Migration Guide · Allen-Bradley SLC 500
Allen-Bradley SLC 500 Migration Guide: 1747 Processors & 1746 I/O to CompactLogix
A practical cross-reference for replacing discontinued SLC 500 processors and 1746 I/O with CompactLogix — which controller maps to which CPU, how to keep DH+ and DH-485 devices talking, and how to reuse the existing rack and field wiring with a 1747-AENTR adapter.
- SLC 500 migration
- 1747-L551 replacement
- SLC 500 to CompactLogix
- 1747-AENTR
- 1746 I/O conversion
- RSLogix 500 to Studio 5000
- DH+ to EtherNet/IP
- obsolete Allen-Bradley parts
Machies Automation Engineering Team · Updated
Why migrate now
Rockwell Automation has discontinued the SLC 500 processors, and the 1746 I/O platform is on the same path. Lines still run on them because they are reliable — but every failed processor now means hunting for a surplus or refurbished spare, and every year that gets slower and more expensive.
The good news is that a migration doesn't have to be a rip-and-replace. With a 1747-AENTR adapter, a CompactLogix controller can run the existing 1746 rack, so the processor can be replaced in one shutdown and the I/O modernised later. Confirm each catalogue number's current status in Rockwell's product lifecycle search before you budget.
Processor cross-reference: 1747 SLC 500 to CompactLogix
Filter by the legacy network the processor sits on — that, more than memory size, drives migration effort. Every part number links to the product or a pre-filled quote request; use RFQ for a like-for-like spare.
5 of 5 processors shown
| Obsolete SLC CPU | Platform & comms | CompactLogix 5380 | CompactLogix 5370 | Bridge / rack retention | Complexity |
|---|---|---|---|---|---|
| 1747-L511RFQ | SLC 5/01 · 1K words DH-485 (programming / slave) Small programs, rarely any messaging. A 1769-L16ER supports a limited number of EtherNet/IP I/O nodes — count the AENTR plus any drives before choosing it. | 1747-AENTR keeps the 1746 I/O. DH-485 peers, if any, need a DH-485 to EtherNet/IP gateway. | Low | ||
| 1747-L524RFQ | SLC 5/02 · 4K words DH-485 Check indexed (#) addressing and STI routines by hand after conversion — they don't always translate cleanly. | 1747-AENTR keeps the 1746 I/O. DH-485 peers need a DH-485 to EtherNet/IP gateway. | Low | ||
| 1747-L532RFQ | SLC 5/03 · 16K words RS-232 DF1 (ch 0) + DH-485 (ch 1) Serial MSG instructions to HMIs, drives or other PLCs must be re-pathed to EtherNet/IP. | 1761-NET-ENI puts existing DF1 serial devices on EtherNet/IP during a phased cut-over; 1747-AENTR keeps the I/O. | Medium | ||
| 1747-L541RFQ | SLC 5/04 · 16K words DH+ (ch 1) + RS-232 DF1 (ch 0) DH+ usually links several PLCs and PanelViews. Plan the network migration before the processor swap, not after. | 1756-DHRIO in a ControlLogix gateway chassis (or a standalone DH+ gateway) keeps DH+ peers online; 1747-AENTR keeps the I/O. | High | ||
| 1747-L551RFQ | SLC 5/05 · 16K words Ethernet (ch 1) + RS-232 DF1 (ch 0) Easiest network-wise. Most of the work is program conversion and re-addressing HMI tags. | No network bridge needed — re-point HMI and MSG paths at the new controller's IP address; 1747-AENTR keeps the I/O. | Medium |
Targets are typical sizing for the original program; size the controller for the new I/O count, EtherNet/IP node count and any added functions.
1746 I/O conversion matrix
Keep these modules on a 1747-AENTR for now, or replace them with the equivalents below when the rack is rewired.
| Legacy 1746 module | Function | CompactLogix 5380 | CompactLogix 5370 | Wiring / harness path | Watch out for |
|---|---|---|---|---|---|
| 1746-IB16RFQ | 16-point 24 V DC sinking digital input | 5069-IB16 | 1769-IQ16 | Re-land field wires on the new terminal block, or use a pre-wired interface module + cable to shorten downtime. | 1769-IQ16 is sink/source — confirm the field device type and common wiring before reuse. |
| 1746-OB16RFQ | 16-point 10–50 V DC sourcing (transistor) output | 5069-OB16 | 1769-OB16 | Re-land on the new terminal block, or pre-wired interface module + cable. | Replacement outputs are 24 V DC class. Loads switched above ~30 V DC need interposing relays. |
| 1746-OW16RFQ | 16-point relay output (AC/DC) | 5069-OW16 | 1769-OW16 | Re-land on the new terminal block; keep separate AC and DC commons segregated as before. | Match per-point and per-common current ratings and the commons grouping before landing wires. |
| 1746-NI8RFQ | 8-channel analog input, voltage or current per channel | 5069-IF8 | 1769-IF8 | Re-land shielded pairs individually; keep shield grounding at one end as on the original. | Scaling changes: SLC raw/proportional counts don't match Logix module data. Re-scale and loop-test every channel. |
| 1746-NO4IRFQ | 4-channel analog current output | 5069-OF4 | 1769-OF4CI | Re-land shielded pairs individually; confirm loop power arrangement matches the new module. | Set the new module's fault and program-mode output states to match what the process expects. |
Pre-wired conversion harnesses and interface modules for 1746 to 1769/5069 cut-overs are available from Rockwell and third-party vendors. The right harness depends on the target module and terminal style — send us your 1746 module list and we'll quote the matching harness alongside the I/O.
