FANUC Ethernet/IP Setup with Allen-Bradley PLCs: EDS Files, Assembly Instances, and Explicit Messaging That Actually Works

You finished the cell wiring, the FANUC robot and the Allen-Bradley PLC are both on the same subnet, both pings respond, and yet the I/O between them is dead. RSLogix even reports the connection as established. This is the bread and butter of Ethernet/IP commissioning on FANUC controllers, and it almost always comes down to three things lining up on both sides of the link. At Probot Systems we commission these links every week, and the pattern is consistent enough that we know within five minutes which knob is wrong.

This post is written for controls engineers and integrators bringing up Ethernet/IP between an R-30iB or R-30iB Plus FANUC controller and a Rockwell PLC (CompactLogix, ControlLogix, or a Micro850 with EIP scanner). The same checklist applies on R-30iA controllers that have the EIP option enabled, with minor menu differences.

How Ethernet/IP between FANUC and Rockwell actually works

FANUC’s Ethernet/IP implementation acts as an EIP adapter (target) by default. The Rockwell PLC, configured as a scanner, opens an implicit messaging connection to the robot at a fixed RPI, exchanges I/O at every cycle, and exposes those bytes to your ladder logic. The data path is essentially: robot DI/DO range -> FANUC EIP rack -> assembly instance -> PLC tag. If the byte counts or the assembly numbers disagree, the connection still opens at the TCP layer (which is why ping works and RSLogix shows green), but no payload moves.

The exact menu path on the robot is MENU > I/O > Ethernet/IP > Connection #x detail, where you set Input Size, Output Size, and the assembly instance numbers. The Reddit thread How to become an expert in one day captures how often new engineers stop at “it’s connected” without checking that those three numbers actually match what they typed into RSLogix. They almost never do on the first try.

On the Rockwell side, you need the FANUC EDS file installed in RSLogix or Studio 5000. FANUC publishes different EDS files for R-30iA, R-30iB, and R-30iB Plus, and the wrong one will let the connection appear without exchanging real I/O.

Most common causes, in order of probability

1. EDS file wrong or missing. The single most frequent fault. R-30iA and R-30iB Plus each have their own EDS, and the EIP option firmware bumps mean newer FANUC firmware may need a newer EDS than the one already in RSLogix. Confirmed in the FANUC to HMI direct thread where the discussion centered on which EDS file matches which controller revision.
2. Assembly instances mismatched between robot and PLC. FANUC’s defaults are 101 for Input (robot to PLC) and 151 for Output (PLC to robot), but engineers regularly type 100/150 or 102/152 from memory and lose hours. Read the actual numbers off the FANUC connection detail screen.
3. Input/Output byte size mismatched. A FANUC slot configured for 8 bytes input must be configured for 8 bytes input on the PLC side, exactly. RSLogix will allow a smaller PLC tag and the connection will still appear up, but the trailing bytes will read garbage or zeros.
4. Two scanners trying to own the connection. If the same FANUC EIP slot is configured in two PLC projects (common on a brownfield retrofit where the old PLC was left in the cabinet), the connection drops a few seconds after it establishes. Only one scanner per FANUC EIP slot.
5. HOSTCOMM or FTP options eating the connection. Older R-30iB firmware sometimes interleaves HOSTCOMM/FTP traffic with EIP and produces transient drops if the network is busy. Separate VLANs help.

How to diagnose in under 10 minutes

Step 1. On the FANUC, go to MENU > I/O > Ethernet/IP > status and read the connection state for each slot. “Run” with nonzero counters is good. “Idle” or “Stopped” with a fault code is your starting point.

Step 2. On the same FANUC screen, press the detail soft key for the problem connection and write down the Input Size, Output Size, Input Assembly, and Output Assembly. Do not work from memory.

Step 3. In RSLogix or Studio 5000, open the FANUC module properties. Match every field to what you wrote down in Step 2. If anything is different, the PLC is the one to change in 90% of cases, because the FANUC side is what is already deployed.

Step 4. Ping the FANUC from the PLC’s subnet. If the ping fails or shows packet loss, stop debugging EIP and fix the network. FANUC EIP is intolerant of switch loops, missing IGMP snooping config, or VLAN mismatches.

Step 5. Walk one DO from the PLC to the robot. Set the PLC tag, watch DI[1] on the robot in MENU > I/O > Digital > IN/OUT. If it does not toggle, the byte mapping is wrong, not the wiring.

How to fix it

EDS file wrong revision. Pull the right EDS from FANUC America’s customer portal for your controller model and firmware. The thread I/O Configuration FANUC robot and Omron PLC describes the symptom: connection looks fine, payload does not move. Install the matching EDS, delete the existing module in RSLogix, re-add it from the catalog, and the assembly defaults will populate correctly.

Assembly instances mismatched. Edit them in RSLogix to match the FANUC. Do not edit them on the FANUC unless you have a documented reason, because other connections on the robot may already depend on the defaults.

Sizes mismatched. Match byte sizes exactly. If the FANUC EIP slot is 32 bytes IN / 32 bytes OUT, the PLC tag must also be 32 bytes IN / 32 bytes OUT. A common mistake is to configure a smaller PLC tag thinking you “only need” 8 bytes. EIP exchanges the full assembly regardless.

Implicit vs explicit confusion. For most cell I/O you want implicit messaging at a fixed RPI, which is what FANUC’s default config provides. Explicit messaging is for one-shot reads/writes from the PLC into specific CIP objects. The cleanest reference for the latter is the Ethernet/IP Explicit Messaging Example Program on robot-forum, which shows the MSG instruction setup against a FANUC target.

Two scanners on one slot. Decommission the old PLC’s EIP connection in its project before powering it up next to the new one. Saw a single shared FANUC slot drop every 30 seconds for two days on a retrofit until we found the old PLC still owning the connection.

When to call a specialist

If you have matched EDS, assembly numbers, and byte sizes, ping is clean, and the connection still will not run, the next suspects are firmware (FANUC EIP option needs a software patch) or a hardware fault on the EIP option card itself. Both require a controlled start on the robot side, plus the right firmware ISO from FANUC, and that is not work to do at 4pm on a Friday before a Monday startup.

For a brownfield retrofit where you are stripping out old PLCs and reusing FANUC cells with new Allen-Bradley logic, getting the EIP map documented on both sides before commissioning is what separates a one-day startup from a one-week startup. contact us if you want a documented EIP I/O map handed over before the cell arrives, or set up a maintenance preventive contract that includes an annual EIP integrity check.

Related errors to check

• PRIO-620 PNIO station deactivated: the Profinet equivalent of an EIP connection drop, same diagnostic mindset.
• PRIO-081 I/O is not initialized: appears at boot if the EIP option file is missing or corrupted.
• SRVO-049: shows up on some controllers when the EIP loss timer is configured to e-stop on disconnect.

Probot Systems is a FANUC integrator based in Lévis, Quebec, supporting Ethernet/IP commissioning on R-30iA, R-30iB, and R-30iB Plus across Canada and the US. If your cell is throwing intermittent EIP drops or you need a clean I/O handoff document, that is a contact us conversation.