On a DH-485 multi-drop trunk serving SLC-500 processors, one node can take every other station offline while looking healthy in isolation. RSLinx may still see that single controller through a local interface, the processor stays in Run, and InTouch or RSView tags for every peer go bad. Disconnecting the suspect DH-485 cable restores the network immediately. That pattern is almost never a ladder or MSG bug. It is a physical-layer or link-layer event—most often a failed 1747-AIC transceiver that holds the differential pair and stops token circulation.

Prove the link coupler first. Program edits and channel rebuilds waste hours when one AIC is electrically locking the bus.

Hardware in scope

Publication 1770-UM018 remains the electrical and protocol reference for DH-485 interface behavior.

Ranked root causes

1. Failed 1747-AIC (or NET-AIC) RS-485 driver that passes single-node tests but corrupts multi-drop token arbitration.
2. Duplicate node address with no link-layer duplicate detection—presents identically as a bus lock.
3. Mid-trunk termination or shield grounded at both ends, collapsing differential margin or biasing common mode.

Baseline before disturbing cables

Record every node address, baud rate (often 19200, sometimes 9600 legacy), maximum node setting, terminator locations, shield bond point, and processor fault bits S:5 / S:6. A faulted SLC can hold its port abnormal independently of the AIC; clear major faults before blaming the network.

Isolation sequence

1. Confirm the suspect CPU is in Run with clear fault words.
2. Hot-swap the entire 1747-AIC with a known-good spare of the same catalog. If RSWho recovers, scrap the original coupler—do not reseat and return it to service.
3. If the network still collapses, swap chassis to separate address/cabling issues from a specific processor.
4. Temporarily readdress the node to an unused number only after AIC swaps fail; update SCADA mappings in the same change window.
5. Verify 150 Ω terminators only at the two physical ends and that AIC terminator jumpers are open on mid-span nodes.
6. If stock spares were questionable, install a factory-sealed AIC. Silent failures on shelf spares are common.

Replacement guidance

Specify 1747-AIC for standard SLC modular chassis isolation and 1747-AICR where the application requires the isolated variant. Match baud and node settings exactly after replacement. Do not leave factory terminators enabled on every AIC in the rack.

After-action documentation

When the network returns, update the DH-485 topology drawing with the replaced AIC serial or install date, confirm every node address against the channel configuration printout, and note whether the failed coupler was a shelf spare or an in-service unit. That record prevents the next outage from starting with another untested spare. If InTouch or RSView scripts cache stale node lists, recycle the DA server after the physical repair so quality bits clear cleanly.

Swap-test the AIC before redesigning the trunk

Success is simultaneous Active status for all nodes in RSWho plus restored HMI tag quality—not merely local visibility of the repaired node. Avoid debugging MSG instructions first; avoid mid-span terminators; avoid dual shield grounds; avoid returning a “maybe good” AIC to the spare drawer without a bench multi-drop test. Keep certified AIC spares with other PLC and PAC communications hardware so a bus lock does not become a production-wide outage.