How to add Ethernet communications to a non-Ethernet PLC-5

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PLC-5 Today you'll find Ethernet in almost every manufacturing facility, and on most all modern automation systems. But many facilities also have an installed base of legacy PLC-5 systems, and often clients call asking how they can add Ethernet connectivity to their legacy PLC-5's.

Before we answer that question, lets first review the communication options available on the PLC-5 processor.

The original processors (5/10, 5/12, 5/15, & 5/25) which were released beginning in 1985, came with two connectivity options built into every model:

  1. Data Highway Plus (DH+, also known as Peer Communication Interface or PCI)
  2. Remote I/O (RIO.)

The second generation of PLC-5 processors (5/11, 5/20, 5/30, 5/40, 5/60, 5/80,) known as the “New Platform” (NP) PLC-5,  had an additional 25 pin RS-232 port, and could also be ordered with an optional Ethernet or ControlNet port.

So for all those PLC-5 processors without the built-in Ethernet port I list five options below which allow you to add Ethernet communications to your non-Ethernet PLC-5:

AN-X2-AB-DHRIOOption 1: EtherNet/IP to Allen Bradley Remote I/O or DH+ Gateway (works with all DH+ devices)

From Prosoft: “The EtherNet/IP to Remote I/O or DH+ (AN-X2-AB-DHRIO) Gateway is designed to help customers initiate a phased migration approach to replacing or upgrading legacy Remote I/O, HMI's or Drives. The AN-X2-AB-DHRIO gateway has multiple operating modes that are selected during initial configuration and are described below..”


Works with any DH+ device


It's not free?

PNs and Prices

AN-X2-AB-DHRIO  EtherNet/IP to Allen Bradley Remote I/O or DH+ Gateway $1,895.

1761-NET-ENIOption 2: DF1 to Ethernet/IP converter: (New Platform PLC-5 models only)

If your New Platform PLC-5’s serial port is available, you can easily add a DF1 to Ethernet/IP converter (1761-NET-ENI.)


Least expensive option

Direct access to PLC-5 Data Table

Has built in email capability (converts PLC MSG to an email)

ENI configuration is done with a free utility


Only three simultaneous bi-directional connections to PLC-5 at any given time

The PLC-5 serial port is limited to 19.2K (low bandwidth)

Requires PLC-5 NP processor with available serial port

If you have an HMI on the serial port you can use the A-B Serial DF1 port splitter (1747-DPS1) but you'll be splitting the bandwidth with the HMI.

PNs and Prices


$893.00 list price from A-B


$74.10 list price from A-B

$25.00 from Amazon

9 Pin to 25 Pin Adaptor

$10 from Amazon

1756-DHRIOOption 3: 1756 DH+ to Ethernet Gateway (works with all DH+ devices)

A very common option to bridge from Ethernet to DH+, CNET, and DNET, a 1756 chassis with an Ethernet module and a single DHRIO module provides for the bridging from Ethernet to two DH+ networks at 57.6K or 115K, or one DH+ at 230K.


One Ethernet gateway to many networks

Direct access to PLC-5 Data Tables

No programming needed to bridge Ethernet to DH+

Add additional modules for additional networks: ControlNet, Devicenet, DH+, DH-485, or 3rd party modules from Prosoft


Expensive solution for a single PLC

PNs and Prices

1756-A7: 7 SLOT CONTROLLOGIX CHASSIS   $504.00 list price from A-B

1756-ENBT: CLX ETHERNET/IP BRIDGE $2150.00 list price from A-B

1756-PA75: 85-265V AC POWER SUPPLY   $911.00 list price from A-B

1756-DHRIO: DH+/RIO BRIDGE $1,910.00 list price from A-B

PLC-5 ENETOption 4: ENET Sidecar: (New Platform PLC-5 models only, firmware upgrade may be needed, )

If your New Platform PLC-5 has the correct firmware, or if you're willing to purchase the firmware upgrade, and you have the ability to move whatever module you have in the first I/O slot (directly to the right of the processor,) the 1785-ENET Ethernet sidecar is an option you can consider.


High Speed connection to PLC-5

Direct access to PLC-5 Data Table

Channel 3a configuration is simple


Need to move and readdress I/O module in slot 1

Will likely need to update PLC-5 firmware chip which can be expensive

PNs and Prices


$3720.00 list price from A-B

5/80 Firmware

~$685.00 from A-B (most expensive version, price circa 2005)

MVI71Option 5: Prosoft Ethernet Module for 1771 Chassis and all PLC-5 Processors: (works will all PLC-5 models)

From Prosoft: “Support for the storage and transfer of up to 4000 registers to/from the PLC processor using the block transfer. The module can send read or write commands to remote devices on the Ethernet/IP network. The MVI71-DFNT also has 20 servers that allows the module to receive commands from remote devices. The MVI71-DFNT Pass-Through server allows the user to edit, download, and upload the PLC ladder logic using an Ethernet connection. Up to 100 commands can be configured by the user.”


Works with any PLC-5 in any SLOT


Requires programming (Prosoft tech support historically has been good)

– Block Transfers needed in PLC-5 Program

PNs and Prices

MVI71-DFNT EhterNet /IP Communication Module ~$2,159 (circa 2005)

I hope you’ve found this article about how to add Ethernet to non-Ethernet versions of the Allen-Bradley PLC-5 helpful. If you have a question or would like to offer your own option please use the “Leave a reply” form below to comment on this article.


Shawn Tierney
Automation Blogger and Trainer

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