How-To Guide  ·  Allen-Bradley ControlLogix  ·  Platform Migration

PLC-5 to ControlLogix Migration: Hardware, I/O, Wiring, and Software

Part Number: 1756-L83E  ·  Bulletin 1785 → 1756 · 4 Migration Paths · Field Wiring Preserved

1. Quick Migration Calculator

Skip the catalog cross-referencing. Pick up to 15 of your existing PLC-5 cards from the dropdowns below. We'll suggest the Rockwell-equivalent ControlLogix part numbers, recommend a chassis size and power supply, and pre-fill an RFQ you can submit in one click — all sourced from Rockwell's I/O Wiring Conversion Selection Guide (1492-SG121) and Modernization Profile (MIGRAT-PP003).

This is an initial BOM that should be qualified by a competent engineer. We offer Automation Consulting Services if you would like help understanding or executing the migration process of your system. You can also contact us at any time. Every part PLC Exchange stocks is backed by our 2.5-year warranty.

2. Why Migrate Now

The PLC-5 was one of the most successful PLC platforms ever produced — but it has reached end-of-life. Rockwell Automation publication MIGRAT-PP003 (The Modernization of PLC-5 Controllers and 1771 I/O to ControlLogix) confirms the formal discontinuance date of June 30, 2017. Every PLC-5 processor (1785-L11B/L20B/L30B/L40B/L60B/L80B/L20E/L40E/L80E and the redundant 1785-L40C15/L80C15/L20C15) and the majority of 1771 I/O modules are now in the Discontinued phase. New units are no longer produced; the secondary market and Rockwell’s repair/exchange services are the only sources.

What's Driving the Migration

Why ControlLogix Specifically?

Rockwell’s recommended PLC-5 migration target is the ControlLogix 1756 platform — not CompactLogix 5380. The reason is form-factor and footprint preservation: PLC-5 systems are chassis-based, often with high I/O counts, sometimes with redundancy, and frequently distributed across multiple chassis in different cabinets. ControlLogix is the only Logix platform that matches this architecture point-for-point: 4/7/10/13/17-slot chassis (1756-A4 through 1756-A17), modular AC and DC power supplies (1756-PA72/PA75/PB72/PB75), redundant controller pairs (1756-RM3 redundancy modules), and the 1492 swing-arm conversion system that lets you reuse PLC-5 field wiring with no touch on the field side.

3. Migration Path Overview

There are four practical migration paths from PLC-5 to ControlLogix. They differ in how much existing hardware (chassis, I/O, field wiring) you keep versus replace, and in how much risk and downtime the cutover carries. Most real projects use a combination — for example, the 1492 swing-arm conversion (path B) on three chassis whose 1771 I/O has direct 1756 equivalents, plus a 1756-RIO bridge (path C) on a fourth chassis with obsolete specialty modules that need a phased programming and tuning effort.

4. Path A: Full Hardware Replacement

Path A is the cleanest architecture but the most labor-intensive. Every component of the legacy PLC-5 system — chassis, processor, I/O modules, swing arms, and field-wiring landings — comes out. New 1756 ControlLogix hardware goes in, with field wires terminated directly on new 1756 RTBs (Removable Terminal Blocks) or on intermediate IFM (Interface Module) terminal blocks.

When Path A Makes Sense

1. The cabinet is being rebuilt anyway. If a panel refresh is already scheduled (corrosion, capacitor aging, branch-circuit upgrade), the marginal cost of full I/O replacement drops dramatically.
2. The 1771 I/O mix is heavy on obsolete specialty modules. 1771 modules without a clean 1756 equivalent (some 1771-N high-resolution analog variants, certain motion modules, custom OEM modules) make swing-arm conversion impractical.
3. Field wiring needs to be replaced anyway. Aged insulation, undersized conductors, or a need to add shielded cabling for analog signals all argue for fresh wiring.
4. The new control panel is in a different physical location. If the controller is moving from a process area to a central control room, you are running new conduit anyway.

