IIoT, SCADA & Edge Connectivity calculator

PLC Data Availability Calculator

Estimate clean PLC tag polls per period. Enter the tags polled per scan cycle (per group on the OPC server), planned scan cycles in the period, the OPC scanner uptime, and the share of polls that return fresh data without retry. The calculator returns gross capacity, good capacity, and the loss buckets.

What this calculator does

  • Estimate how many PLC tags can be polled cleanly per period from tags polled per scan cycle, planned scan cycles in the period, scanner uptime, and the share of polls that return fresh data without retry.
  • Use it when a controls engineer is sizing the PLC scanner load on Kepware, FactoryTalk Linx, or a vendor OPC UA server before adding more tags or another machine to the line.
  • It returns the PLC tag polls per period the OPC scanner can deliver cleanly after downtime and retry losses.

Formula used

  • Gross PLC poll capacity = tags per scan cycle × planned cycles
  • Good PLC poll capacity = gross capacity × scanner uptime × clean-poll rate

Inputs explained

  • Tags polled per scan cycle: Use the count of tags configured in the active OPC scan group (typical 100 to 5000 tags per group depending on PLC and bandwidth).
  • Planned scan cycles in the period: Use the planned cycles in the period (60 per minute at 1 second scan, 600 per minute at 100 ms scan).
  • OPC scanner uptime: Use the share of the period the OPC scanner was actually running (typical 99+ percent on a healthy server; lower if the PLC went into program mode or the network dropped).
  • Clean-poll rate without retry: Use the share of polls that returned fresh data on the first attempt (typical 98 to 99.9 percent for a stable PLC; lower if the PLC is overloaded or on a noisy network).

How to use the result

  • Use it before adding tags to a scan group, before bumping the scan rate from 1 second to 100 ms, or when an OEE dashboard is showing intermittent gaps that point at scanner load.
  • It is a poll-capacity view. It does not say whether the PLC processor itself can support the scan group; check PLC scan time and CPU usage on the PLC, not just the scanner.

Common questions

  • What scan rate should I model? Use the actual scan rate of the OPC group. Fast process tags often run at 100 to 250 ms (10 to 4 cycles per second), discrete machine tags at 250 ms to 1 s, and slow analog tags at 1 to 5 s. Match the group, not a single global rate.
  • What is a healthy clean-poll rate? 98 to 99.9 percent. Below 95 percent usually means the PLC processor is at the edge, the OPC group has too many tags, or the Ethernet path has retransmits.
  • How do I model multiple PLCs on one OPC server? Run the calculator once per PLC scan group. Roll up the goods totals to get total OPC server capacity.
  • Should I include alarm and event polling? No. AC&E uses a different path. Scope this calculator to data tag polling; size AC&E load with the alarm rationalization workload calculator.

Last reviewed 2026-05-12.