Siemens Comfort Panel — No Connection to the PLC

Siemens · intermediate · 30 min

A Siemens Comfort Panel showing 'no connection' to an S7-1500 is most often the CPU's PUT/GET security setting or a cached IP after a change — not a wiring fault. Work these checks in order.

ℹ️ Since S7-1500 firmware V2.9, HMI access is blocked by default. If a panel that used to work stopped after a CPU update, check the security setting before touching cables.

The two causes that explain most "no connection" cases

1. CPU access security (most common on S7-1500 V2.9+)

S7-1500 CPUs on firmware V2.9 or newer enable enhanced security that blocks HMI access by default. This is the number-one cause when a panel that previously worked goes offline after a CPU firmware update.

In TIA Portal, open CPU Properties → Protection & Security → Connection mechanisms and enable "Permit access with PUT/GET communication from remote partner (PG/PC/HMI)". Then confirm every PLC tag the HMI reads has the "Accessible from HMI/OPC UA" / "Writable from HMI" attribute set. Re-download the PLC.

2. Cached connection after an IP change

HMI/WinCC connections do not re-establish automatically after the PLC IP address changes in TIA Portal — the panel keeps the old cached connection parameters. After any PLC IP change you must re-download the complete HMI configuration so it picks up the new address.

Step-by-step checks

1. Physical + link. Confirm the Ethernet cable is in the correct port on both devices and the link LEDs are on. A panel with multiple ports must use the one configured for the PLC connection.
2. IP / subnet. Verify the panel and CPU are on the same subnet and that the configured HMI connection IP matches the CPU's actual IP. For PROFINET, set the device name as well — using PROFINET device names instead of raw IPs reduces re-config after address changes.
3. CPU security. Apply the PUT/GET setting and per-tag HMI-access attribute above (S7-1500 V2.9+). Re-download the PLC.
4. Re-download the HMI. After any PLC IP or security change, re-download the full panel project so cached parameters are refreshed.
5. Time/date. Set the date and time on both the HMI and PLC — a large clock mismatch can interfere with secure connections.
6. Factory reset (last resort). If it still won't connect, on the panel go Online → Reset device to factory settings, then re-download the complete configuration to clear all cached data and force a clean handshake.

Verify

After the fix, the HMI connection status should go online and tags should update live. If it connects but tags read as invalid, the per-tag HMI-access attribute is still missing on those specific tags.