Modbus RTU Timing and Poll Time Calculator
Work out how long Modbus RTU messages take at your baud rate and serial format, the required silent intervals, and how fast you can poll a network of slaves.
- Bits per character = start + 8 data + parity + stop = 11
- Character time = bits ÷ baud
- Above 19200 baud the Modbus serial line specification fixes t1.5 = 0.75 ms and t3.5 = 1.75 ms
- Cycle = slaves × (request + t3.5 + response delay + response + t3.5)
Modbus RTU Timing
t3.5 = 3.5 character times (fixed 1.75 ms above 19200 baud)
Response bytes for FC 03 = 5 + 2 × registers
Worked example
At 19200 baud, 8E1: a character takes 0.573 ms, t3.5 = 2.0 ms. Reading 10 registers from 8 slaves with 10 ms response delay takes about 263 ms per cycle.
Frequently Asked Questions
What is t3.5 in Modbus RTU?
The minimum silent time between frames, 3.5 character times, which marks the end of a message.
What serial format does Modbus RTU use?
11 bits per character: 1 start, 8 data, 1 parity (even by default) and 1 stop, or no parity with 2 stop bits.
How can I make polling faster?
Read blocks of consecutive registers in one request, reduce slave response delays and avoid polling slow data every cycle.
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