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This article introduces the knowledge of "how to understand the simple communication protocol between C language and the Internet of things". In the operation of practical cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

When using C language to develop the Internet of things gateway, it is often necessary to read data from some sensors through serial port, 485 interface and so on. due to the complex and diverse environment of gateway devices and sensors, electromagnetic interference often destroys the transmitted data. in order to ensure the reliability of the transmitted data, some strategies are usually adopted: data verification + timeout retransmission. The specific process is as follows:

When sending data, the sender adds check data to form a request packet (data pack) based on the metadata, then sends the request packet and starts the guard timer (Guard Timer).

After receiving the request data packet, the receiver calculates the metadata with the same algorithm as the sender to get the check data, and then compares it with the received check data. If it is the same, it shows that the data is reliable and can be used; on the contrary, it means that the data is destroyed and discarded directly. If the data is reliable, the receiver makes a response packet in the same format and replies to the sender; if the data is found to be corrupted, the receiver does not reply to any data.

If the sender correctly receives the response packet replied by the receiver before the guard timer expires, stop the guard timer and carry out subsequent processing; if the sender does not receive the response packet replied by the receiver when the guard timer expires, resend "1." The packet in is repeated until it is sent correctly or the number of retransmissions is reached.

Step 1-3 focuses on how to ensure reliable data transmission. In a real scenario, in order to break the packet correctly (identify the packet), the packet is usually defined as follows:

Package start identification: fixed data is usually used, such as 0xFA, 0xAA, 0xA5, etc.

Metadata length: the packet is broken according to the metadata length. For example, the length of metadata is 1 byte, and the parity data is 2 bytes. When the metadata length is 10:00, the packet size is 14 bytes.

Metadata: application data sent or received with a certain format or meaning

Check data: the value calculated according to a check algorithm (such as checksum,crc, etc.).

# define MAX_DATA_LEN 100 / * assume that the maximum length of metadata is 100byte * / typedef struct tag_data_pack {uint8_t sop; / * packet start identifier, (Start Of Pack) * / uint8_t len; / * metadata length, and assume 1 byte * / uint8_t dataset [Max _ DATA_LEN + 2] / * metadata + 2 bytes of parity data * /} data_pack_t

In bus communication, the address of the target device is usually added to the packet to determine which device on the bus the data is sent to. The reference structure of the packet is as follows:

Described in C language as follows:

# define MAX_DATA_LEN 100 / * assume that the maximum length of metadata is 100byte * / typedef struct tag_data_pack {uint8_t sop; / * packet start identifier, (Start Of Pack) * / uint8_t addr; / * destination device address, assume 1 byte * / uint8_t len / * metadata length, assuming 1 byte * / uint8_t dataset [Max _ DATA_LEN + 2]; / * metadata + 2 bytes of parity data * /} data_pack_t; "how to understand the simple communication protocol between C language and the Internet of things" ends here. Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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