Calculate Modbus RTU character time, T1.5, T3.5 silent interval, and frame transmission time from baud rate and serial format settings.
Enter Modbus RTU parameters to calculate timing values.
Modbus RTU uses serial characters and silent intervals to separate frames. Timing depends on baud rate and the configured character format.
Character time is the serial time for one full character (start bit, data bits, optional parity, and stop bits). T1.5 and T3.5 are 1.5 and 3.5 character times.
First compute bits per character, then divide by baud rate for character time. Multiply that by 3.5 to get T3.5, which is the typical silent interval threshold.
| Baud | Format | Bits per char | Character time | T3.5 | Notes |
|---|---|---|---|---|---|
| 9600 | 8E1 | 11 | 1.146 ms | 4.010 ms | Common Modbus RTU setting |
| 19200 | 8E1 | 11 | 0.573 ms | 2.005 ms | Common higher baud setting |
| 38400 | 8N1 | 10 | 0.260 ms | 0.911 ms | Character-based timing example |
| 115200 | 8N1 | 10 | 0.0868 ms | 0.304 ms | High speed serial example |
Many implementations keep character-based timing, but some use fixed timers above 19200 bps. Always check the device stack or protocol profile used in your project.
T3.5 is a silent interval of 3.5 character times, commonly used to mark frame boundaries in Modbus RTU communication.
Compute bits per character from serial format, then divide by baud rate. For example, 8E1 has 11 bits per character.
Yes. Enabling parity adds one bit per character, so character time, T1.5, T3.5, and frame time all become slightly longer.
A practical timeout starts from frame time plus T3.5, then adds a margin for processing and bus latency.