Modbus Poll Rate Estimator
Estimate the minimum Modbus polling cycle time for a given number of registers, baud rate and device count.
Modbus
poll rate
scan time
cycle time
response time
baud rate
polling
Input values
⚙
Enter values and click Calculate to see results here.
Formula
T_frame = (Req_bytes + Resp_bytes) × 11 / baud
T_total = N_devices × (T_frame + T3.5 + T_response)
Worked Example
Given: 3 devices on one RS-485 bus at 9600 baud. Reading 10 holding registers each. Response time 50 ms.
Step 1: Request: 8 bytes (addr + FC + start + count + CRC) = 8 × 11/9600 = 9.17 ms
Step 2: Response: 25 bytes (addr + FC + byte count + 20 data + CRC) = 25 × 11/9600 = 28.6 ms
Step 3: Per device: 9.17 + 28.6 + 4.0 (T3.5) + 50 = 91.8 ms
Step 4: Total: 3 × 91.8 = 275 ms → 3.6 Hz poll rate
Engineering Notes
- Bottleneck: The baud rate is usually the bottleneck. At 9600 baud, a 25-byte response takes ~26 ms. At 115200 baud, it takes ~2.4 ms.
- Batch reads: Reading 100 registers in one request is faster than 10 requests of 10 registers. The per-request overhead is significant.
- Device count: Each device on an RS-485 bus adds its response time to the total scan time. Limit devices per bus to keep scan time acceptable.
- Response time: Check the device datasheet for response time. Some devices need 100+ ms to process requests.
- Timeout: Set the master timeout to 3-5× the expected response time to account for noise and retries.
References
- MODBUS over Serial Line Specification V1.02