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Migrating Allen-Bradley SLC 500 to CompactLogix

Enhanced Control Solutions · · 9 min read

Rockwell has stopped selling SLC 500 processors. Rockwell’s product pages list the SLC 5/03 (1747-L531) as discontinued in August 2018, the SLC 5/05 16K (1747-L551) in December 2021, and the remaining 5/04 and 5/05 processors (1747-L542, L552, L553) on 31 March 2024. Chassis, power supplies and many 1746 I/O modules have followed. Repair and exchange may still be available, but new supply is ending or has ended.

That does not mean every SLC on site needs replacing this year. It means a failed processor is now a search for surplus stock, and the time to plan the move is before that search starts. This guide covers the routes Rockwell supports, what the software conversion does and does not do, and how to stage the work around production.

Where to migrate to

Rockwell’s recommended path is CompactLogix 5380 with Compact 5000 (5069) I/O. ControlLogix 5580 suits larger systems or sites that already standardise on it. CompactLogix 5370 with 1769 Compact I/O is still listed as a mapping in Rockwell’s hardware migration manual (1746-RM003), but Rockwell now says some 5370 controllers are discontinued and points to the 5380, so it is a weaker long-term choice.

The I/O platform follows the controller. 1769 I/O needs a CompactLogix 5370; 5069 I/O needs a CompactLogix 5380, 5480 or ControlLogix 5580.

Three ways to handle the I/O

The I/O decision drives cost, downtime and risk more than the controller does.

1. Keep the 1746 I/O for now. The 1747-AENTR EtherNet/IP adapter goes in slot 0 of the existing SLC chassis in place of the processor, and the new Logix controller runs the old I/O over Ethernet. Field wiring is untouched and the change can be reversed. Limits worth knowing: up to 3 chassis and 30 modules per adapter, it draws 470 mA from the existing power supply, and some modules are not supported, including the 1747-SN, 1747-SDN, 1747-DCM and 1746-HSRV. More importantly, Rockwell lists the 1747-AENTR itself as End of Life, discontinued from 15 February 2027. Treat this as a stage, not a destination.

2. Replace the I/O but keep the field wiring. Rockwell’s 1492 conversion system replaces each 1746 chassis with a conversion chassis carrying 5069 modules. The existing SLC terminal block plugs into a conversion module, so, in Rockwell’s words, the conversion is done “without removing any field wires from the existing terminal block”. It covers the common digital and analogue modules (for example 1746-IB16, OB16, OW16, NI4, NI8, NO4I). Some modules need two 5069 modules (the 32-point cards), single-ended sensors are not supported on the NI8 conversion, and a 5069-FPD is needed where field voltages differ. Modules not on Rockwell’s list are not covered.

3. Replace the I/O and rewire. The cleanest result and the longest outage. It makes most sense where the panel is being rebuilt anyway, or the wiring is in poor condition.

Rockwell’s migration manual supports doing this in stages, and says I/O “can be swapped one rack at a time”. A common sequence is to convert the code, swap the processor for an AENTR, then replace the I/O.

Converting the program

The current tool is built into RSLogix 500 itself. From version 12 (released December 2018), an SLC project can be saved as a Logix Designer .ACD file. You choose the target controller and whether to keep the existing I/O (through the AENTR) or update all I/O; Rockwell describes updating all I/O as the preferred option. The older standalone translation tool is no longer the route.

Rockwell’s own description is the right expectation: the migrator “automatically converts the program logic, but it is not the complete solution.” In practice:

  • Only ladder converts. SFC and structured text routines need removing or rewriting first.
  • Data files become arrays. N7 becomes an integer array, B files become DINT arrays (B3:4/1 becomes B3[4].1), F files become REAL arrays. Address comments become alias tags; instruction comments do not carry over.
  • I/O tags arrive empty. Converted tags such as I_007 have to be mapped to the real module data, usually with CPS or MOV. Logix updates I/O asynchronously to the scan, so logic that relied on SLC scan behaviour needs buffering.
  • Problem rungs are marked. Anything the converter could not translate becomes a PCE (Program Conversion Error) instruction. Each one has to be found, understood and replaced with working logic. A project that verifies is not necessarily correct.
  • Timers change base. Logix timers run on 1 ms. Presets are rescaled during conversion, but logic that reads or writes .PRE or .ACC directly is flagged and needs checking.
  • The status file does not exist. S: references are rebuilt with GSV instructions, and SLC fault routines are not called the way they were.
  • Interrupts are rebuilt. STI instructions are not supported, and DII routines become event tasks on suitable 5069 input modules.
  • Messages need finishing. MSG instructions are only partly converted, and Modbus MSG configurations are not supported.
  • Indirect file addressing does not convert. Word and bit indirection mostly does; indirect file numbers need redesigning, often as a two-dimensional array.
  • Some instructions have no equivalent. High-speed counter, PTO/PWM, sequencer compare and others become PCE instructions. PID converts, but the parameters need checking.

Rockwell’s rule of thumb is that a full 32 KB SLC program needs around 360 KB of Logix memory, and that overall scan time typically falls by 50 to 80%. A faster scan is usually good news, but timing-sensitive logic should be tested rather than assumed.

Communications and HMIs

The SLC serial port maps to a 5069-SERIAL module, and the CompactLogix 5380 supports DF1 and DH-485. DH+ and Remote I/O are harder: Rockwell’s migration manual lists no CompactLogix replacement for the 1747-SN scanner, and the ControlLogix DH+/RIO module (1756-DHRIO) was itself discontinued at the end of 2024. Anything still on DH+ or RIO needs a decision early, usually to move those devices to EtherNet/IP as part of the project.

PanelView Standard terminals move to PanelView Plus 7 by importing the PanelBuilder32 application into FactoryTalk View ME. Most screens and tags carry over, but the tags have to be readdressed to the new Logix tags, some objects do not map, long alarm messages may be truncated, and PanelView Plus 7 is Ethernet only. Check the panel cut-out too: no PanelView Plus 7 fits a PanelView 300 cut-out.

Before you start

  1. Know exactly what is installed. Every processor, module, adapter and HMI, with catalogue numbers and firmware, and every comms link. Unsupported modules and DH+ links are where plans fall over.
  2. Get a current backup of every program and confirm it matches what is running. Converting an old copy converts old logic.
  3. Clean the project. Remove unused memory and addresses, and SFC or structured text routines, before exporting.
  4. Decide the I/O route per rack, using Rockwell’s Integrated Architecture Builder SLC migration wizard to check module support and power loading.
  5. Plan the test, not just the cutover. I/O mapping, start-up and shutdown, alarms, permissives, safety interactions, device comms, and recovery after a fault or power cycle.
  6. Check licences. You need RSLogix 500 v12 for the export and a Studio 5000 Logix Designer edition that covers the target controller. The Lite edition does not cover ControlLogix, so check the edition against the target before ordering.

How ECS approaches it

Most SLC migrations we see are driven by a failure that nearly happened. The work that makes them go smoothly happens before any code is converted: a Capture audit records every device on the line, takes the backups and flags which modules are discontinued, so the plan is built on what is really installed. The converted programs, versions and cables then live in Command Centre, so the new system does not end up as undocumented as the old one.

We carry out the conversion, panel work and commissioning in-house as part of our automation projects, usually staged around production. If an SLC fails before the plan is in place, emergency support is the first call.

Sources

Lifecycle statuses and dates change. Check a specific catalogue number on Rockwell’s site before buying or planning around it.

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