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Protocol & FramingModbus Term

Inter-Frame Delay (3.5 Character Times)

The minimum silence on the bus that marks the end of a Modbus RTU frame.

Definition

Modbus RTU uses silent time to delimit frames rather than special start/end characters. A gap of at least 3.5 character times (at the current baud rate) with no activity signals that the current frame has ended and the next byte begins a new frame. At 9600 baud a character time is approximately 1.04 ms, so the inter-frame gap is about 3.6 ms. Missing this timing is a common cause of framing errors.

Quick Facts

Minimum gap3.5 character times
At 9600 baud≈ 3.6 ms
At 19200 baud≈ 1.8 ms
At 115200 baud≈ 0.3 ms (1.75 ms minimum)

Related Terms

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ModPackQT Modbus Glossary · Updated 2026