Features and Differences between CAN Bus and RS485

CAN bus features:

1. International standard industrial level field bus, reliable transmission, high real-time;

2. Long transmission distance (up to 10km), fast transmission rate (up to 1MHz bps);

3. A single bus can connect up to 110 nodes, and the number of nodes can be easily expanded;

4. Multi master structure, equal status of all nodes, convenient regional networking, high bus utilization;

5. High real-time, non-destructive bus arbitration technology, no delay for nodes with high priority;

6. The wrong CAN node will automatically close and cut off the connection with the bus, without affecting the bus communication;

7. The message is of short frame structure and has hardware CRC check, with low probability of interference and extremely low data error rate;

8. Automatically detect whether the message is sent successfully, and the hardware can automatically retransmit, with high transmission reliability;

9. The hardware message filtering function can only receive the necessary information, reduce the burden of the CPU, and simplify the software preparation;

10. Common twisted pair, coaxial cable or optical fiber can be used as communication media;

11. The CAN bus system has a simple structure and high cost performance.

 

RS485 features:

1. Electrical characteristics of RS485: logic "1" is represented by+(2-6) V voltage difference between two lines; Logic "0" is represented by the voltage difference between two lines as - (2-6) V. If the interface signal level is lower than RS-232-C, it is not easy to damage the chip of the interface circuit, and this level is compatible with the TTL level, which can facilitate the connection with the TTL circuit;

2. The maximum data transmission rate of RS485 is 10Mbps;

3. RS485 interface is a combination of balanced driver and differential receiver, which enhances the ability to resist common mode interference, that is, good noise interference;

4. The maximum transmission distance standard value of RS485 interface is 4000 feet, which can actually reach 3000 meters. In addition, only one transceiver is allowed to be connected to RS-232-C interface on the bus, that is, single station capacity. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has the ability of multiple stations, so users can use a single RS-485 interface to easily establish the device network. However, only one transmitter can transmit on the RS-485 bus at any time;

5. RS485 interface is the preferred serial interface because of its good noise immunity, long transmission distance and multi station capability.;

6. Because the half duplex network composed of RS485 interfaces generally requires only two wires, RS485 interfaces are transmitted by shielded twisted pair.

Differences between CAN bus and RS485:

1. Speed and distance: The distance between CAN and RS485 transmitted at a high speed of 1Mbit/S is not more than 100M, which can be said to be similar in high-speed. However, at low speed, when the CAN is 5Kbit/S, the distance can reach 10KM, and at the lowest speed of 485, it can only reach about 1219m (no relay). It can be seen that CAN has absolute advantages in long-distance transmission;

2. Bus utilization: RS485 is a single master slave structure, that is, there can only be one master on a bus, and communication is initiated by it. It does not issue a command, and the following nodes cannot send it, and it needs to send a reply immediately. After receiving a reply, the host asks the next node. This is to prevent multiple nodes from sending data to the bus, causing data confusion. The CAN bus is a multi master slave structure, and each node has a CAN controller. When multiple nodes send, they will automatically arbitrate with the ID number sent, so that the bus data can be good and messy. After one node sends, another node can detect that the bus is free and send it immediately, which saves the host's query, improves the bus utilization rate, and enhances the rapidity. Therefore, CAN bus or other similar buses are used in systems with high practicality requirements such as automobiles;

3. Error detection mechanism: RS485 only specifies the physical layer, but not the data link layer, so it cannot identify errors unless there are some short circuits and other physical errors. In this way, it is easy to destroy a node and send data to the bus desperately (sending 1 all the time), which will paralyze the whole bus. Therefore, if an RS485 node fails, the bus network will hang up. The CAN bus has a CAN controller, which can detect any bus error. If the error exceeds 128, it will be automatically locked. Protect the bus. If other nodes or their own errors are detected, error frames will be sent to the bus to remind other nodes that the data is incorrect. Be careful, everyone. In this way, once a node CPU program of CAN bus runs away, its controller will automatically lock and protect the bus. Therefore, in the network with high security requirements, CAN is very strong;

4. Price and training cost: The price of CAN devices is about twice that of 485. In this way, 485 communication is very convenient in terms of software. As long as you understand serial communication, you can program. While CAN requires the bottom engineer to understand the complex layer of CAN, and the upper computer software also needs to understand the CAN protocol. It can be said that the training cost is high;

5. The CAN bus is connected to the physical bus through the CANH and CANL of the two output terminals of the CAN controller interface chip 82C250. The CANH terminal can only be in a high level or suspended state, and the CANL terminal can only be in a low level or suspended state. This ensures that, as in the RS-485 network, when the system has errors and multiple nodes send data to the bus at the same time, the bus will be short circuited, thus damaging some nodes. In addition, the CAN node has the function of automatically closing the output when the error is serious, so that the operation of other nodes on the bus will not be affected, so as to ensure that there will be no problems in the network, and the bus will be in the "deadlock" state due to the problems of individual nodes;

6. CAN has perfect communication protocol, which can be realized by CAN controller chip and its interface chip, thus greatly reducing the difficulty of system development and shortening the development cycle, which is incomparable to RS-485 only with electrical protocol.

 

Shenzhen Zhongling Technology Co., Ltd., since its establishment in 2013, has been committed to the wheel robot industry, developing, producing and selling wheel hub servo motors and drivers with stable performance. Its high-performance servo hub motor drivers, ZLAC8015, ZLAC8015D and ZLAC8030L, adopt CAN/RS485 bus communication, respectively support CiA301 and CiA402 sub protocols of CANopen protocol/modbus RTU protocol, and can mount up to 16 devices; It supports position control, speed control, torque control and other working modes, and is suitable for robots in various occasions, greatly promoting the development of the robot industry.