When a live machine develops a fault, go online to inspect what the controller recorded. Its diagnostic buffer lists diagnostic events with timestamps, newest first. Read those entries alongside a watch table of live tag values to investigate the fault. Use the controls below to introduce events and watch the corresponding entries appear in the log.
The buffer is on the left; a watch table on the right monitors four tags. Switch mode, trip an overload or pull a module using the controls below. Each action adds a timestamped event and event code to this simulated buffer. When a fault sends the CPU to STOP, look for the entry with its cause to begin tracing what happened before the stop.
| Tag | Value | Force |
|---|---|---|
| "Conv_Motor":P | FALSE | |
| "Motor".current | 3.2 A | unavailable |
| "Line".speed | 0 /min | unavailable |
| "ET200".online | TRUE | unavailable |
A forced value can override the program's normal control of an input or output. The CPU shows a warning while a force is active. Forcing can cause unexpected machine movement: use the approved test procedure and never leave a force as a fault repair. Safety functions must remain under the safety relay or safety PLC.
Begin the investigation with these checks: go online → read the diagnostic buffer → watch the suspect tags. Treat forcing as a separate test action on a live machine, subject to the approved procedure and precautions against unexpected movement. The Trace tool adds high-speed recording, and the built-in web server lets you view the buffer through a browser.
📓 Diagnostic buffer: read the timestamped entries for mode changes, module faults and programming errors. These give you a starting point for the investigation.
👁️ Watch & force tables: monitor tag values while the program runs. Forcing holds supported inputs or outputs at selected values for testing and requires precautions against unexpected movement.
📈 Trace: record selected tags at the cycle clock to capture a glitch that is too fast to observe in a watch table, then inspect it in a scope plot.
🌐 Web server: view the CPU's diagnostics page over HTTP. You can inspect the buffer and module status in a browser without opening TIA.
A machine tripped to STOP overnight. Nobody saw it. What's the single fastest first step to find out why?