The configured data path is RealVNC Viewer on the remote PC through the Ethernet switch to the VNC service associated with the X20 1584 CPU. The T50 is another client on that network. Giving both clients the same IP address breaks endpoint identity before VNC can establish a reliable session.
Where does the packet path stop?
Follow the packet from physical link to application. The PC transmits an Ethernet frame to the switch, resolves the destination through IP and ARP, opens the transport connection to the configured VNC destination port, and then starts the VNC session. A duplicate client IP disrupts the ARP stage: the CPU and other devices can associate that IP with either the PC MAC address or the T50 MAC address.
| Layer | Required condition | Failure indication |
|---|---|---|
| Physical Ethernet | CPU, PC, and T50 have active switch links |
No link indication or no traffic counters |
| Addressing | Every simultaneously connected interface has a unique IP | Intermittent reachability or replies from the wrong MAC address |
| Transport | The configured VNC destination port is reachable | Ping works, but the VNC connection fails |
| VNC session | A server session slot is available | Connection rejection or a session that never completes |
A switch does not correct duplicate IP addresses. It learns MAC forwarding locations, while hosts independently maintain IP-to-MAC neighbor mappings.
Gate check: With only the PC connected, confirm that its link is active and that the CPU responds at the expected CPU address before changing any VNC setting.
How should the three Ethernet links be connected?
Layer one first. Connect the X20 1584, remote PC, and T50 to separate switch ports. Inspect link state at both ends and verify that moving or disconnecting one client does not drop the CPU link. A link indication proves only the cable and Ethernet negotiation; it does not prove correct addressing.
- Disconnect either the PC or
T50if both currently use the same IP. - Keep the CPU connected to the switch.
- Connect the remaining client and verify its link state.
- Ping the CPU from the PC when the PC is the active client.
- Record the exact ping result and any
RealVNC Viewererror separately.
No ping directs the investigation toward cabling, interface state, IP address, subnet mask, routing, or firewall rules. Successful ping with a failed VNC session moves the investigation to the configured port, server state, or client limit.
Gate check: Proceed only after the active client has a stable physical link and repeatable IP reachability to the CPU.
How should the PC and T50 addresses be assigned?
Assign one address to each active interface. Two VNC clients do not need the same source IP to reach one server. When all three devices share a switched local subnet, retain the CPU address and subnet plan from the working configuration, then give the PC and T50 different unused addresses within that plan.
| Device | Address requirement | Connection role |
|---|---|---|
X20 1584 |
Retain the configured server address | VNC destination and controller |
T50 |
Unique client address | Panel-side VNC client |
| Remote PC | Different unique client address |
RealVNC Viewer client |
- Read the CPU, panel, and PC address and subnet-mask settings.
- Check the network for an unused address before assigning it.
- Configure the PC and
T50with different addresses. - Reconnect both devices and test each one independently.
- Inspect the CPU diagnostics or logger for connection failures and duplicate-address symptoms.
Gate check: From the PC, verify stable CPU reachability while the T50 remains connected. Loss of communication when the second client joins means the addressing conflict is not resolved.
How do I clear stale state after using a duplicate IP?
Changing or unplugging an endpoint does not immediately remove every network and application record. ARP state belongs to each device's network stack; the RealVNC Viewer cache is separate. Reinstalling the viewer therefore does not reliably clear the CPU's neighbor entry or an occupied VNC server session.
A manual swap in which the PC and T50 reuse one address can fail because the CPU still maps that address to the previously connected MAC address. A VNC server configured for one parallel client can also reject the replacement client if the previous session did not close cleanly.
- Remove the duplicate by configuring unique addresses, or leave only one duplicate-address device physically connected during recovery.
- Close the VNC session cleanly on the active client where possible.
- Clear the PC's neighbor state through its operating-system network tools, then disable and re-enable its interface if permitted by the installation.
- Check the CPU logger and VNC configuration for a retained session and the maximum number of parallel clients.
- If the CPU provides no supported way to clear that state, restart it only through the site's process-safe shutdown and startup procedure. Restart the affected client as required.
Gate check: Reconnect from the PC and confirm that the CPU records a new session from the PC's unique address rather than the former shared address.
Why can the SCADA alarm return with unique addresses?
The reported SCADA alarm occurs when the T50 is treated as disconnected. That supervision function and the VNC connection solve different problems. If the Allen-Bradley SCADA through the Schneider OPC server tests only a particular IP endpoint, giving the PC that address may satisfy the reachability test while falsely identifying the PC as the panel.
| Observation | Diagnostic meaning | Required check |
|---|---|---|
| Alarm clears when another device uses the panel address | Supervision may be testing address reachability rather than panel identity | Inspect the alarm's monitored endpoint and condition |
| Ordinary variables work remotely | The VNC display and basic input path are operational | Test command-specific input behavior |
| On/off commands fail | The failure is limited to commands requiring a press longer than 5 seconds | Observe press, hold, and release states |
Correct the alarm logic in the SCADA, OPC, or HMI project so it supervises the intended device. If the project sources and server configuration are unavailable, obtain them or request the delivering company to modify the application. Reusing the panel IP on the PC is not a valid simultaneous-operation fix.
Gate check: With unique addresses and both clients connected, confirm that the supervision alarm represents the actual T50 connection state.
How do I verify VNC and the long-press commands?
The final test must separate network connectivity from HMI command behavior. Since ordinary variables respond remotely, investigate how the viewer delivers a sustained pointer-down event, whether focus leaves the control, and whether release occurs early. Compare the same operation locally on the T50.
- Start one PC VNC session while the
T50remains connected with its unique address. - Exercise several ordinary controls and confirm that their displayed and controlled states change correctly.
- Operate an on/off control continuously for more than 5 seconds without moving the pointer outside its active area.
- Observe the corresponding controller variable or application diagnostic throughout press, hold, and release.
- Repeat the test locally on the
T50and compare the event sequence. - If the remote hold terminates early, inspect viewer input settings and the HMI control implementation. Changing the required hold behavior requires access to the applicable project configuration.
- Disconnect and reconnect each client once, confirming that neither operation disrupts the other endpoint or leaves the VNC server unavailable.
Gate check: Accept the commissioning result only when both unique-address clients remain reachable, the SCADA reports the true panel state, and the remote press remains active for the full required interval.
FAQ
How do I recover VNC after assigning the same IP twice?
Disconnect one conflicting endpoint, assign unique PC and T50 addresses, clear stale neighbor state, and check the CPU for a retained VNC session. Restart the CPU only through the process-safe procedure when its session state cannot otherwise be cleared.
How do I use the remote PC as a T50 backup?
Keep the PC and T50 on different IP addresses and point both VNC clients to the configured X20 1584 destination. Check the VNC server's parallel-client limit if both must connect at once.
How do I prove a five-second VNC button works?
Hold the remote control for more than 5 seconds while observing its controller variable through press, hold, and release, then repeat locally on the T50. Complete verification by disconnecting and reconnecting each client and repeating the full-duration command.