Adding 1785-ENET to PLC-5: Slot Addressing and Firmware
Add 1785-ENET to a PLC-5: slot addressing rules, minimum firmware revisions for ControlLogix CIP, and 1785-ENET module compatibility matrix.
The 1785-ENET Ethernet interface is a sidecar module for Allen-Bradley PLC-5 processors. It mounts on the right edge of the CPU and presents 10 Mbps A-B Ethernet (TCP/IP) with CSPv4 messaging. Because the assembly is mechanical and electrical, not a 1771 backplane card, it never appears in the I/O image table—yet it still forces a physical slot change that rewrites how the local rack is addressed. Plants that treat the upgrade as “plug Ethernet and go” often spend the next shift chasing phantom I/O faults that are really displaced slot mapping.
Sidecar topology: vacate the processor’s adjacent slot, seat the ENET flush, then reassign every address that lived in the vacated group before restoring power.
Prerequisites and publications
Confirm a compatible PLC-5 series and firmware, a 1771 chassis with a free position immediately to the right of the processor, RSLogix 5 matched to that CPU series, and RSLinx Classic with Ethernet Devices support. Ethernet plant media should be 10Base-T class with a switch appropriate to the segment. When CIP messaging toward ControlLogix or PanelView Plus is required, both the processor and the ENET must clear the CIP minimum revisions listed below. The authoritative module reference is publication 1785-UM019.
Slot addressing: one-slot versus two-slot
Under single-slot addressing each 1771 module owns one input word and one output word. Only the card sitting in physical slot 0 must move farther right; higher slots keep their offsets. Under two-slot (rack-optimized) addressing, slot pairs 0/1, 2/3, and so on share one input word and one output word. Installing the ENET consumes the processor-adjacent position, so the entire 0/1 pair must be vacated and both words reassigned to a free pair elsewhere in the chassis or a remote rack.
| Addressing mode | What must move | Logic impact |
|---|---|---|
| 1-slot | Module in physical slot 0 only | Update that card’s I/O addresses |
| 2-slot | Both modules in the 0/1 pair | Reassign the shared input and output words |
Always verify the mode in RSLogix 5 under Processor Properties → I/O Configuration before labeling wires. A two-slot plant that only relocates one card will leave half of a shared group orphaned.
Physical installation sequence
- Power down the chassis and document slot 0 wiring against the live I/O addresses.
- Remove the displaced module(s), insulate field conductors, and reinstall them in the planned free slot or pair.
- Update ladder and HMI references for every address that changed (for example I:000 to I:002).
- Seat the 1785-ENET against the processor, tighten captive screws, and confirm the bezel is flush.
- Connect RJ-45 plant Ethernet, restore power, and wait for the OK LED to go solid green after self-test.
Online channel configuration
ENET parameters are configured online through the attached processor; there is no meaningful offline-only channel editor path that can replace a live sidecar. Go online in RSLogix 5, open the Ethernet channel for the sidecar, assign IP address, subnet mask, and gateway, enable the CSPv4 / EtherNet/IP messaging options required by the application, then download. The module resets and rejoins the network on the new addressing.
Firmware floors for sidecar acceptance
Per Rockwell Knowledgebase G14998, the processor must meet series-specific minimum firmware before it will accept and operate the ENET. Series A processors need Revision F or later within Series A; Series B needs Revision J or later; Series C needs Revision N; Series D needs Revision E; Series E needs Revision D. Firmware upgrades are paid catalog kits and stay inside a series—Series B cannot become Series C by flash alone.
| PLC-5 series | Min FW for ENET | CIP to Logix |
|---|---|---|
| A / B | See series table above | Not supported |
| C Rev N+ | ENET A Rev D+ or B Rev A+ | Yes |
| D Rev E+ | ENET B Rev A+ preferred | Yes |
| E Rev D+ | ENET B Rev A+ | Yes |
Series A and B processors can host an ENET for CSPv4 traffic, but CIP multi-hop messaging to ControlLogix fails with a multi-hop messaging fault. Plants that need PanelView Plus or Logix MSG peers must budget a Series C/D/E CPU upgrade, not only a sidecar.
Slot map and firmware gate before go-live
After addressing updates, force every relocated input and stroke every relocated output before returning to production. Browse the ENET from RSLinx Ethernet Devices and confirm the configured IP is unique on the VLAN. Typical failures include leaving two-slot pairs half-moved, assigning a duplicate IP, seating the sidecar with bent connector pins, and attempting CIP MSG instructions on Series A/B hardware. When the Ethernet path is healthy but CIP still faults, re-check both processor and ENET revisions against Knowledgebase G16915 rather than rebuilding the switch.
Extending a PLC-5 onto plant Ethernet is a legitimate life-extension step, but it should sit inside the same sparing and migration plan used for other PLC and PAC systems on the floor.
About the Author
Mark Townsend | Senior Automation Engineer – Allen-Bradley Systems
Mark Townsend is a senior automation engineer with more than 18 years on Allen-Bradley platforms spanning ControlLogix, CompactLogix, and legacy SLC-500. His day-to-day work is RSLogix / Studio 5000 logic and FactoryTalk View HMI bring-up on aging and mixed fleets.
