PLC Scan Time Calculator
ANA›Life Services Authority›National Calculator Authority›PLC Scan Time Calculator
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PLC Scan Time Calculator
Estimate the PLC scan time based on program instruction count, I/O module count, and processor execution speed.
Number of Instructions (rungs/instructions)
Average Instruction Execution Time (µs per instruction)
Total I/O Points (discrete + analog combined)
I/O Update Time per Point (µs per point)
System Overhead Time (µs) — housekeeping, comms, diagnostics
Calculate
function plcCalc() { var resultDiv = document.getElementById('plc-result'); resultDiv.style.display = 'none'; resultDiv.innerHTML = '';
var instructions = parseFloat(document.getElementById('plc-instructions').value); var execTime = parseFloat(document.getElementById('plc-exec-time').value); var ioPoints = parseFloat(document.getElementById('plc-io-points').value); var ioTime = parseFloat(document.getElementById('plc-io-time').value); var overhead = parseFloat(document.getElementById('plc-overhead').value);
// --- Validation --- var errors = []; if (isNaN(instructions) || instructions 0 µs."); if (isNaN(ioPoints) || ioPoints 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Input Errors:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- Core Formula --- // T_scan (µs) = (N_instr × T_exec) + (N_io × T_io) + T_overhead var programTime = instructions * execTime; // µs var ioScanTime = ioPoints * ioTime; // µs var totalUs = programTime + ioScanTime + overhead; // µs var totalMs = totalUs / 1000.0; // ms var maxFreqHz = (totalUs > 0) ? (1e6 / totalUs) : Infinity; // Hz var maxFreqKHz = maxFreqHz / 1000.0;
// Utilisation breakdown percentages var pctProgram = (totalUs > 0) ? (programTime / totalUs * 100) : 0; var pctIO = (totalUs > 0) ? (ioScanTime / totalUs * 100) : 0; var pctOverhead = (totalUs > 0) ? (overhead / totalUs * 100) : 0;
// Recommended safety margin: scan time should be ≤ 1/3 of fastest process event // Typical guideline: scan time 100) { warning = '⚠ Warning: Scan time exceeds 100 ms. This may be too slow for most real-time control applications.
'; } else if (totalMs > 10) { warning = '⚠ Caution: Scan time exceeds 10 ms. Verify this is acceptable for your process dynamics.
'; } else { warning = '✔ Good: Scan time is within typical acceptable range (≤ 10 ms).
'; }
resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Results ' + warning + '' + 'Program Execution Time' + programTime.toFixed(2) + ' µs (' + pctProgram.toFixed(1) + '%)' + 'I/O Scan Time' + ioScanTime.toFixed(2) + ' µs (' + pctIO.toFixed(1) + '%)' + 'System Overhead' + overhead.toFixed(2) + ' µs (' + pctOverhead.toFixed(1) + '%)' + 'Total Scan Time' + totalUs.toFixed(2) + ' µs (' + totalMs.toFixed(3) + ' ms)' + 'Max Scan Rate' + maxFreqHz.toFixed(1) + ' Hz (' + maxFreqKHz.toFixed(3) + ' kHz)' + ''; }
#### Formula
Tscan = (Ninstr × Texec) + (Nio × Tio) + Toverhead
- Tscan — Total PLC scan time (µs)
- Ninstr — Number of ladder logic instructions / rungs
- Texec — Average execution time per instruction (µs); found in processor datasheet (e.g. 0.05–1 µs)
- Nio — Total number of discrete and analog I/O points
- Tio — I/O update time per point (µs); depends on backplane speed and module type
- Toverhead — Fixed system overhead (µs): communications, diagnostics, housekeeping tasks
- Max Scan Rate (Hz) = 1,000,000 / Tscan(µs)
#### Assumptions & References
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