Why can't the Modbus RTU request interval be 1ms?
The Modbus RTU protocol's request interval cannot be set to 1 millisecond (ms). This is primarily due to its close relationship with the protocol's time interval specifications, data frame transmission mechanism, and physical layer limitations. The following is a detailed analysis:
1. Protocol Specification Time Interval Requirements
• Frame Interval Limitation: Modbus RTU requires a minimum interval of 3.5 character times between two consecutive frames. Otherwise, the receiver will treat the continuous data streams as a single frame, leading to parsing errors.
• Character Interval Limitation: The time interval between two bytes within the same frame cannot exceed 1.5 character times. Otherwise, the receiver will consider the frame transmission interrupted and discard the current frame.
If the request interval is set to 1ms, the character time needs to be calculated based on the specific baud rate:
• Each character contains 11 bits (1 start bit + 8 data bits + 1 stop bit).
Character Time = 11/115200 ≈ 0.0953 ms.
Therefore, 3.5 character times ≈ 0.0953 × 3.5 ≈ 0.3336 ms.
Theoretically, a 1ms interval is much larger than 0.3336ms, seemingly meeting the requirement. However, in practical applications, specific protocol rules must be followed.
• When the baud rate exceeds 19200, Modbus RTU no longer calculates dynamic character times but uses fixed time intervals:
Frame interval is fixed at 1.75ms (a substitute for 3.5 character times). Character interval is fixed at 0.75ms (a substitute for 1.5 character times).
This rule is to reduce the CPU load caused by frequent calculation of time intervals, which is especially efficient in high-speed communication.
Applicability of 115200 baud rate:
Since the 115200 baud rate is much larger than 19200, the fixed time rule must be followed. Even though the theoretically calculated 3.5-character time is only 0.3336ms, the protocol still requires a frame interval of at least 1.75ms. Therefore, a 1ms interval does not meet the protocol requirements and may lead to frame merging or verification failures.
The above is only a theoretical calculation of the frame interval based on the standard. In addition, the Modbus RTU master-slave mode and half-duplex transmission characteristics must be considered.
Master-slave mode: There is only one master and multiple slaves on the bus. The master uses a unique address to address the slaves and initiates requests. The slaves passively respond and cannot actively send data.
Half-duplex: Only one device can occupy the bus to transmit data at a time. Data can only be sent or received at a time, not simultaneously. After sending a request, the master must wait for the slave's response. During this period, other devices are prohibited from occupying the bus to avoid conflicts.
The two figures above show the waveforms of reading and writing data using RS-485 PDO messages. It can be seen that the interval between each request is 7ms, and the interval between the request and response is approximately 1.5ms, with a clear message interval between the request and response. We've also tested a 6ms interval; the previous response and the next request are practically consecutive, posing a certain risk.
2. Physical Layer and Communication Stability
• RS-485 Bus Characteristics: Modbus RTU is typically based on the RS-485 physical layer, and its signal transmission needs to handle electrical delays and interference. Short intervals may lead to incomplete signal stabilization or noise interference, affecting verification and parsing.
3. Consequences of Protocol Errors
• Frame Merging or Loss: If the frame interval is too short, the receiver may misinterpret two independent frames as the same frame, leading to data parsing confusion or verification failure.
• CRC Check Failure: CRC check relies on complete frame data. If the frame interval is insufficient, causing data truncation, the check result will inevitably be incorrect, triggering an abnormal response.
