In modern systems, most instruments and equipment are controlled through host computer software, which makes debugging easier and improves operational efficiency. This process relies heavily on communication protocols. Based on the author's experience in developing several devices, this article outlines a general approach to writing communication programs, covering both upper and lower level communication.
**1. Custom Data Communication Protocol**
A data protocol is a packet format built on top of the physical layer. The physical layer includes common communication methods such as RS232, RS485, infrared, fiber optics, and wireless. At this level, the underlying software provides two basic functions: sending one byte of data and receiving one byte of data. All higher-level protocols are based on these operations.
Data is typically transmitted in packets, known as frames. Similar to the TCP/IP protocol in network communication, a reliable communication protocol usually includes the following components: frame header, address information, data type, data length, data block, checksum, and frame tail.
The frame header and frame tail are used to identify the integrity of the data packet. A fixed-length sequence is often chosen for this purpose. The goal is to minimize the probability of matching random bytes in the entire data stream. This can be achieved by either reducing the likelihood of matching feature bytes or increasing the length of the feature bytes. The first method is commonly used because the data in the system is predictable and can be manually avoided. However, increasing the feature byte length is more versatile and suitable for systems where data is not predictable.
Address information is essential in multi-device communication, allowing each device to be uniquely identified. In a one-to-many system, only the destination address may be included. In many-to-many systems, both source and destination addresses are necessary.
The data type, data length, and data block contain the main content of the message. The data type indicates whether the following data is a command or raw data, while the data length specifies the number of valid bytes.
The checksum ensures the correctness and integrity of the data. Common methods include simple accumulation or more complex CRC calculations. The choice depends on factors like speed and error tolerance.
**2. Data Transmission Between Upper and Lower Computers**
At the physical communication layer, two fundamental operations are available: sending and receiving single bytes of data. Data transmission involves sending each byte of the packet sequentially. Different methods exist, such as direct transmission or interrupt-based transmission.
In microcontroller systems, it’s common to use the serial port to send a single byte directly. While this method requires the processor to manage the entire transmission, it ensures immediate data availability on the communication line. Another approach is to use interrupts, where data is stored in a buffer and sent via an interrupt routine. This reduces CPU usage but introduces slight delays. For 51-series microcontrollers, direct transmission is often preferred due to its simplicity.
Here is an example of a function for sending a single byte in a 51-series microcontroller:
```c
void SendByte(unsigned char ch) {
SBUF = ch;
while (TI == 0);
TI = 0;
}
```
On the host computer side, serial communication can be handled in various ways. Windows provides built-in serial communication controls, while APIs allow direct access to serial ports. On Linux, serial devices are treated as files. The CSerialPort class is a useful tool that simplifies serial communication with functions like `WriteToPort(char* string, int len)`.
**3. Data Reception and Protocol Analysis in the Lower Computer**
The lower computer can receive data using either polling or interrupt methods. Interrupt reception is generally more efficient and less resource-intensive. Once the serial port is initialized, the received data can be parsed within the interrupt service routine or stored in a buffer for later processing.
For example, in a simple system, the protocol parsing can occur directly in the interrupt handler. If the protocol is more complex, the data is stored in a buffer, and the main program handles the parsing. This allows for greater flexibility and scalability.
An example of a packet format might look like this:
`0x55, 0xAA, 0x7E, 0x12, 0xF0, 0x02, 0x23, 0x45, SUM, XOR, 0x0D`
Each part of the packet has a specific role: the header identifies the start, the address fields identify the sender and receiver, the data length defines the size of the payload, and the checksum ensures data integrity.
The state machine in the code tracks the progress of the packet reception, ensuring that each byte is correctly interpreted and validated. If any unexpected byte is received, the state machine resets, preventing errors from propagating.
**4. Data Reception and Command Processing on the Host Computer**
On the host side, the data reception process is similar to the lower computer, but the implementation varies depending on the serial port interface. Using a thread for monitoring serial data is recommended, especially when working with blocking read functions.
The CSerialPort class, for instance, uses a separate thread to monitor incoming data, stores it in a buffer, and sends a message to the parent window. This allows the main application to process the data asynchronously.
The message `WM_COMM_RXCHAR` is used to notify the application that a character has been received. The response function then processes the data, performing protocol checks and extracting meaningful information.
**5. Conclusion**
This article presents a basic framework for designing a communication system. While the examples provided are simplified, they demonstrate a practical and effective approach. Real-world communication protocols are more complex, involving features like packet acknowledgment, error handling, and timing adjustments. With the foundation laid here, more advanced and robust systems can be developed.
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