Bill of Materials Pattern

A typical full-replacement BOM for a single 16-slot PLC-5 chassis with mixed digital and analog I/O looks like this (exact catalog numbers depend on I/O mix — see the I/O equivalents section below):

5. Path B: Swing-Arm 1492 Wiring Conversion

Path B is the marquee strategy in Rockwell’s migration profile (MIGRAT-PP003) and is the path most PLC-5 conversions ultimately take. The premise is simple: do not touch the field wiring. Instead, leave every field wire on its existing 1771 swing arm. Replace the 1771 chassis itself with a 1492 conversion mounting assembly (a base plate that has the same footprint and bolt pattern as the 1771 chassis it replaces). Plug the 1771 swing arm — with all field wires still attached — onto a 1492 conversion module (which has the same edge connector as the 1771 module the swing arm came from). A pre-wired 1492 conversion cable carries the I/O signals from the conversion module to a 1756 RTB, which plugs onto the new 1756 module mounted on a 1756 chassis above the conversion plate. Field wires never move. Documentation per MIGRAT-PP003 page 7.

How the Conversion Mounting Assembly Works

The 1492 conversion mounting assembly is a metal base plate with two functions: (1) it mounts in the cabinet using the same bolt holes as the 1771 chassis it replaces, so no drilling and tapping is required, and (2) it carries a row of 1492 conversion modules where the 1771 modules used to plug in. Above the conversion modules sits a cover plate with pre-drilled holes for the new 1756 chassis. The cover plate supports left-justified, right-justified, or centered 1756 chassis mounting because the 1756 chassis is sometimes wider than the 1771 chassis it replaces (per 1492-SG121 page 4). Combined depth is 10.25 inches with the controller key, 10.0 inches without.

Mounting Assembly Selection

Selection follows the original 1771 chassis size:

Note: the 1771-A3B1 is for 19-inch instrumentation panels and uses the 1492-MUA3B-A10-A13 assembly. Source: 1492-SG121 page 4.

The Eight-Step Conversion Sequence

Per MIGRAT-PP003 page 7, the actual conversion of one chassis follows these eight steps:

1. Document the existing 1771 system. Take photos of the chassis layout, swing-arm wiring, and field-side terminations. Capture I/O addressing in RSLogix 5.
2. Remove the 1771 swing arms from the 1771 modules (field wires stay attached to the swing arm).
3. Remove the 1771 chassis from the cabinet (it has been depopulated — only the empty chassis comes out).
4. Install the 1492 conversion mounting assembly base plate in the cabinet using the original 1771 mounting holes. No new holes drilled.
5. Install the 1492 conversion modules (one per 1771 module being converted) on the base plate. Each conversion module is selected by the 1771 module it accepts — e.g., 1492-CM1771-LD007 for a 1771-IA, 1492-CM1771-LA002 for a 1771-IFE in differential mode.
6. Plug the 1771 swing arm — field wires still attached — onto the conversion module’s edge connector. Field wiring has now been transferred without a single wire being touched.
7. Connect the 1492 conversion cable from the conversion module to the 1756 RTB on the new 1756 I/O module. The cable is pre-wired to handle the pinout translation.
8. Install the 1756 chassis on the cover plate, populate with 1756 I/O modules in the order matching the 1492 conversion modules, install the new 1756-PA72/PB72 power supply, and the 1756-L83E (or chosen) processor.

Conversion Cable Length Options

The 1492 conversion cables ship in two standard lengths, and combination cables (for 2-module-to-1-module conversions) come in mix-and-match length pairs:

6. Path C: 1756-RIO Remote I/O Bridge

Path C is the phased cutover strategy. Instead of replacing PLC-5 hardware in one event, you replace only the processor first — leaving the entire 1771 I/O chassis tree, all 1771 modules, and all field wiring exactly where they are. The new ControlLogix processor in a separate 1756 chassis communicates with the legacy 1771 I/O over the existing PLC-5 Remote I/O (RIO) network, using a 1756-RIO module as a network scanner in the new chassis.

