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Description
Application Scenarios
Picture a food processing plant where the main line still runs on a PLC-5/40 with six Remote I/O racks scattered across three buildings, and the maintenance team has been told the processor is no longer supportable. A full rip-and-replace would mean new I/O racks in every panel, a complete rewire, and a multi-day shutdown — easily a six-figure project with a week of lost production. Instead, the integrator installed a small ControlLogix chassis with a 1756-DHRIO in slot 2 and an 1756-L73 processor in slot 0. Channel A was configured as DH+ and connected to the existing DH+ trunk; Channel B was configured as RIO scanner and took over the six racks. The PLC-5/40 stayed in the chassis as a logic processor while the ControlLogix took over the I/O scan, and the plant ran the next shutdown on the new platform while the old racks were converted one at a time to EtherNet/IP.The real pain point was not the hardware — it was the risk. Nobody wanted to be the engineer who explained a week-long shutdown for a panel that had run flawlessly for twenty years. With the 1756-DHRIO, the cutover was reversible: if a rack misbehaved, the configuration was changed back and production resumed. The plant’s control engineer later described it as “the only migration path where the legacy system is still the backup plan.” Three years into the project, four of the six racks have been converted to 1794-AENT Flex I/O on EtherNet/IP; the remaining two stay on the 1756-DHRIO until their next scheduled shutdown, and the module has not logged a single communication fault.
Parameter
| Main Parameters | Value / Description |
|---|---|
| Product Model | 1756-DHRIO |
| Manufacturer | Allen-Bradley (Rockwell Automation) |
| Product Category | DH+/RIO Bridge and Scanner Communication Module (ControlLogix 1756 family) |
| Number of Channels | 2 (Channel A and Channel B — fully independent protocol, baud rate, and configuration) |
| Supported Protocols | Data Highway Plus (DH+); Remote I/O (RIO) in scanner or adapter mode |
| Channel Configuration | Each channel independently selectable: DH+ only, RIO scanner, or RIO adapter |
| DH+ Baud Rates | 57.6 kbps, 115.2 kbps, 230.4 kbps (all devices on a trunk must match) |
| DH+ Node Capacity | Up to 32 nodes per DH+ link; 64 nodes total across both channels |
| DH+ Data Capacity | Up to 1,024 words of input and 1,024 words of output data per channel |
| DH+ Network Length | 10,000 ft (3,048 m) @ 57.6 kbps; 5,000 ft (1,524 m) @ 230.4 kbps (with repeaters) |
| RIO Connections | 32 per channel; 64 total across both channels |
| RIO Scan Time | Programmable 5…255 ms per scan (default 10 ms); actual scan determined by rack count and block-transfer load |
| Block Transfer | 16 per channel; DH+ “Block Transfer Pass-Through” messages supported for RIO block transfers |
| Backplane Power | 24 V DC, 300 mA (7.2 W) — backplane-powered, no external supply required |
| Current Draw (typical distributor spec) | 850 mA @ 24 V DC — verify against the specific revision and chassis power-supply sizing |
| Physical Connection | One 3-position removable terminal block per channel; RS-232 port on front for programming-terminal connection |
| Module Location | Any slot except the controller slot; cannot be placed in the same chassis as a redundant controller |
| Configuration Software | RSLogix 5000 (v16 or later) or Studio 5000 Logix Designer; RSLinx Classic for routing tables |
| Diagnostic Access | Controller tags, controller properties dialog, module web page, front-panel status LEDs |
| Indicators | Module status, network status, and active/I/O status LEDs per channel |
| Operating Temperature | 0…60 °C (32…140 °F) |
| Storage Temperature | -40…85 °C (-40…185 °F) |
| Relative Humidity | 5…95%, non-condensing |
| Dimensions (W × H × D) | 30.9 × 141.1 × 119.9 mm (single-width 1756 module) |
| Certifications | UL Listed, cUL Listed, CE Marked, C-Tick, Ex, FM; compliant with EN 50081-2, EN 50082-2 |
| Compatible Chassis | 1756-A4, 1756-A7, 1756-A10, 1756-A13, 1756-A17 (and harsh-environment -K variants) |
| Compatible Processors | 1756-L61/L62/L63/L64/L65, 1756-L71/L72/L73/L74/L75, 1756-L81E/L82E/L83E/L84E/L85E; GuardLogix 5570 |
| Compatible DH+ Peers | PLC-5, SLC 5/04, MicroLogix 1400, PanelView 550/600/900/1000/1400e, 1785-KA5, 1771-KA2, 1756-DHRIO, 1756-DH485, 1756-DHRP |
| Compatible RIO Devices | 1771-A1B/A2B/A3B/A4B adapters; 1747-SN; 1794-ASB; 1794-ASB/E; 1771-ALX; 1794-ALB; 1797-ALB |
| Lifecycle Status | Discontinued for new architectures — the only standard Logix product with native DH+ and RIO; no direct factory replacement |
