Symptom
Values on one or more displays stay constant while the process is known to be changing. Alarms may not be arriving. The values may be plausible, which is what makes the symptom dangerous.
The short answer
Values Frozen On Screen
A frozen value is a value that has stopped updating somewhere between the field and the pixel. Work back along the path: is the controller in run and scanning, is the communication driver connected to it, is the tag server updating the tag, is the client connected to the server, is the display bound to the tag at all. The tag quality and timestamp, which every SCADA platform records, tell you at which link the update stopped.
Key points
- Check the tag timestamp and quality first; they say when the value last updated and whether the driver considers it good.
- If every tag from one controller is frozen, the communication link or the controller is the problem, not the display.
- If every tag on every controller is frozen, the tag server or the client connection stopped.
- If one tag is frozen and its neighbors update, the tag is misconfigured, bound wrong, or the controller value really is static.
- A controller in program mode or faulted delivers values that never change and reports a healthy connection.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Controller faulted or in program mode | The connection is up and the values are static because the controller is not scanning. Look at the processor lights and the SCADA driver status. |
| Communication driver disconnected | Driver diagnostics show the device off line or timing out. Every tag from that device is stale with bad quality. |
| Tag server or service stopped | Every tag from every device is stale. The SCADA runtime or the OPC server has stopped or hung. The service status on the server shows it. |
| Client lost its connection to the server | On a client and server architecture the display is running but not receiving updates. Other clients update normally. The client connection status shows it. |
| License expired or limit reached | Some platforms stop updating tags, or stop accepting new connections, when the license lapses or the tag count exceeds it. A message on the server says so. |
| Scan class or polling disabled | A tag group set to a slow rate, or a scan class paused, or a device disabled in the driver. Tags in other groups update. |
| Display bound to a static tag or the wrong tag | One value on one display never updates while the same tag on another display does. The display object is bound to a memory tag, a constant, or the wrong name. |
| Duplicate IP address or device address | Two devices answering the same address, so the driver reads an unrelated device that happens to return constant data, or alternates between two. |
| The value really is static | A flow of zero at night, a tank at its overflow, a controller holding a bad-quality input. Confirm from the field before chasing SCADA. |
Frozen is worse than blank
A display that shows nothing gets a phone call. A display that shows a plausible level from an hour ago gets trusted, and the operator makes decisions on it until something floods. Most SCADA platforms can show stale data differently, greyed, hatched, or flagged with the quality, and the first fix on a system where this has happened once is to turn that on so that frozen never looks like live again.
Locate the break
- 1
Read the timestamp and the quality.
Every tag carries the time it last updated and a quality from the driver. A tag with a timestamp from an hour ago and bad quality tells you the driver stopped getting it; a current timestamp with good quality on a value that is not changing tells you the value is not changing.
- 2
Find the scope.
One tag, all the tags from one controller, or every tag on the system. One tag is the tag configuration. One controller is the link or the controller. Everything is the server or the client.
- 3
Check the controller.
Run light on, no fault, I/O light solid. A faulted controller answers the driver perfectly well with values that never change.
- 4
Check the driver.
The driver diagnostics show each device connected or failed, with error counts and the last good read. A device in a failed state, or one with a rising timeout count, is the break.
- 5
Check the server and the client.
Service running, license valid, tag count within limits, and, on a client, the connection to the server up. A client showing a disconnected banner is a client that has been ignored.
- 6
Check the display binding.
For a single frozen object, open the display in the designer and read the tag it is bound to. Compare with the tag browser and the controller.
The link
The connection between the driver and the controller is where most freezes happen. A radio path that faded, a switch that rebooted, a cellular modem that lost registration, a controller Ethernet port that hung, an address conflict. The driver retries, gives up, and marks the device failed; some drivers mark it and some just stop updating. The network and radio troubleshooting pages cover the physical causes; from the SCADA side, the driver diagnostics say which device and since when.
After it comes back
When the updates resume, look at the alarm summary and the historian for the gap. Alarms that occurred during the freeze may have been missed or may arrive all at once; events in the controller that a DNP3 outstation would have buffered are gone on a polled protocol. Note the outage in the log with its start, its end, and its cause, and if the cause was a link, watch the driver counters for the next week.
Frequently asked questions
- How can I tell whether the controller or SCADA is the problem?
- Watch the tag in the controller with the programming software. If it changes there and not on the screen, the path between them is broken. If it is static there too, the controller or the field is the problem and SCADA is showing the truth.
- Why does the driver show connected but the values do not update?
- The controller is answering but not scanning: faulted or in program mode. Or the tag is in a scan group that is disabled or set to a very slow rate. Or the value genuinely is not changing.
- Why did every screen freeze at once?
- The tag server or the SCADA runtime stopped, the license lapsed, or the client lost the server. Check the service on the server and the connection banner on the client.
- How do I make stale data obvious to operators?
- Use the platform feature that displays bad or stale quality differently, and put a communication status indicator for each controller on the overview display. Frozen data that looks live is the failure to design out.
Related topics
- Tag Shows Bad QualityA tag flagged bad or uncertain by the driver. What the quality codes mean, the difference between a device that is not answering and an address that does not exist, and how to clear each.
- Processor FaultedA PLC with its fault light on and no logic running. How to read what it is telling you, the causes in order of likelihood, and how to restart it without repeating the fault.
- Ethernet Device Drops OfflineAn industrial Ethernet device that disappears and comes back. Duplex mismatch, cabling, spanning tree, and the switch counters that identify it in minutes.
- Remote Site Stops CommunicatingOne radio site silent while its neighbors answer: site power, the radio, antenna and coax after a storm, a reset configuration, the controller behind it, or a changed path. What the master diagnostics say before anyone drives, and the order of checks on site.
- What Is SCADA?Supervisory control and data acquisition: the layer that collects data from controllers across a system, presents it to operators, records it, and lets them intervene.
- Alarms Not AnnunciatingThe condition is real and nothing happened on the screen or the phone: where an alarm can be lost between the controller and the operator, from the alarm bit and the tag quality to the alarm server, the display filter, the shelved and suppressed lists, the audible, and the notifier.
Direct contact
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