Troubleshooting

HMI Shows #### or "Communication Lost": Finding the Broken Link

Every value on the screen turned into ####. Is it the network, the PLC, or the HMI project? A short sequence separates the three.

· 7 min read · By TryPLC Team

  • HMI
  • Communication
  • Troubleshooting
  • SCADA

The symptom

The operator panel shows #### (or *, or red crosses) in all value fields, or a banner such as Connection to PLC lost. Sometimes only one screen or a few tags are affected.

All values or some values?

This single observation halves the search.

  • All tags on all screens: the HMI has no connection to the PLC. Look at the network and connection settings.
  • Only some tags: the connection works, but those tags are invalid — wrong address, deleted variable, or a value out of format.

    • When all values are lost

  • Physical link: check the link LED on the HMI port and the switch. Reseat or replace the cable.
  • Reachability: from the HMI's system menu or a laptop, ping the PLC's IP address. Confirm both devices are in the same subnet.
  • Connection settings: in the HMI project, check the PLC IP address, the slot/rack (Siemens S7: rack 0, slot 1 for S7-1500; check for older CPUs), the driver type and the station name. A replaced CPU with a new IP breaks every HMI that pointed to the old one.
  • PLC side access: on an S7-1200/1500, PUT/GET communication must be allowed for some older HMI drivers, and the protection level must permit HMI access. On a Rockwell system, check the controller path in the HMI shortcut.
  • Connection limits: each PLC accepts a limited number of HMI/SCADA connections. An extra laptop or historian can push it over the limit — the most recent connection fails.

    • When some values are lost

  • The tag points to a variable that was renamed or deleted in the PLC. After a PLC download, recompile and download the HMI too.
  • The tag's data type does not match the variable (for example the HMI expects INT and the PLC now uses REAL).
  • On optimized data blocks, absolute addressing from an older HMI project is no longer valid; switch the tags to symbolic access.
  • The display format is too short: a value of 10000 in a 4-digit field shows ####. That is formatting, not communication.

    • Intermittent loss

    If the banner appears and disappears, check for network load (a broadcast storm or a looping cable), a duplicate IP, and the HMI update rate. Polling hundreds of tags at 100 ms when 1 s is enough loads both PLC and network.

    Prevent the next one

    Show a clear communication-status indicator on every screen, and keep PLC and HMI projects versioned together so a download of one prompts a check of the other. See the HMI programming guide.

    Frequently asked questions

    Why does my HMI show ####?

    Either the HMI lost connection to the PLC (all values affected) or specific tags are invalid or too long for their display field (some values affected).

    Why did the HMI stop working after a PLC download?

    Renamed or deleted variables, changed data types or a changed PLC IP address break HMI tags. Recompile and download the HMI project to match.