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3. Key Technical Specifications
| Parameter | Value |
|---|---|
| Number of Channels | 2 (independently configurable for DH+ or RIO) |
| Communication Rates | 57.6 kbps, 115.2 kbps, 230.4 kbps |
| DH+ Connections | 32 messages per module |
| RIO Connections | 32 logical racks per channel; 16 block transfers per channel |
| Max Recommended Logix Connections | 32 |
| Backplane Current @ 5.1 V DC | 850 mA |
| Backplane Current @ 24 V DC | 1.7 mA |
| Power Dissipation, Max | 4.5 W |
| Thermal Dissipation | 15.4 BTU/hr |
| Isolation Voltage | 30 V continuous, basic insulation |
| Cable Type | Belden 9463 twinaxial |
| Max Cable Length | 3048 m (57.6 kbps) / 1524 m (115.2 kbps) / 762 m (230.4 kbps) |
| Operating Temperature | 0 °C to +60 °C |
| Slot Width | 1 |
4. Product Introduction
The Allen-Bradley 1756-DHRIO is a dual-channel communication module for ControlLogix systems that supports both Data Highway Plus (DH+) messaging and Remote I/O (RIO) scanning. Each channel can be independently configured for DH+ or RIO operation.
It enables ControlLogix controllers to exchange messages with PLC-5, SLC 500, and other DH+ devices, and to scan remote I/O racks as a scanner only. Maximum recommended Logix connections are 32. The module is commonly used to integrate legacy DH+ and RIO networks into modern ControlLogix architectures.
1756-DHRIO
1756-DHRIO
5. Troubleshooting Quick Reference
| Symptom | Possible Cause | Relevance to this Part | Quick Check Method | Recommendation |
|---|---|---|---|---|
| Channel status LED red / faulted | Incorrect network type switch or node address | ✅ High | Verify rotary switches behind door (DH+ vs RIO, node 00–77 octal) | Set switches correctly; cycle power |
| No DH+ messaging | Baud rate mismatch or missing terminators | ✅ High | Confirm baud rate and 150 Ω / 82 Ω terminators | Match baud rate; install correct end-of-line resistors |
| RIO racks not updating | Scanner configuration or rack address error | ✅ High | Verify RIO rack numbers and group sizes in Studio 5000 | Correct rack mapping and re-download |
| Module OK LED off | Backplane power or seating issue | ❌ Medium | Measure 5.1 V rail; reseat module | Confirm chassis power supply capacity |
| Intermittent communication | Cable length exceeded or noisy media | ✅ Medium | Check total twinax length vs baud rate limits | Shorten trunk or lower baud rate |
| Block transfers failing | Exceeded 16 BT connections per channel | ✅ High | Count active block-transfer messages | Reduce concurrent BTs or add second module |
If diagnostics remain unclear, capture the alphanumeric display codes, switch settings, and channel status LEDs before contacting support.
6. Frequently Asked Questions (FAQ)
Q: Is the 1756-DHRIO still available new from Rockwell? Yes. It remains available for legacy DH+ and Remote I/O integration into ControlLogix systems.
Q: Can both channels run RIO at the same time? Yes. Each channel can be independently set to RIO scanner mode, providing two separate RIO networks.
Q: Can the 1756-DHRIO act as a Remote I/O adapter? No. It operates as an RIO scanner only. For adapter functionality use the 1756-RIO module.
Q: What is the maximum number of RIO racks per channel? 32 logical rack connections per channel, plus up to 16 block-transfer connections per channel.
Q: What cable and terminators are required? Use Belden 9463 (or equivalent) twinaxial cable. Install 150 Ω terminators for 57.6/115.2 kbps or 82 Ω terminators for 230.4 kbps at both ends of the trunk.
Q: Does Channel A support 230.4 kbps DH+? Yes. Channel A supports all three baud rates for DH+. When Channel A is set to RIO, Channel B DH+ is limited to 57.6 kbps.
Q: Why is New Surplus sometimes priced lower? These are genuine Allen-Bradley modules from excess inventory or cancelled projects. They are unused and typically supplied with anti-static protection. They are not refurbished unless specifically stated.
