*2.2. CAN Bus with CANopen Protocol*

The exchange of data packets in the system was based on a CAN bus. However, ISO 11,783 was specifically designed for tractor control system development [23–25]. In China, fewer products have been developed using subprotocols, especially in sensors and on-board computers. The on-board computer (eMT3070B) used in this paper had a CAN interface and conformed to the CANopen protocol. To test the designed system as soon as possible and to enhance the scalability at the present stage, the CANopen protocol was used as the basic protocol. CANopen is a high-level communication protocol based on the controller area network. It includes a communication subprotocol and a device subprotocol and has often been presented in embedded systems and industrial controls [26]. The CANopen protocol usually consists of three parts: a user application layer, an object dictionary, and communication. The core part is the object dictionary, which describes the relationship between the application object and the CANopen message. The user application layer in this paper refers to the application interface downloaded to the eMT3070B using EasyBuilder Pro development software provided by Weintek. Figure 3 shows the partial display interface design of the monitoring software for the eighteen-row seeding. In the communication layer, considering the field working environment, the wellestablished TJA1050 chip was selected as the transceiver of the CAN bus. This chip can work normally even with electrostatic interference and in voltage-mutating and high-noise environments and communicates with electricity.

To be stable, reliable, and controllable, the CANopen network needs to be set up with a network management master (NMT-Master) that controls the start and stop of all nodes. Communication between the on-board computer as the NMT host and the NMT slave via the NMT network management message is responsible for the layer management, network management, and ID distribution services. NMT management involves six states of a CANopen node following power-up: initializing, application reset, communication reset, preoperational, operational, and stopped. The NMT management state transition diagram is shown in Figure 4.


**Figure 3.** Partial human–computer interaction software interface.

**Figure 4.** NMT management state transition diagram.

The object dictionary is the most important part of a device specification. It is an ordered set of parameters and variables, including all parameters of device description and device network state. The CANopen protocol uses an object dictionary with 16-bit indices and 8-bit subindices, and all parameters of the device can be accessed through the object dictionary. The parameter object dictionary of the system is defined in the 2000H–5000H (H represents hexadecimal system) index region according to the CANopen CiA 301 document. Real-time data use the process data object (PDO) for asynchronous one-way transmission without a node response. The service data object (SDO) is mainly used for the parameter configuration of slave nodes in the CANopen master station. Service validation is the largest feature of an SDO, generating a response for each message to ensure the accuracy of data transmission. The CAN bus system in this paper consisted of a master node and eighteen slave nodes with 104 object dictionaries. Partial object dictionary descriptions are shown in Table 1. The CAN bus data transmission mode is shown in Figure 5.


**Table 1.** Partial object dictionary description.

**Figure 5.** CAN bus data transmission mode.
