OX400 — Communication Fault / No Update to DCS (Decision Tree)
Use when DCS shows bad quality, comms alarm, or values are frozen.
Decision Tree
1) Local value changes?
If local display changes normally but DCS is frozen → issue is output/comms, not measurement.
If local display changes normally but DCS is frozen → issue is output/comms, not measurement.
2) Output type?
Identify if site uses 4–20mA or digital (Modbus/Ethernet). Follow relevant steps below.
Identify if site uses 4–20mA or digital (Modbus/Ethernet). Follow relevant steps below.
3) For 4–20mA
Check wiring polarity, loop power, terminal tightness, and that DCS AI card is healthy. Confirm range/scaling matches analyzer configuration.
Check wiring polarity, loop power, terminal tightness, and that DCS AI card is healthy. Confirm range/scaling matches analyzer configuration.
4) For digital comms
Check IP/serial settings, cable, switch port, link LEDs, and that polling is active. Confirm register mapping and quality/status bits.
Check IP/serial settings, cable, switch port, link LEDs, and that polling is active. Confirm register mapping and quality/status bits.
5) Analyzer diagnostics
Confirm no internal error flags. Reboot only if permitted by site procedure.
Confirm no internal error flags. Reboot only if permitted by site procedure.
6) Still failing?
Capture settings screenshots/logs (IP, mapping, scale) and escalate to controls/instrument team.
Capture settings screenshots/logs (IP, mapping, scale) and escalate to controls/instrument team.
Fast Checks
- Compare local reading vs DCS reading
- Check loop power / AI card / wiring (4–20mA)
- Check network link/ping and mapping (digital)
- Confirm range/scaling matches