Getting a Legacy OPC DA Server Through DCOM Without Losing a Day to It
Why OPC Classic (DA/HDA) links fail across DCOM — RPC endpoint mapper on 135, dynamic port ranges, server identity — and the fastest way through it.
Articles
Practical SCADA notes
Field-oriented references for HMI/SCADA systems, industrial communication, alarms, tags, historians, and project delivery.
Why Your SCADA Client Shows an OPC UA Struct as a Blob — and How to Decode It
A PLC exposes a UDT over OPC UA and your client shows a ByteString. What an ExtensionObject actually holds, and how clients decode structured DataTypes.
How OPC UA Republish Recovers Dropped Notifications Before They Become Trend Gaps
Sequence numbers and the Republish service turn a dropped notification into zero data loss — but only if the client watches them and the queue is sized.
OPC UA Browse Skipped Half Your Tags? You Dropped a Continuation Point
Browse returns success but the tag import is short. The OPC UA Part 4 v1.05 continuation point rules, MaxBrowseContinuationPoints, and how to find the cause.
OPC UA PubSub over UDP: Prove the Packet Path Before You Trust a Single Tag
Commissioning OPC UA PubSub (Part 14, UADP over UDP) without shipping stale values as live: dataset contracts, VLAN checks, and reboot-only failures.
Why an OPC UA Setpoint That Browses Writable Still Rejects the Operator
AccessLevel vs UserAccessLevel vs WriteMask: why a setpoint that browses writable returns BadUserAccessDenied, and testing with the production identity.
Which OPC UA Name Should Your SCADA Store: NodeId, BrowseName, or DisplayName?
NodeIds regenerate, browse paths move, DisplayName is a label. Picking the reference your HMI and historian store so firmware doesn't blank a screen.
How to Set OPC UA Deadbands Without Hiding a Drifting Transmitter
Choosing DataChangeFilter settings — absolute versus percent deadband, DataChangeTrigger, queue size — so a filter cuts noise, not real movement.
Backfilling a Historian Gap with OPC UA HistoryRead Without Double-Counting
Repairing a historian gap with OPC UA HistoryRead: scoping the window, letting returnBounds handle edges, keeping Bad quality, and paging safely.
Which OPC UA Timeout Actually Killed Your Session?
The secure channel, the session and the subscription each expire on their own clock. Which one fired tells you to blame the firewall, load or keepalive.
Good Quality, Stale Value: Reading OPC UA StatusCode and Timestamps
Using StatusCode severity bits, SourceTimestamp and ServerTimestamp to catch frozen data, cached gateway values and clock drift a value-only HMI hides.
Why OPC UA Rejects a Server You Can Ping
Client won't connect though ping and port 4840 are open? Usually an endpoint URL, hostname SAN or ApplicationUri mismatch. Reading the status codes.
How to Catch an Intermittent SCADA Comms Fault in a Packet Capture
Where to tap, what to filter, and how to read the first thirty seconds of a capture when comms drop and recover before anyone can get to a keyboard.
OPC UA Alarms & Conditions: Getting Retain, Acknowledge, and Reconnect Right in SCADA
Commissioning OPC UA Alarm & Condition subscriptions: event filters, the Retain flag, ConditionRefresh, and the failures that appear on reconnect.
In OPC UA, a Trusted Certificate Is Not Permission to Write
How OPC UA separates certificate trust, user identity and Part 18 role mapping — and commissioning writes so the wrong session can't move a setpoint.
OPC UA Reverse Connect: When the Server Has to Dial the Client
How reverse connect flips the TCP direction through a cell firewall, and the certificate, endpoint and diagnostic traps that still bite afterward.
Why Your OPC UA Client Drops Fast Tag Changes — and Which Queue Settings Fix It
How monitored item queues, discard policy and the Overflow status bit decide whether your client sees every fast tag change or only the latest one.
OPC UA Method Calls From the HMI: Why 'Good' Doesn't Mean the Command Ran
An OPC UA method call can return Good while the machine never moves. Wiring HMI calls so operators see acceptance, execution and a real failure reason.
Commissioning SCADA Firewall Rules: Test the Application, Not the Port
Commissioning SCADA firewall rules from data flows rather than port lists — the supporting services that break at cutover, and proving each rule for real.
How to Test OPC UA Redundant Failover Before the Plant Depends On It
A field-tested procedure for OPC UA redundant failover: ServiceLevel, subscription recovery, certificates, and catching stale data before operators do.
It Connects with None but Fails with Basic256Sha256: Debugging OPC UA Trust
Why a client browses fine with security off but drops the secure channel, and how to fix trust stores, endpoints and certificate names in a project.
OPC UA Sampling vs Publishing: Why Connected Doesn't Mean Good Data
How sampling interval, publishing interval, queue size, deadband and the KeepAlive/Lifetime pair decide whether your client and historian see the process.
OPC UA Connects Fine but Every Tag Reads Bad: the NodeId Moved
After a PLC download the session opens and every item returns Bad_NodeIdUnknown. How namespace indexes and client caches shift, and what to check first.
OPC UA Certificate Renewal Checklist
A practical checklist for renewing OPC UA application certificates without breaking SCADA clients, historians, gateways, or production HMI connections.
SCADA Network Troubleshooting Checklist
A layered checklist for troubleshooting SCADA communication problems from physical link to protocol behavior.
OPC UA Client Commissioning Checklist
A practical checklist for commissioning OPC UA client connections from SCADA, HMI, historian, or gateway software to PLCs and automation servers.
OPC UA vs MQTT
How to think about OPC UA and MQTT in industrial systems without turning the comparison into a vendor argument.