Sampling interval
The sampling interval is the rate at which a server samples one MonitoredItem's attribute and evaluates its filter. It is per item, not per subscription. Zero means as fast as the server can, minus one means inherit the subscription's publishing interval. It controls acquisition; the publishing interval controls delivery of what was acquired.
Per item, and negotiated twice over
You request a sampling interval per MonitoredItem and the server returns a revised value. It is bounded from below by the Variable's MinimumSamplingInterval attribute, which tells you the fastest that node can honestly be sampled, and by whatever discrete rates the server's internal scheduler supports. Read MinimumSamplingInterval before designing a fast collection: it is the server telling you the answer in advance.
The relationship with the publishing interval
Sampling faster than publishing produces several samples per cycle, and all of them are delivered only if the queue is large enough - size it as publishing divided by sampling, rounded up. Sampling slower than publishing means cycles with nothing new. Using -1 makes the item inherit the publishing interval, which is the right default for anything that does not need its own rate and keeps the configuration honest.
What the number really means on a PLC
Few device-embedded servers poll each item independently. They map monitored items onto an underlying scan, a device subscription or a change-detection mechanism, so the effective rate is the device's, and a requested 10 ms quietly becomes the PLC's task cycle. The revised value plus a measured notification cadence, not the requested value, is what tells you the real resolution of your data.
Example
Three items, one subscription, three sampling strategies
Subscription PublishingInterval 1000 ms
item SamplingInterval samples/cycle QueueSize needed
Motor.Speed 100 ms 10 10
Tank.Level 1000 ms 1 1
Line.State -1 (inherit) 1 1
Vibration.RMS 0 (fastest) server-defined size generously, watch overflow
SamplingInterval 0 is not a promise of speed - it is a request. Read the revised value.Defined in: Part 4 - Services, CreateMonitoredItems; Part 3 - Address Space Model for the MinimumSamplingInterval attribute.
Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.