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Modbus RTU Polling Time & Register Calculator
A Modbus RTU poll cycle is the total time to read every register from every device on a serial bus. Enter your device count, data points, register width, baud rate and slave response delay to estimate how many requests are needed, the bytes on the wire and how often each device can be refreshed.
HOW IT IS CALCULATED
The formulas
- Registers per device = data points × registers per point
- Requests per device = registers per device ÷ maximum registers per request, rounded up (function code 03, holding registers)
- Request frame = 8 bytes; response frame = 5 + 2 × registers bytes
- Time per request = (request bytes + response bytes + 7 silent-interval characters) × bits per character ÷ baud rate + slave response delay
- Poll cycle time = sum of request times for all devices; refresh rate = 1 ÷ poll cycle time
Worked example
With the default values (10 devices, 20 data points of 2 registers each, 19,200 baud, 11 bits per character, 10 ms slave delay) each device needs one 40-register request. That is 400 registers and 930 bytes per cycle, a poll cycle of about 672.9 ms and a refresh rate of about 1.49 updates per second.
USING THE RESULT
What to do with the estimate
Serial Modbus is often limited by the bus, not the software. If the cycle time is too long, group registers into fewer contiguous blocks, raise the baud rate where cabling and devices allow, split devices across more than one serial port, or move to a gateway or Modbus TCP network. Read only the registers that operators or analytics actually need.
COMMON QUESTIONS
Frequently asked questions
What is the maximum number of registers in one Modbus request?
A single read holding registers request (function code 03) can return up to 125 registers. Larger blocks must be split into several requests.
Why does Modbus RTU use 11 bits per character?
The Modbus serial specification defines an 11-bit character: one start bit, eight data bits, one parity bit and one stop bit, or two stop bits when no parity is used. Many devices also support 8N1, which uses 10 bits.
How can I make Modbus polling faster?
Read fewer registers, group registers into contiguous blocks, increase the baud rate, reduce slave response delays, or split devices across multiple serial ports or a Modbus TCP gateway.
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