Allen-Bradley DH+ to EtherNet/IP: Network Migration Guide

Data Highway Plus (DH+) is Allen-Bradley's legacy plant-floor network, introduced in the 1980s for PLC-5 and SLC 5/04 communication. It is a token-passing network running at 57.6, 115.2, or 230.4 Kbaud — thousands of times slower than modern EtherNet/IP. No new DH+ hardware is manufactured. Migration to EtherNet/IP is not optional — it is a matter of when, not if.

DH+ vs. EtherNet/IP Comparison#

Migration Option A: Bridge (Coexistence)#

Use a ControlLogix chassis as a gateway between EtherNet/IP and DH+:

Hardware needed:

• 1756 chassis (4-slot minimum) + power supply
• 1756-EN2T (EtherNet/IP module)
• 1756-DHRIO (DH+ interface, 2 channels per module)
• No ControlLogix CPU required — the chassis acts as a pure gateway

How it works: The 1756-DHRIO connects to the existing DH+ network. The 1756-EN2T connects to EtherNet/IP. RSLinx Classic configures routing tables between the networks. EtherNet/IP devices can browse through the gateway to access all DH+ nodes.

When to use: As a transition strategy while migrating PLC-5/SLC processors one at a time. Allows new ControlLogix/CompactLogix PLCs and PanelView Plus HMIs to communicate with legacy DH+ devices during the migration period.

Third-party alternatives:

• Prosoft AN-X2-AB-DHRIO (~$2,500) — standalone gateway with web configuration
• Automation Networks ANC-100e (~$1,495) — compact EtherNet/IP to DH+ converter

Migration Option B: Full Migration#

Replace all DH+ devices with EtherNet/IP equivalents:

MSG Instruction Conversion#

Every MSG instruction that references a DH+ address must be reconfigured:

PLC-5 MSG (DH+ path):

Channel: 1A (DH+ Channel A)
Target node: 5
Destination: N7:0 (in target PLC)

ControlLogix MSG (EtherNet/IP CIP path):

Path: 1,0,2,192.168.1.5,1,0
Target tag: MyData[0]
Message type: CIP Data Table Read/Write

The path format 1,slot,2,IP_address,1,slot represents: backplane(1), local EN2T slot, Ethernet port(2), target IP, target backplane(1), target CPU slot.

Phased Migration Approach#

1. Install gateway (1756 chassis with ENBT + DHRIO) — connects both networks
2. Migrate HMIs first — replace PanelViews with PanelView Plus on EtherNet/IP, connecting to PLC-5s through the gateway
3. Migrate PLCs one at a time — replace each PLC-5/SLC with ControlLogix/CompactLogix
4. Remove gateway — when the last DH+ device is replaced, the DHRIO module is no longer needed

Frequently Asked Questions#

Can DH+ and EtherNet/IP devices communicate directly?

Not directly. They need a gateway (1756-DHRIO or third-party converter). The gateway translates between DH+ token-passing protocol and EtherNet/IP CIP protocol.

How many DH+ channels can one DHRIO module bridge?

Two. Each 1756-DHRIO module has Channel A and Channel B, each supporting one independent DH+ network.

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