What Does ## ## Mean on a Siemens SIMATIC HMI? | PLC Communication Fault Guide

Siemens SIMATIC HMI Showing ## ##? How to Restore PLC Communication

If your Siemens SIMATIC HMI displays ## ## instead of machine information, the HMI is not receiving data from the PLC (Programmable Logic Controller).

This indicates a communication fault between the HMI and the PLC network. The fault may be caused by a disconnected network cable, loss of power, damaged communications hardware, or a PLC communication issue

How Communication Works on the Machine

The HMI located in the main control panel is connected directly to the PLC via an Ethernet network cable.

The cabin HMI receives its information through the main control panel HMI, which acts as a communication relay between the PLC and the operator station.

For normal operation:

✅ The PLC must be powered and operational

✅ The main control panel HMI must have power

✅ All network connections must be secure

✅ Communication cables must be undamaged

If any part of this communication path fails, the HMI may display ## ##

Troubleshooting Checklist
1. Check All Network Cable Connections

Inspect all Ethernet network connections.

Check:

PLC Ethernet port
Main control panel HMI connection
Cabin HMI connection

Ensure all connectors are fully inserted and securely locked.

A loose Ethernet cable is one of the most common causes of HMI communication errors.

2. Check Power to the Main Control Panel HMI

Verify that the Siemens SIMATIC HMI in the main control panel is powered on.

Because this HMI acts as the communication relay, loss of power will prevent machine data reaching the cabin HMI.

If the downstairs HMI has no display or no power indicators illuminated, investigate the HMI power supply before proceeding.

3. Check the PLC Status

Verify that the PLC is powered and operating correctly.

Look for:

Normal PLC status lights
PLC in RUN mode
No active fault indicators

A PLC in STOP mode or fault condition may prevent communications with the HMI

4. Connect the Cabin HMI Directly to the PLC
This test helps determine whether the fault lies with the communication network or with one of the HMI units.

Disconnect the cabin HMI network cable.
Connect the cabin HMI directly to the PLC.
Restore power if required.
Check whether communication is restored.

What Does the Result Mean?
Cabin HMI Works When Connected Directly

If communications return when the cabin HMI is connected directly to the PLC:

✅ PLC is operating correctly

✅ Cabin HMI is operating correctly

✅ Fault is likely between the main control panel HMI and cabin HMI

Inspect:

Network cable condition
RJ45 connectors
Cable routing
Connection points at both ends

Cabin HMI Still Displays ## ##

If the fault remains after connecting directly to the PLC:

Verify the PLC is powered and in RUN mode
Check PLC network settings
Inspect PLC Ethernet port
Contact M&K Technical Support

Communication configuration or PLC faults can also prevent connection establishment.

Common Causes of the ## ## Error
Main control panel HMI has no power
Loose Ethernet cable
Damaged network cable
Faulty connector
PLC not powered
PLC in fault or STOP mode
Network communication failure
Incorrect communication settings following maintenance work

System Communication Layout

┌─────────────────┐
│ PLC │
│ Main Controller│
└────────┬────────┘
│
│ Ethernet / Network Cable
│
┌────────▼────────┐
│ Siemens SIMATIC │
│ HMI (Control │
│ Panel) │
└────────┬────────┘
│
│ Ethernet / Network Cable
│
┌────────▼────────┐
│ Siemens SIMATIC │
│ HMI (Cabin) │
│ Operator Screen │
└─────────────────┘

Communication Fault Diagnostic

HMI Displaying ## ##

│
▼

Is Cabin HMI Displaying ## ## ?
│
▼
Check Network Cables
│
▼
Check Power to Control
Panel HMI
│
▼
Control Panel HMI Powered?
┌──┴──┐
No Yes
│ │
Restore ▼
Power Check PLC Status
│
▼
PLC in RUN Mode?
┌──┴──┐
No Yes
│ │
Resolve ▼
PLC Fault Connect Cabin HMI
Directly to PLC
│
▼
Communication Restored?
┌──┴──┐
Yes No
│ │
│ └─ Check PLC,
│ Network Settings,
│ Contact Support
│
└─ Fault between
Control Panel HMI
and Cabin HMI
“

The ## ## display indicates that the HMI cannot communicate with the PLC and is not receiving live machine data. This is typically caused by a network connection issue, loss of power to the control panel HMI, or a PLC communication fault.

Yes. A loose, damaged, or disconnected Ethernet cable is one of the most common causes of HMI communication faults. Check all cable connections between the PLC, control panel HMI, and cabin HMI before investigating other causes.

The control panel Siemens SIMATIC HMI acts as a communication relay between the PLC and the cabin HMI. If the control panel HMI loses power, machine data cannot be passed to the cabin HMI, resulting in a communication fault and the ## ## display.

Connect the cabin HMI directly to the PLC. If communication is restored, the PLC and HMI are functioning correctly and the fault is likely within the network cable or connections between the HMIs. If communication is not restored, further investigation of the PLC or communication settings may be required.

No. In most cases, the PLC is operating normally and the issue is caused by a communication interruption. Always check network cables, HMI power supplies, and PLC status indicators before assuming a PLC failure.

Before contacting support, verify:

All Ethernet cables are securely connected
The control panel HMI has power
The PLC is powered and in RUN mode
The cabin HMI has been tested directly against the PLC
No damaged cables or connectors are present
Completing these checks can often identify the root cause without requiring further assistance.

Yes. Damaged or partially connected Ethernet cables can cause intermittent communication faults, where the HMI periodically loses data from the PLC. Inspect cables for physical damage, crushed sections, moisture ingress, or loose connectors.

Most communication faults can be diagnosed within a few minutes by checking HMI power, inspecting network connections, and performing a direct PLC connection test. These simple checks resolve the majority of ## ## display issues.