PLC-5 systems are still running in many gas stations, power plants and production lines in Vietnam. The hardware still works, but the fallback options are narrowing: RSLogix 5 programming software has been discontinued since 31 December 2025, and the 1756-DHRIO bridge module — which lets ControlLogix scan legacy 1771 I/O racks — has been discontinued since 31 December 2024 (product pages on rockwellautomation.com). This article collects what to know before moving a PLC-5 system to ControlLogix: what the conversion tool does, what it does not do, and whether to keep or replace the 1771 I/O.
Two separate jobs: program and I/O
A PLC-5 upgrade always has two parts, and they should be planned separately:
- Program: convert the RSLogix 5 file into a Studio 5000 Logix Designer project (.ACD), then review, correct and test it.
- I/O hardware: keep the 1771 racks during a transition period, or replace them fully with ControlLogix I/O / distributed I/O on EtherNet/IP.
The two decisions are linked from the very first step: Rockwell's tool asks for the I/O strategy before it generates the new project.
Converting the program with Logix Designer Export
Logix Designer Export is built into RSLogix 5: open the project, Save As, and choose the .ACD format (1756-RM085I p. 11). Before running it, Rockwell recommends cleaning up the source file: delete unused memory and unused addresses (Tools > Delete Unused Memory, Tools > Database > Delete Unused Addresses), and remove SFC and structured text routines, to avoid syntax errors — the tool does not convert these anyway (p. 9, 11).
In the export dialog the engineer selects the controller family, the controller catalog number and the target firmware/software revision (p. 13), then the I/O strategy (p. 14–15):
- Keep Existing I/O — retains the I/O modules and devices of the old project; the tool automatically adds a bridge device so that Logix can scan the existing PLC-5 I/O.
- Update All I/O — Rockwell's preferred option: migrates the modules to the latest ControlLogix/CompactLogix I/O family, with no bridge device. If the I/O is placed in remote racks, all remote I/O generated by the tool is on EtherNet/IP.
What the tool does, and what stays manual
Rockwell is explicit: the result is syntactically correct, but the intent of the original program can be lost, because of different rules for precedence, indexed addressing and I/O addressing (p. 9). Suspect rungs are flagged with a PCE instruction; instructions with no equivalent become UNK instructions. Both must be deleted and replaced with correct logic (p. 16–17).
| Item | How the tool handles it | Work required afterwards | Source (1756-RM085I) |
|---|---|---|---|
| Languages | Converts ladder only | Rewrite SFC and structured text | p. 9, 22 |
| 16-bit → 32-bit architecture | N files become DINT, sign-extended; B files become DINT with the upper 16 bits zero-filled | Review instructions that use masks, as they may behave differently; rework BSL, BSR and BTD on B files | p. 21, 25–26, 31 |
| Timers | Converted to a 1 ms time base, presets scaled from the old time base (e.g. 0.01 s, preset 20 → 200) | Every reference to .PRE/.ACC generates a PCE and must be reviewed | p. 21, 32 |
| I/O | Creates matching I and O arrays (I:002 → I[2]) but leaves them empty | Map the new I/O tags into the arrays with CPS, BTD or MOV; Logix I/O updates asynchronously to the scan, so buffer it with CPS | p. 10, 19–20 |
| Block transfers (BTR/BTW) | Converted to MSG; rack/group/module ignored | Reconfigure the path; adjust 16 → 32 bit with RES/FAL | p. 57–58 |
| ControlNet I/O (CIO) | Converted to MSG, PCE generated | Reconfigure | p. 58 |
| MSG instructions | Only partially converted | Complete the Communication tab and the path to the device | p. 20, 41 |
| PID | Structure converted | Many parameters do not map directly — the loop must be verified | p. 41, PCE 114 |
| Main control programs (MCP) | Become event tasks, called by EVENT instructions in the continuous task | Check execution order | p. 23 |
| STI interrupt | Becomes a periodic task | Set its priority; S:2/1 (STI enable/disable) generates a PCE | p. 23 |
| PII input interrupt | Not supported in Logix; converted to a subroutine | Call the routine from your logic | p. 23–24 |
| Indirect addressing | Element and bit indirection converted; indirect file numbers (N[N7:0]:5) are not | Rewrite, for example with a multi-dimensional array if the files are consecutive | p. 46–47 |
| FOR/NXT/BRK | Structure is fundamentally different in Logix | Logic usually has to be restructured | p. 76 |
| AGA3, AGA7, API routines (CIR/COR instructions) | Not converted, PCE generated | Rebuild the gas flow calculations | p. 58, 76 |
| Comments | Instruction comments not converted; address comments become alias tags | Delete unused aliases to save memory | p. 21 |
The AGA3/AGA7/API row matters for gas stations: if the PLC-5 calculates flow with these routines, that part does not come across with the tool and has to be rebuilt and verified separately.
