The purpose of communication is to transmit information! The opposite of communication is hardwired control. For example, to start and stop a motor, we need to run two signal wires. If there are ten motors, we need twenty wires, which is obviously very cumbersome. However, if we encode these twenty start and stop signals into a series of codes, send the codes out, and the receiving end translates the codes into start and stop signals according to the rules, we can control the motors. This is communication. Therefore, communication and hardwired control are the same; they just transmit information differently! So, no matter what you do, you must understand the principles! Only then can you apply your knowledge to other situations and achieve a comprehensive understanding! Many people reject communication because they think of it as mysterious. In fact, communication isn't that mysterious.
Today, I'll introduce some basic knowledge about communications.
1. Communication Protocol
There are various communication protocols, such as PROFIBUS-DP, CANopen, and DEVICENET, etc. Essentially, they are just different communication protocols. A communication protocol can be understood as a language; if they share the same language, they can communicate without barriers. But what about different languages? They cannot. Therefore, different protocols cannot communicate with each other. Different manufacturers support different communication protocols. PROFIBUS-DP, CANopen, and DEVICENET are the three most commonly used bus communication protocols. PROFIBUS-DP is primarily promoted by Siemens, so Siemens products support it. CANopen was first proposed by Bosch and is mainly used in automobiles. Now, Schneider Electric, Lenze, and many other companies support CANopen, including many domestic manufacturers, and it is also used in various embedded systems. DEVICENET is mainly promoted by AB and Omron; its hardware layer is actually the same as CANopen.
Therefore, communication protocols are very important. Different protocols cannot communicate with each other. Of course, there are many third-party devices on the market that can convert between different protocols, but I haven't used them and can't comment on them.
2. Baud Rate
Baud rate, in essence, is the communication speed. If we compare communication to language, then baud rate is the speaking speed! Communication can only proceed normally when both parties have the same baud rate. Just like speaking, if one person speaks very fast and another speaks very slowly, they cannot communicate properly.
Baud rates are typically a series of numbers, such as 9.6K, 18.4K, 125K, 500K, 1M, etc. Both communicating parties must set their baud rates to be consistent; otherwise, communication will not succeed. Of course, Siemens PROFIBUS-DP communication supports adaptive settings. A higher baud rate results in faster communication speeds and higher efficiency, but also less stable communication and shorter transmission distances. Therefore, the baud rate should be chosen as low as possible while still meeting communication requirements.
3. Master station and slave station
Bus communication typically uses a broadcast format, meaning one master station connects to multiple slave stations. The master station can send and read information from slave stations, but slave stations cannot read information from the master station, nor can they read information from each other. In other words, there can only be one master station. PROFIBUS-DP uses this format. CANopen communication, however, is different; it doesn't have a strict master-slave distinction, and all stations can act as master stations.
4. Topology
Topology refers to the way the various stations in a network are interconnected. In bus communication, a daisy-chain topology is used, which is a parallel structure where connections are made hand-in-hand. This structure is the most stable. Many people prefer non-star topologies when wiring, but this type of topology is highly unstable.
As shown in the diagram, this is the CANopen communication structure of a Schneider PLC. As you can see, all devices are connected in parallel, one after another. The red circles indicate the terminating resistors. This topology is the most stable!