How the RIO Bridge Works

The PLC-5 Remote I/O network (often called “Blue Hose” from the cable jacket color) is a Rockwell-proprietary serial network that carries I/O image data between a PLC-5 processor (the RIO scanner) and one or more 1771-ASB adapter modules in remote 1771 chassis. The 1756-RIO module is a ControlLogix module that plays the role of the PLC-5 processor on this network — it scans the 1771-ASB adapters, builds the same I/O image, and presents it to the ControlLogix controller as a tag-based I/O tree.

Per MIGRAT-PP003 page 6, the 1756-RIO module “enables communication and data transfer between a ControlLogix controller and other devices on your existing RIO network. It can be used to upgrade an existing PLC-5, PLC-3, or SLC system to a ControlLogix system. The advantages of using the 1756-RIO module in a phased modernization include allowing the existing Remote I/O network to remain in place — allowing the new application to be tested before switch over — and to switch back to the old application in minutes.”

Why This Path Is the Risk-Reduction Champion

1. Test against live I/O before cutover. The new ControlLogix can be powered, programmed (with RSLogix Project Migrator-converted code), and commissioned against the existing 1771 I/O over the existing RIO network — before pulling the legacy PLC-5 processor.
2. Two-minute switchback. If the new logic misbehaves during commissioning, swap the RIO scanner role back to the original PLC-5 processor and you are back in production. This is the lowest-risk single cutover possible.
3. Phased I/O replacement. After the controller is proven stable, you can convert individual 1771 chassis to path B (1492 swing-arm conversion) one at a time over multiple shutdowns — without any further controller changes.
4. Multi-chassis support. The 1756-RIO module supports the same RIO network rack scan capacity as a PLC-5 processor, so you can keep all existing 1771 chassis in the network if your topology has multiple drops.

Bill of Materials Pattern

7. Path D: EtherNet/IP Distributed I/O

Path D is the modern greenfield strategy. Instead of mounting the new 1756 I/O directly above the conversion plate where the 1771 chassis used to be (path B), you put the controller in a central location and the I/O drops at remote locations — connected over EtherNet/IP. This is the architecture every new Rockwell capital project uses today: a small central chassis with the processor, distributed I/O drops in the process area.

When Path D Makes Sense

1. Network refresh is part of the project. If the cabinet upgrade is the first time anyone has run modern Cat6 industrial Ethernet to the panel, the marginal cost of putting the controller in a central control room is small.
2. The site has multiple PLC-5 chassis in different physical areas. Combining them under a single ControlLogix controller (instead of replacing each PLC-5 with its own ControlLogix) saves licensing, reduces engineering complexity, and centralizes the control architecture.
3. DLR ring resilience matters. EtherNet/IP DLR (Device Level Ring) provides <3 ms recovery from a single network fault. Critical processes that previously required ControlNet for determinism can use DLR EtherNet/IP today.
4. You want OPC UA, MES, and historian on the same wire. EtherNet/IP runs alongside enterprise traffic on the same cable plant when properly segmented. Legacy DH+ and RIO cannot.

Architecture Pattern

A typical path D system has a small central 1756 chassis (controller + EtherNet/IP modules), plus one or more remote chassis at the I/O drops. Each remote chassis has a 1756 EtherNet/IP adapter (operating as an EtherNet/IP target) plus 1756 I/O modules:

Combining Path D with Path B

Path D and path B are not mutually exclusive. A common pattern: keep the 1771 chassis footprint (path B with 1492 conversion modules) at the original I/O location, but instead of running an I/O backplane back to the controller, install a 1756-EN2TR adapter in that remote 1756 chassis. This gives you the field-wiring preservation benefit of path B and the network architecture benefit of path D. The controller can be anywhere on the EtherNet/IP network.

8. Choosing the ControlLogix Controller

Selecting the right ControlLogix processor is the most consequential decision in a PLC-5 migration. Undersized memory or node-count headroom forces a re-buy a few years into the system’s life. The mapping below is the conservative recommendation Rockwell uses for typical PLC-5 capital migrations, calibrated against the published specs in 1756-TD001 (ControlLogix Controller Specifications).

PLC-5 to ControlLogix 5580 Recommended Mapping

Note: PLC-5 user memory in 16-bit words does not translate cleanly to ControlLogix 32-bit memory in MB. The mapping above accounts for the different addressing models (file/element vs tag/UDT) and the typical 2–3x logic expansion that happens when RSLogix 5 is converted via the RSLogix Project Migrator (because integer arithmetic with B/N/S file structures gets re-expressed as native UDT and structured-text constructs).

