Automatic translation utilities suggest that a PLC-5/250 program already speaks Logix. Production experience says otherwise. Integer files become structures, Remote I/O block transfers disappear, and asynchronous status bits that 6200-series software highlighted have no identical home on a MESSAGE tag. The migration succeeds when engineers treat it as a data-movement redesign guided by publications such as 1756-RM085, not as a search-and-replace on mnemonics.

BTR/BTW control bits do not survive intact—rebuild MSG instructions and rewrite every .AE/.AD consumer.

PLC-5 asynchronous bits versus MSG bits

On PLC-5/250, block-transfer control elements expose .AD (Asynchronous Done) and .AE (Asynchronous Error) alongside the more familiar .DN/.ER family. Those asynchronous flags are not synonyms for Logix MSG .DN/.ER in every edge case—PLC-5 can show mixed partial-completion states that CIP messaging simply does not reproduce. Document rung-condition-out behavior before deleting BT rungs.

Mechanical substitution (then rewrite the rung)
BTW N7:0 .AD  ->  MSG_tag.DN
BTW N7:0 .AE  ->  MSG_tag.ER
BTR N10:50 .AD -> MSG_tag.DN
BTR N10:50 .AE -> MSG_tag.ER

Architectural replacements for RIO block transfers

• Implicit I/O connections — POINT, FLEX, ArmorBlock over EtherNet/IP with RPI-driven updates and .Faulted status
• Explicit MSG — CIP Data Table Read/Write (or typed services only when the peer truly remains PLC-5)
• Produced/Consumed tags — peer Logix controllers exchanging data without a polled MSG
• 1756-DHRIO bridge — temporary home for remaining 1771 racks during phased cutover

Do not paste a BTR rung into Studio 5000 and rename the mnemonic. The instruction family is gone; the MSG must be created with a valid CIP path, service type, source/destination elements, and timeout.

Retain the PLC-5 image read-only until the ControlLogix cell survives a full production week.

Step-by-step conversion

1. Inventory every BTR/BTW: file, element, rack/group/slot, length, and status-bit consumers
2. Confirm target firmware supports the messaging services and EN2T/EN4TR paths you need
3. Run Translate PLC-5/SLC 500 where useful, then open every generated MSG configuration
4. Set Communication Path (backplane → ENxT → IP → remote slot), Service Type, and elements
5. Replace remaining .AE/.AD references with .ER/.DN on the new MESSAGE tags
6. Latch .ER into a fault structure; clear/retrigger deliberately—do not leave orphaned enables
7. Map N/B/T/C files into UDTs or arrays operators still recognize
8. Force former MCR/zone assumptions during FAT; scan order surprises appear here

MSG .ER/.DN mapping after cutover

Compare legacy BT lengths and update rates to MSG timeouts (≥5× expected service time is a common starting rule). Trend .DN/.ER alongside process values during soak tests. Prove DHRIO node octal addressing with a known pattern register before enabling writes. Keep the PLC-5 processor available as a rollback until KPIs match for a production week.

Migration hardware—controllers, EN2T-class modules, and temporary DHRIO—should be stocked under ControlLogix and PAC standards so the cutover is not waiting on a single surplus ENBT.