PLC-5 to ControlLogix: Instruction Set Conversion Table
Conversion reference for Allen-Bradley PLC-5 instructions to ControlLogix equivalents. Covers file instructions, Block Transfers, indirect addressing, and PID changes.
PLC-5 to ControlLogix: Instruction Set Conversion Table
Most PLC-5 instructions have direct equivalents in ControlLogix. The key differences are in file handling, Block Transfers, indirect addressing, and communication instructions.
Instructions That Convert Directly
All basic bit, timer, counter, math, and comparison instructions (XIC, XIO, OTE, OTL, OTU, TON, TOF, RTO, CTU, CTD, ADD, SUB, MUL, DIV, EQU, NEQ, GRT, LES, MOV, COP, FLL) work identically in ControlLogix. The only change is addressing: file-based (N7:0) becomes tag-based (MyTag).
Instructions That Require Changes
| PLC-5 Instruction | ControlLogix Equivalent | Change Required |
|---|---|---|
| BTR (Block Transfer Read) | MSG (Message) | Complete restructure — BTR/BTW replaced by MSG with CIP path |
| BTW (Block Transfer Write) | MSG (Message) | Same — MSG handles all communication |
| MSG (DH+ based) | MSG (EtherNet/IP CIP path) | Path syntax changes from DH+ node to IP address/CIP path |
File indirect N7:[N10:0] | Array tag MyArray[MyIndex] | Syntax change — same concept |
| PID (PLC-5 structure) | PIDE (enhanced PID) | Different structure, more parameters, anti-windup built in |
| SFC structures | Sequential steps | Re-implementation recommended |
| ASCII instructions (AHL, ARD, AWT) | ASCII serial port instructions | Similar but different parameter structure |
Indirect Addressing Conversion
PLC-5 indirect addressing is powerful but converts to a different syntax:
| PLC-5 | ControlLogix | Notes |
|---|---|---|
N7:[N10:0] | MyArray[MyIndex] | Tag-based array with integer index |
#N7:0 | MyArray[0] (with length) | File reference → array with explicit length |
B3:[N10:0]/0 | MyBoolArray[MyIndex].0 | Bit-in-word with computed index |
Migration trap: PLC-5 allows nested indirect addressing (N7:[N10:[N12:0]]). ControlLogix does not support nested indirection — you must unroll to intermediate variables.
Block Transfer → MSG Conversion
This is the most labor-intensive conversion for PLC-5 programs with analog or specialty I/O:
| PLC-5 | ControlLogix |
|---|---|
| BTR with rack/group/module | MSG with CIP path to module |
| BTW with data file reference | MSG with source tag |
| Continuous BTR/BTW scheduling | Periodic MSG trigger logic |
On ControlLogix, most analog and specialty modules are configured in the I/O tree and produce/consume data automatically — no MSG instructions needed. BTR/BTW are only required for legacy Remote I/O modules accessed via 1756-DHRIO bridge.
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