Modbus RTU Frame Timing Calculator
Calculate Modbus RTU inter-character and inter-frame timing requirements for a given baud rate.
Modbus
RTU
timing
baud rate
silence
T3.5
timeout
serial
Input values
⚙
Enter values and click Calculate to see results here.
Formula
T_char = 11 / baud_rate (1 start + 8 data + 1 parity + 1 stop)
T3.5 = 3.5 × T_char (inter-frame silence)
T1.5 = 1.5 × T_char (inter-character, max)
Worked Example
Given: Baud rate = 9600, 8 data bits, even parity, 1 stop bit.
Step 1: T_char = 11 / 9600 = 1.146 ms
Step 2: T3.5 (min silence) = 3.5 × 1.146 = 4.01 ms
Step 3: T1.5 (max inter-char) = 1.5 × 1.146 = 1.72 ms
At 9600 baud, the minimum silence between frames is ~4 ms. If silence < 3.5 chars, the device may interpret it as a continuation of the same frame.
Engineering Notes
- T3.5 rule: A silence of ≥ 3.5 character times indicates a frame boundary. If two frames are too close together, they will be merged and rejected as a CRC error.
- T1.5 rule: Characters within a frame must not be separated by more than 1.5 character times. Exceeding this causes a framing error.
- Auto-baud detection: Many Modbus devices use the T3.5 silence to detect baud rate automatically. Ensure your master respects the silence timing.
- High baud rates: Above 19200 baud, the T3.5 timing is fixed at a minimum of ~1.75 ms. Check your device documentation for exact values.
- Response timeout: Typical master timeout is 1-3 seconds. Slave must respond within this time or the master will retry.
References
- MODBUS over Serial Line Specification V1.02 — Section 2.5