ControlLogix 5580 Family at a Glance

Legacy ControlLogix 5570 (1756-L7x) Option

The previous-generation ControlLogix 5570 family is still supported for spares and is sometimes the right choice when matching an existing in-plant 5570 standard. The two most common 5570 controllers in this scenario are the 1756-L73 (8 MB user memory) and the 1756-L75 (32 MB user memory). For new capital projects, 5580 (L8xE) is the recommended choice because of the integrated Gigabit Ethernet, OPC UA support, and CIP Security.

9. Chassis and Power Supply Selection

Chassis selection is straightforward in path B (where the conversion mounting assembly dictates the 1756 chassis size) and almost as simple in paths A, C, and D — pick the smallest 1756 chassis that fits the controller, comm modules, and I/O modules with a slot or two of growth headroom.

1756 Chassis Sizes

Source: 1492-SG121 page 4, ControlLogix chassis dimensions table.

Power Supply Selection

ControlLogix power supplies are picked on (1) input voltage source and (2) backplane current draw. Most PLC-5 retrofits inherit the cabinet’s existing supply — 120/240 VAC for traditional plant cabinets, 24 VDC for modern integrated panels:

10. 1771 to 1756 I/O Module Equivalents

This is the master table of 1771 I/O modules and their 1756 ControlLogix equivalents, plus the 1492 conversion module and conversion cable for path-B swing-arm conversion. Source: 1492-SG121 (I/O Wiring Conversion Systems — PLC-5 1771 to ControlLogix 1756 Selection Guide) pages 5–8. The 1492 cables shown are the recommended 0.5 m default length (suffix 005); 1.0 m and combination lengths are available as listed in the path B section.

Digital Inputs

Digital Outputs

Analog I/O

Source: 1492-SG121 pages 5–7. The full I/O equivalents matrix has additional 1771 module variants (1771-IC, 1771-IG, 1771-IH, 1771-IM, 1771-IN, 1771-IT, 1771-IV, 1771-OC, 1771-OG, 1771-OM, 1771-ON, 1771-OQ, 1771-OR, 1771-OVN, 1771-OWNA) plus the 1771-N high-resolution isolated analog modules — consult the full selection guide for all options. The conversion modules and cables shown above are stocked at PLC Exchange.

11. Software Migration: RSLogix 5 to Studio 5000

Hardware is half the migration. The other half is converting the RSLogix 5 program to Studio 5000 Logix Designer, including all logic, I/O addressing, data files, and any motion or PID configuration. Rockwell provides a free conversion utility — the RSLogix Project Migrator — that handles the bulk of the conversion mechanically; the rest is engineering effort that varies with program complexity.

RSLogix Project Migrator

Per MIGRAT-PP003 page 6, the RSLogix Project Migrator is a “standalone software tool for converting an RSLogix 5 or 500 project export file for import into the Studio 5000 Logix Designer application.” The workflow is: (1) export the RSLogix 5 project from the live PLC-5 to a .PC5 file; (2) run the Project Migrator on the .PC5 to produce a Studio 5000 .L5K import file; (3) import the .L5K into a new Studio 5000 project; (4) review and remediate the conversion notes. Per MIGRAT-PP003 page 7, the tool typically converts 80–100% of the code automatically. The remaining 0–20% is manual remediation — messages, certain special instructions (PID with non-default tuning), and timing/scan differences.

What Converts Automatically

1. Ladder logic rungs — XIC, XIO, OTE, OTL, OTU, TON, TOF, RTO, CTU, CTD, MOV, ADD, SUB, MUL, DIV, EQU, NEQ, GEQ, LEQ, GRT, LES, AND, OR, XOR, NOT.
2. Data files — B, N, F, T, C, R, ST, A, D files convert to native Logix tags with corresponding data types (BOOL, INT, REAL, TIMER, COUNTER, CONTROL, STRING).
3. I/O addressing — PLC-5 file/word/bit addresses (e.g., I:001/00, O:002/15) get rewritten to Logix tag-based addresses (e.g., Local:1:I.Data.0, Local:2:O.Data.15) when the I/O configuration is built first.
4. Subroutines (SBR/JSR/RET) — preserved with parameter passing.
5. SCP, FAL, FFL/FFU, LFL/LFU, BSL/BSR — structured arithmetic and shift instructions translate to their Logix equivalents.