Quote a harnessModernization sequence
A sequence that keeps the line recoverable at every step — from the .RSS backup to the final I/O swap.
Back up the running system
Capture exactly what's in the processor before touching hardware.
- Go online with RSLogix 500, upload the program and save the .RSS file together with its documentation (symbols, rung and address comments).
- Record the processor catalogue number, series and firmware (FRN), plus the slot-by-slot chassis layout.
- Save live data table values (setpoints, recipes, counters) that aren't initialised by logic.
- Back up HMI applications and photograph panel wiring and terminal markings.
Audit I/O, networks and messaging
Know every device the SLC talks to before choosing the controller.
- List every 1746 module and check each against the 1747-AENTR supported-module list — most standard digital and analog modules are supported, some specialty and communication modules are not.
- Map every DH+, DH-485 and DF1 node, and every MSG instruction with its target.
- Count EtherNet/IP nodes (adapters, drives, HMIs) against the target controller's node limit.
Parts1747-AENTRRFQConvert the .RSS program to Studio 5000
Automated conversion, then a line-by-line review.
- Use Rockwell's PLC-5/SLC migration tooling in Studio 5000 Logix Designer (Project Migrator; older versions used Translate PLC-5/SLC) to turn the .RSS into a Logix project.
- Review by hand: indexed and indirect addressing, STI/DII interrupts, PID, MSG, timers and any logic reading status file (S:) bits.
- Use PLC/SLC message mapping so existing HMIs and peers that address N7:0-style files keep working during the transition.
Bridge DH+ and DH-485 to EtherNet/IP
Keep legacy peers online so the line can migrate in phases.
- DH+ (SLC 5/04): a 1756-DHRIO module in a ControlLogix chassis, or a standalone DH+ gateway, bridges DH+ nodes to EtherNet/IP.
- DF1 serial (SLC 5/03 channel 0 and similar): a 1761-NET-ENI connects DF1 devices to EtherNet/IP.
- Ethernet (SLC 5/05): no bridge needed — update HMI and MSG paths to the new controller's IP address.
Retain the rack with a 1747-AENTR
Swap the processor, keep the chassis, power supply, I/O and field wiring.
- Remove the SLC processor and install the 1747-AENTR EtherNet/IP adapter in slot 0 of the local 1746 chassis.
- Add the adapter and each 1746 module to the Logix I/O configuration (install the AENTR Add-on Profile), then alias converted tags to the module data.
- Keep the existing 1746-P2 (or matching) power supply, and the same chassis and expansion-cable layout.
Commission, then replace I/O in later shutdowns
Prove the new controller first; modernise the I/O on your schedule.
- Point-to-point check every discrete I/O, and loop-test every analog channel against the new scaling.
- Keep the removed SLC processor and the original .RSS as a rollback for the first production runs.
- Swap 1746 I/O to 5069 or 1769 modules during planned shutdowns, using the I/O conversion matrix above.
1-Page SLC 500 Migration Reference Sheet (PDF)
Both cross-reference tables and the six-step sequence on a single A4 page — print it for the panel shop, attach it to a CAPEX request, or send it to your integrator.
Frequently asked questions
Can a CompactLogix PLC control 1746 I/O?
Yes. Replace the SLC processor in slot 0 of the 1746 chassis with a 1747-AENTR EtherNet/IP adapter, and a CompactLogix 5370 (1769-L3xER) or CompactLogix 5380 (5069-L3xxER) controller scans the existing 1746 I/O over EtherNet/IP — the chassis, power supply and field wiring stay in place. Check each module against the 1747-AENTR supported-module list first: most standard digital and analog 1746 modules are supported, but some specialty and communication modules are not.
What is the direct replacement for a 1747-L551 processor?
There is no drop-in successor that fits the SLC 500 chassis. The migration path for the SLC 5/05 (1747-L551) is a Logix controller — typically a CompactLogix 5380 such as the 5069-L320ER, or a CompactLogix 5370 1769-L33ER — with a 1747-AENTR adapter in slot 0 so the existing 1746 I/O keeps working. If you need a like-for-like 1747-L551 to keep a line running while the migration is planned, Machies Automation can source one.
Can I convert an RSLogix 500 .RSS file to Studio 5000?
Yes. Rockwell's PLC-5/SLC migration tooling in Studio 5000 Logix Designer converts an .RSS program into a Logix project. Conversion is not fully automatic: indexed and indirect addressing, interrupts, PID, MSG instructions and logic that reads status file bits need to be reviewed and tested by hand.
How do I connect a DH+ network to EtherNet/IP?
The usual approach is a 1756-DHRIO module in a ControlLogix chassis acting as a gateway, or a standalone DH+ to EtherNet/IP gateway. Either lets legacy DH+ PLCs and HMIs keep communicating with the new CompactLogix controller while the rest of the network is migrated in phases.
Do I have to replace all the 1746 I/O at the same time as the processor?
No. With a 1747-AENTR the processor can be replaced first while the 1746 I/O and field wiring stay untouched. The I/O can then be moved to 5069 or 1769 modules during later planned shutdowns, one rack or one module group at a time.
Keeping an SLC 500 line running while you plan?
We source hard-to-find 1747 processors, 1746 I/O and power supplies as well as the CompactLogix hardware to replace them. Send the part numbers and quantities — we reply with price and availability in 24–36 hours.
Allen-Bradley parts in our catalogue
Allen-Bradley, SLC, CompactLogix, ControlLogix, RSLogix and Studio 5000 are trademarks of Rockwell Automation, Inc. Machies Automation is an independent supplier and is not affiliated with Rockwell Automation. This guide is general engineering guidance — verify module compatibility, firmware and lifecycle status against current Rockwell documentation before ordering.