Industrial protocols

Modbus RTU frame time and polling-cycle calculator

Calculate character, inter-character and inter-frame timing, then estimate one exchange and a complete multi-device polling cycle.

Calculate

Serial framing
The Modbus serial guide uses one stop bit with parity and two without parity.
Polling exchange
bytes
Include address and the two CRC bytes.
bytes
Include address and the two CRC bytes.
ms

Worked example

Complete polling cycle, estimated

 ms

Character time
 ms
t1.5 inter-character limit
 ms
t3.5 inter-frame minimum
 ms
One request and response exchange
 ms
Complete cycles per minute
 cycles/min
Share of the exchange spent transmitting bytes
 %

The parity and stop-bit combination follows the 11-bit Modbus serial framing recommendation. Character-derived t1.5 and t3.5 timings are applied.

Underlined figures are rounded. Select one to copy its full value.

Method and assumptions

Formula

tchar = framing bits ÷ baud; texchange = request + max(response delay, t3.5) + response + t3.5

At or below 19,200 bit/s, t1.5 and t3.5 follow character time. Above 19,200 bit/s, the serial guide recommends fixed 750 µs and 1.750 ms timers.

Limits of this result

  • The cycle estimate excludes retries, timeouts, broadcasts, operating-system scheduling and other masters.
  • The entered response delay must represent the device from request end to response start; the calculation enforces at least t3.5.
  • This is an ideal sequential estimate with one identically sized exchange per device. Nonconforming 10- or 12-bit framing results are exploratory timing arithmetic, not Modbus conformance approval.

Technical sources

  1. MODBUS over Serial Line Specification and Implementation Guide V1.02Modbus Organization · 2006-12-20
  2. REF601/REJ601 Modbus Communication Protocol ManualABB · 1MDU072231-YN

Put the value to work

Read the related engineering guidance, or carry this task into System Builder to identify the signal path and EpiSensor products.

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