Take one point, because RS232 communication can only be used for point to point. For example, computer to PLC, if the system needs to communicate with each other the number of nodes more than two, it can not directly meet the requirements, for this reason, "EIA" (Electronic Industries Association) developed a new interface standard RS485, which can support a point to multi-point communication.
The electrical standard for RS485 communication is the RS422 communication standard.RS485 is a variation of RS422A.RS422A is full duplex, two pairs of balanced differential signal lines are used for transmitting and receiving at the same time.RS485 is half-duplex, only one pair of balanced differential signal lines can not be used for transmitting and receiving at the same time.
Emphasis: RS485 is half-duplex, master-slave communication.
The use of RS485 communication interface and twisted pair can be composed of serial communication network, constitute a distributed system, the system can have up to 32 stations. If the number of devices connected to a network more than 32, you can use the RS485 repeater, some can allow the connection of 128 stations.
Emphasis: RS485 supports multi-station and multi-device communication.
RS485 Communication Wiring
In order to effectively suppress interference, shielded twisted-pair cables are generally used as the communication medium, e.g. Profibus DP cables from Siemens:
Each device's RS485 communication wiring is very worry-free, the premise to ensure that each device is "hand in hand" string down (very costly line or cable, more equipment, if the use of hubs or repeaters will save trouble), in order to avoid communication failures, to reduce interference and other factors, do not have a star connection or fork.
Perhaps you will ask, how the device is to do "hand in hand" string down, then please see this:
Siemens specializes in RS485 communication cable connector (Profibus DP connector/plug), this connector also has a terminating resistor, easy to use, that is, too expensive; some of the equipment in the RS485 communication wiring is in the form of terminal wiring, the connection can be referred to the cable connector connection.
RS485 Communication Bus Wiring
In the simple application of RS485 communication, the general use of bus-type topology wiring. However, in more complex systems, the bus topology wiring construction is not only very cumbersome, but also a waste of a large number of wires. Flexible use of 485 hubs or 485 repeaters, you can connect the bus topology into a star or tree topology. It greatly facilitates the preliminary construction and later maintenance work.
Connection Method of Terminal Resistor
In the practical application of 485 bus, when the transmission distance exceeds a certain length, the anti-interference ability of the bus will decline, in this case, it is necessary to access the 120 ohm terminal matching resistor at the first and last ends of the 485 bus to ensure the stability of the 485 bus.
The proper way to connect the termination matching resistors is to connect a 120 ohm termination resistor to the outlet of the device at the first end of the 485 bus and to the inlet of the device at the end of the 485 bus, which is connected in parallel between the positive and negative wires of the 485 bus.
RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems, as defined by the Telecommunications Industry Association and the Electronic Industries Alliance. Digital communication networks using this standard are capable of efficiently transmitting signals over long distances and in electrically noisy environments.
