PROFINET Update Time, Send Clock, Reduction Ratio
When you configure a PROFINET device you set how often its data refreshes, and that number is not arbitrary - it is the send clock multiplied by a reduction ratio. Understanding this lets you give a fast drive and a slow temperature input each the right rate without flooding the network. This page explains the send clock, the reduction ratio, and how they combine into an update time.
PROFINET Update Time in one line: The PROFINET update time for a device is its send clock multiplied by a reduction ratio. The send clock is the network's base timing tick shared by all devices, and the reduction ratio is a per-device factor that sends that device's data every Nth tick. A fast device uses a small ratio for a short update time; a slow device uses a large ratio, so each gets an appropriate rate on one network.
The Send Clock Is the Base Tick
PROFINET IO organizes cyclic traffic around a common base interval called the send clock. Every device on the network shares the same send clock, and it defines the smallest cyclic slot in which real-time frames are scheduled. Think of it as the heartbeat of the network: nothing cyclic happens faster than one send clock, and every device's update is timed as a multiple of it. Keeping one shared base tick is what lets the network schedule many devices without collisions.
The send clock is a network-wide setting, so shortening it speeds up the fastest possible update but also raises the baseline load, because more slots occur per second. This is why you do not simply set the send clock to its minimum; you set it fast enough for your fastest device and then let slower devices ride at a fraction of that rate through their reduction ratio.
The Reduction Ratio Sets Each Device's Rate
The reduction ratio is a per-device factor that says send this device's cyclic data every Nth send clock. A reduction ratio of 1 means the device updates every send clock - the fastest rate. A ratio of 8 means it updates once every eight send clocks - an eighth of the rate and an eighth of the network load for that device. Multiplying the send clock by the reduction ratio gives that device's actual update time.
This two-part scheme is what makes a mixed network efficient. A servo drive that needs a tight loop gets a reduction ratio of 1; a temperature transmitter that changes slowly gets a large ratio and consumes far less bandwidth. Choosing ratios is a real engineering decision: too fast wastes bandwidth and can push the network toward its limit, too slow adds latency to the control loop. For hard motion timing you move beyond this into scheduled slots, which is the province of PROFINET IRT versus RT.
Tuning Update Times in Practice
The practical method is to work from your fastest requirement. Pick a send clock short enough to satisfy the most demanding device, then assign each other device the largest reduction ratio that still meets its own timing need, so it consumes the least bandwidth it can. The engineering tool typically shows the resulting network load, and a design that runs near the limit leaves no headroom for jitter or added devices.
For a monitoring layer, these settings explain observed data freshness. A tag updated at an eighth of the base rate will look less lively in a trend than one at the base rate, and that is by design, not a fault. A platform such as Merobix trends the values the controller holds; the update time governs how fresh those values are at the controller before SCADA ever samples them, which is worth knowing when a slow-moving trend is expected rather than broken. Excess timing variability shows up as network jitter if the send clock is set too aggressively.
Frequently Asked Questions
How is PROFINET update time calculated?
Update time equals the send clock multiplied by the device's reduction ratio. The send clock is the shared base tick for the whole network; the reduction ratio is a per-device factor that sends that device's data every Nth tick. So a device with a large reduction ratio updates less often and uses less bandwidth than one with a ratio of 1.
Why not set every device to the fastest update time?
Because each fast update consumes network bandwidth. Running every device at a reduction ratio of 1 can push the network toward its limit and leave no headroom for jitter or growth. You give fast devices short update times and let slow-changing devices ride at a larger reduction ratio, matching each device's rate to its real need.
Is the send clock the same for every PROFINET device?
Yes. The send clock is a network-wide base interval shared by all devices; it defines the smallest cyclic slot. Individual devices then differ in how often they actually send, set by their reduction ratio. Shortening the send clock speeds up the fastest achievable update but raises the baseline load on the whole network.
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