
| Lifecycle Status | Discontinued by Rockwell (end-of-life 12/31/2024) |
Technical Principles and Innovative Values
Innovation Point 1: Dual-Channel Flexibility in a Single Slot. The 1756-DHRIO does not force a choice between DH+ and RIO at the module level. Each channel is independently switched: Channel A can run DH+ messaging while Channel B scans RIO racks, or both channels can serve the same protocol. This flexibility is set by physical switches on the module before installation, not by firmware configuration, ensuring deterministic behavior from power-up .
Innovation Point 2: Native RIO Scanning Without External Gateways. When configured as an RIO scanner, the 1756-DHRIO establishes connections directly between the ControlLogix controller and legacy I/O adapters such as the 1771-ASB or 1794-ASB. Each channel supports up to 32 logical rack connections, distributing control so that the controller owns its I/O without an intermediate communication bridge . For plants with substantial 1771 or Flex I/O infrastructure, this preserves the existing I/O investment while migrating the control layer to ControlLogix.
Innovation Point 3: Routing Table Architecture for Multi-Network Messaging. The 1756-DHRIO maintains a routing table that allows DH+ devices to reach destinations across ControlLogix backplanes and additional networks. A message can traverse up to four communication networks and three chassis, enabling a PLC-5 on a DH+ link to send data to a controller in a remote ControlLogix chassis via an EtherNet/IP path . This routing capability is configured once per chassis in RSLinx and applies to every 1756-DHRIO in that backplane .
