How Do Distinguish Between RS232, RS485, RJ45, and Modbus?(4)
A few related questions to explain:
1）For some fieldbuses, tokens are used to solve the problem of control of the bus.
It is easy to think that if the slave station has an urgent matter that needs to be served by the master station, but MODBUS stipulates the polling rules, it may be too late when it waits for itself. So many fieldbuses invented a special thing called a token. The token is short, only one byte, and it can be passed on the bus very quickly. Tokens are passed in each site, and whoever gets the token is the master site and can publish information. If the station has nothing to issue, the token will be handed over to the next station, thus solving the problem of bus occupancy.
2）When the link is disconnected, in order to avoid communication interruption, dual master measures can be used. The dual master stations (the RS485 interfaces of the two master stations of the PLC) are connected by a handshake line. Usually, the main RS485 is opened, and the auxiliary RS485 is floating. Although the floating RS485 is connected to the bus, it is in a high-impedance state, which is equivalent to being completely disconnected. When disconnection occurs, the communication is opened immediately after the slave station confirms it, and the connection communication is performed from both ends of the link.
Sometimes, ring communication measures are also taken. Due to space limitations, no introduction will be given.
3）MODBUS can work at the network layer. At this time, the protocol becomes MODBUS-TCP, but it still conforms to the master-slave structure.
4）The MODBUS protocol was invented by the American company Modicon. The company's purpose is: the MODBUS protocol is a free and open protocol. Later, Modicon was acquired by Schneider, and Schneider inherited the practice of Modicon. MODBUS is a free public agreement. Now that MODBUS has become Schneider's protocol, Schneider extended it to the network layer and constructed the MODBUS-TCP protocol of the network layer, as well as the internal dedicated MODBUS-PLUS protocol. Due to space limitations, the descriptions of these two protocols are omitted here.
5）About the difference between RS232 and RS485
Anyone who has studied analog and digital electricity knows differential circuits. Differential circuits have a common-mode rejection ratio that eliminates common-mode errors. RS485 interface has this feature. Therefore, the transmission distance of the RS232 interface is only ten meters, while the transmission distance of the RS485/RS422 interface is 1200 meters. Although the shapes of the RS232 and RS485 interfaces are the same, their performance and information exchange modes are different, so their anti-interference capabilities are also different.
