First: prove the PLC is alive (60 seconds)
Look at the controller itself — RUN light on, no fault LED — and ping it from a laptop on the same drop. Or faster: put the free LineKeeper app on the same network; it speaks EtherNet/IP, Siemens S7 and Modbus TCP. If the app sees the controller, your PLC and network are healthy and the fault is inside the HMI or its configuration — you've just cut the problem in half.
The universal checklist
| Check | Classic failure | Brand notes |
|---|---|---|
| 1 · Cable and link lights | Re-patched switch, broken lock tab, flooded conduit | All brands equally |
| 2 · IP and subnet | Duplicate IP after a device swap; DHCP surprise after power cycle | Check from the HMI's own network settings, then ping from a laptop |
| 3 · Driver / connection config | Points at the old controller, wrong rack/slot, wrong station | AB: the RSLinx Enterprise shortcut — dedicated PanelView guide. Siemens: connection partner, rack/slot. Modbus HMIs (Weintek, Delta): unit ID, station number |
| 4 · CPU generation mismatch | Comms died the day the CPU was upgraded | Siemens: S7-1200/1500 ship with PUT/GET disabled — enable it in CPU protection settings for classic connections. AB: old terminal firmware doesn't know the new Logix generation |
| 5 · PLC program changes | Specific values error while others work | Renamed tags (AB), moved or re-numbered DBs (Siemens) — and S7 optimized data blocks break absolute addressing entirely: the block must be non-optimized for classic HMI access |
| 6 · Connection limits | Comms drop when SCADA or a programmer connects | Every controller caps simultaneous connections; count the clients |
Reading the error language
- Every display errors at once → transport problem: cable, IP, dead controller port, connection limit.
- Specific values error, rest works → addressing problem: renamed tags, moved DBs, optimized blocks, wrong station on one device.
- Worked until a hardware swap → identity problem: duplicate IP, changed rack/slot, protocol the new CPU speaks differently (PUT/GET, firmware generation).
- Weintek "PLC no response" with a code → the code names the driver conversation that failed — station, IP or protocol mismatch on that specific device rather than the network: the dedicated Weintek guide.
If the HMI itself is the casualty
Comm hardware does die — a port killed by a nearby lightning event, a failed interface board. Before buying, check whether the panel is worth fixing at all: the black-screen triage covers the display-side faults, and for Allen-Bradley and Siemens units the PanelView timeline and Siemens replacement guide tell you whether you'd be repairing into a dead end.
Keep producing while you wait
Have the panel project export? Drop a FactoryTalk View, TIA Portal, Weintek or CSV export into our free HMI project converter — it runs in your browser, the file never leaves your PC, and every display becomes a page of buttons, indicators and values for the app, with the real tag names already in place.
Today, from a phone: the free LineKeeper app for iPhone and iPad speaks EtherNet/IP, Siemens S7 and Modbus TCP. On Allen-Bradley Logix it reads the controller's own tag list automatically; on S7 and Modbus you add the addresses you care about. Build a dashboard and an operator panel with start/stop buttons in about 15 minutes, mount a spare iPad in full-screen panel mode, and the operator has a screen before the first repair quote arrives. Reading is free; writes are a one-time unlock with confirmation and limit checks on every value.
The 15-minute temporary panel
Zero-config tag browser for Logix, drag-and-drop dashboard, operator panel, alarms — no changes to the PLC program.
On a desktop? Scan with your iPhone or iPad to get the free app.For the machine itself: the LineKeeper device — in development — is a self-contained 7″ or 10″ touchscreen built to live on the machine permanently — the same drag-and-drop screens, plus what a phone shouldn't carry:
- Live view of every tag, refreshed twice a second — plus write access with a full audit log
- Operator panel: start/stop, indicators, setpoints — hold-to-run buttons release safely
- Stays useful after the repair: data logging, dashboards, machine diagnostics
Small hardware that helps while the panel is out
- GL.iNet Opal travel router — Wi-Fi into the machine network from a spare switch port, so the phone app can stand in as the screen today.
- Anker USB-C to gigabit Ethernet adapter — wired straight into a spare switch port, the phone sees the controller in seconds; USB-C iPhones and iPads need no drivers.
- VIVO pole-mount VESA arm — puts the temporary screen at eye level on a machine post instead of a chair.
- TICONN IP67 enclosure with clear cover — houses the gateway or Wi-Fi bridge on the machine without bare boards on the rail.
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