Keep or replace the 1771 I/O
Keep the 1771 racks during a transition. The 1756-DHRIO module lets ControlLogix scan Remote I/O racks (1771-ASB…) and exchange messages with PLC-5 over DH+ (1756-UM514C p. 13–15, p. 101). This allows the processor to be replaced first and the I/O later. Channel limits must be respected: up to 32 rack numbers per channel, but only 16 if the network includes a 1771-ASB/A, 1771-ASB/B, 1771-AS, 1771-DCM or 1771-AF; 57.6/115.2/230.4 kbps correspond to total cable lengths of 3,048/1,524/762 m (p. 102–103). Since the 1756-DHRIO is discontinued, this route only suits plants that still hold the modules.
Replace the I/O fully. This is the option Rockwell recommends in the conversion tool ("Update All I/O"). The new I/O sits in the ControlLogix chassis or is distributed on EtherNet/IP; if the old system uses ControlNet, the network should move to EtherNet/IP at the same time (see Upgrade paths for legacy Allen-Bradley systems). Existing enclosures can be kept and refitted inside; field wiring is re-terminated to the new drawings.
The HMI that comes with it
PLC-5 usually comes with RSView32, Citect or a legacy PanelView. The paths for moving HMI/SCADA to FactoryTalk View, AVEVA or Ignition are collected in Legacy HMI/SCADA: what to replace it with. It is best to do the HMI in the same phase, because tags and addresses all change when the program moves to Logix.
How TDL does it
- Start from the program that is actually running: upload from the PLC-5 on site and compare it with the stored offline copy before converting.
- Inventory: racks, I/O and block-transfer modules, networks (DH+, Remote I/O, ControlNet), MSG list, third-party devices, HMI.
- Convert and review: run Logix Designer Export, resolve every PCE/UNK, map the I/O, rewrite what the tool does not convert.
- Bench test: the new program runs on a real controller with simulated I/O and is checked function by function against the old system.
- FAT with operations, then cut over in a shutdown window; the cutover plan should include a fallback to the old system, agreed with the plant in advance.
- Handover: Logix project, as-built drawings, documentation and backup.
PLC-5 projects completed by TDL
- Pha Lai II Thermal Power Plant subsidiary systems (2020): PLC-5 and Citect to ControlLogix and FactoryTalk View SE (Rockwell Automation as EPC contractor; TDL built the panels, carried out PLC/HMI programming and networking, and commissioned) for ash handling, flue-gas desulphurisation (FGD), electrostatic precipitator, soot blower, water and wastewater treatment.
- Phu My Gas Distribution Station (2019): PLC-5/HIMA to an ICSS on redundant ControlLogix and AADvance.
- PV Gas, other stations: an LBV station (2013), a loading arm system (2014), and the piping monitoring system at Ba Ria Gas Distribution Station (2014, PLC-5/RSView32 to redundant ControlLogix and FactoryTalk View).
SLC 500 follows the same approach — for example Dinh Co Gas Processing Plant, CC01 turbo expander (2019).
Frequently asked questions
Are RSLogix 5 and the 1756-DHRIO still sold?
According to the product pages on rockwellautomation.com, RSLogix 5 programming software has been discontinued since 31 December 2025 and the 1756-DHRIO bridge module since 31 December 2024. PLC-5 hardware still works, but the fallback options are narrowing.
Which tool converts a PLC-5 program to ControlLogix?
Logix Designer Export, built into RSLogix 5: open the project, Save As, and choose the .ACD format (1756-RM085I p. 11). Before running it, delete unused memory and addresses and remove SFC and structured text routines; in the export dialog you select the target controller and the I/O strategy.
Does Logix Designer Export convert the whole PLC-5 program?
No. The tool converts ladder only, and the result is syntactically correct but the intent of the original program can be lost (1756-RM085I p. 9); suspect rungs are flagged with PCE, instructions with no equivalent become UNK, and AGA3, AGA7 and API routines are not converted. All of these must be reviewed and rewritten by hand.
Can the 1771 I/O racks be kept when moving to ControlLogix?
Yes, during a transition: the 1756-DHRIO module lets ControlLogix scan 1771 Remote I/O racks and exchange messages with PLC-5 over DH+ (1756-UM514C pp. 13–15, 101). Each channel takes up to 32 rack numbers, but only 16 if the network includes a 1771-ASB/A, 1771-ASB/B, 1771-AS, 1771-DCM or 1771-AF; since the 1756-DHRIO is discontinued, this route only suits plants that still hold the modules.
Sources
- Rockwell Automation, 1756-RM085I-EN-P, Converting PLC-5 or SLC 500 Logic to Logix-Based Logic, 09/2025 — pp. 9–11, 13–17, 19–26, 31–32, 41, 46–47, 57–58, 76
- Rockwell Automation, 1756-UM514C-EN-P, ControlLogix Data Highway Plus-Remote I/O Communication Interface Module User Manual, 06/2014 — pp. 13–15, 101–103
- Rockwell Automation, RSLogix 5 and 1756-DHRIO product pages on rockwellautomation.com
This article paraphrases the sources and does not quote them; read the original documents before designing or working on a system. Links open the latest edition in Rockwell's literature library; page numbers refer to the edition cited.