What Requires Manual Review

Address Structure: File-and-Element to Tag-Based

The biggest conceptual change in the migration is the shift from PLC-5’s file-and-element address model to ControlLogix’s tag-based model. A PLC-5 program references “N7:0” (file 7, element 0); ControlLogix references a named tag like MainPump_Speed with a defined data type. The RSLogix Project Migrator preserves the original file naming as a prefix (so N7:0 may end up as N7_0 or similar), but the engineering opportunity in a migration is to rename these to meaningful aliases during conversion. This is the single highest-value engineering investment in the migration project — future maintenance becomes dramatically easier when tags read like English.

12. Sample Migration: 100-Point Plant Floor

To make the choices concrete, here is a worked example: a single 16-slot 1771-A4B chassis with a 1785-L40B processor, mixed digital and analog I/O totaling about 100 points, no redundancy, no motion, plant air-handling and water-loop control. The recommended path-B (swing-arm conversion) BOM:

Existing PLC-5 Configuration

ControlLogix Path-B Replacement BOM

Recommended Cutover Sequence

1. T − 8 weeks: Run the RSLogix Project Migrator on the production PLC-5 program. Review conversion notes. Build the Studio 5000 project skeleton with the I/O tree pre-configured for path B target modules.
2. T − 4 weeks: Stage the 1492 conversion mounting assembly + 1756 chassis on the bench, fully populated with new I/O modules. Power up. Verify all module LEDs and Studio 5000 communication. This is offline rehearsal.
3. T − 1 week: Final code conversion run against the most recent PLC-5 program. Re-test on the bench setup with simulated I/O. Sign off the program with operations.
4. Cutover (planned outage, typically 8–16 hours for a 100-point system): Power down the cabinet. Remove 1771 swing arms. Pull the 1771 chassis. Mount the 1492 conversion plate using the original 1771 bolt holes. Install conversion modules + cables + 1756 chassis. Plug all swing arms onto conversion modules — field wiring untouched. Power up the new chassis. Download the new ControlLogix program. Function-test every input and output. Restart the process under operations supervision.
5. T + 1 day: Operations runs 24-hour soak test under nominal load. Engineering on-call for any anomalies. The pulled 1785-L40B and 1771 modules stay packaged at the cabinet for emergency swapback.
6. T + 2 weeks: Once the system is proven stable, the legacy PLC-5 hardware is officially decommissioned and the 1771 chassis can be moved to spare-parts inventory or scrap.

13. Related Guides

These guides cover related ControlLogix products and topics:

1. SLC 500 to ControlLogix Migration Guide — Companion guide for the 1746/1747 SLC 500 family. Covers the 1747-AENTR adapter strategy, 1492 swing-arm conversion for 1746 I/O, and RSLogix 500 to Studio 5000 conversion.
2. 1756-L85E ControlLogix 5580 for PlantPAx & Batch Process Guide — The premium ControlLogix controller for batch-sized process projects, including L85E vs L85EP comparison, PlantPAx architecture, redundancy, and Studio 5000 process project setup.
3. 5069-AENTR Compact 5000 EtherNet/IP Adapter Guide — The Compact 5000 equivalent of path D, used when the migration target is CompactLogix 5380 instead of ControlLogix.

For the authoritative source documents, download MIGRAT-PP003 (The Modernization of PLC-5 Controllers and 1771 I/O to ControlLogix) and 1492-SG121 (I/O Wiring Conversion Systems — PLC-5 1771 to ControlLogix 1756 Selection Guide) from Rockwell Automation’s literature library.

Reference Documentation

The following Rockwell Automation publications were used as references for this guide. These are the official manufacturer documents for the hardware covered in this article.