Modbus RTU polling time calculator: cycle and traffic

Over Modbus RTU, a master polls devices one by one, and at low baud rates a full cycle can take a noticeable amount of time — which determines how fast SCADA sees fresh data. Set the baud rate, register count, and device count — the calculator computes the time for one transaction and for the whole polling cycle.

Input data

Baud rate Frame formatHow many bits go into one byte on the line, including overhead. 8N1 (no parity) is 10 bits; with parity or two stop bits, 11 bits. Registers per request, N Number of devices on the bus шт Slave response delay processing time at the device, typically 0–20 ms мс

Result

Response size — байт

Transaction timeHow long a single request-response exchange with one device takes. It determines how often every device can be polled. — мс

Polls per second — 1/с

Full cycle for all devicesThe time to poll every device in turn — this is how often SCADA data refreshes. For fast loops, cut it by raising the baud rate or reducing the register count per poll. — мс

An estimate for reading registers (FC03) that doesn't account for error retries or RS-485/USB converter delays. Cycle time grows linearly with device count — for fast loops, raise the baud rate or reduce the number of registers per poll.

How it's calculated

Calculate frame lengths: a read-registers request is 8 bytes, a response is 5 overhead bytes plus 2 bytes per register.
Byte time is the number of bits per character divided by the baud rate; two inter-frame gaps and the device's processing time are added to the exchange.
The polling cycle is one transaction's time multiplied by the number of devices on the bus — and it's what determines how fast SCADA sees fresh data.

Example: 9600 baud, 11 bits per character, 10 registers: a 25-byte response, a 45.8 ms transaction — about 22 polls per second.

Request frame (FC03) — 8 bytes, response — 5 + 2·N bytes. Byte time = bits/byte ÷ baud rate; the t3.5 gap = 3.5 character times (≤ 19200 baud) or 1.75 ms (above). Transaction = (request+response)·tbyte + 2·t3.5 + slave response delay.

This is an estimate

Verify final values against current standards, the project design, and equipment nameplates.

Explained in the article Modbus: регистры и адресация

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