The difference between RS232 interface and RS485 interface
1、 Physical structure of the interface
1. RS232 interface:
Generally speaking, there will be two pins in the comdb-232 (comdb-232) interface, which is called comdb-2 (comdb) interface.
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2、RS485：
The RS485 interface is not used according to the actual situation of the project.
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2、 Electronic characteristics of the interface
1. RS232: the signal level value of the transmission level signal interface is high (signal "1" is "- 3V to - 15V, signal" 0 "is" 3 to 15V "), which is easy to damage the chip of interface circuit. Because it is not compatible with TTL level (0 ~" < 0.8V ", 1 ~" > 2.0V "), it is necessary to use level conversion circuit to connect with TTL circuit. In addition, the anti-interference ability is poor.
2. The difference between the two transmission lines is represented by "RSV: 2" as the logic difference between the two lines. The interface signal level is lower than RS-232, so it is not easy to damage the chip of interface circuit. Moreover, the level is compatible with TTL level and can be easily connected with TTL circuit.
3、 Communication distance
1. RS232: RS232 transmission distance is limited, the maximum transmission distance standard value is 15 meters, and can only point-to-point communication, the maximum transmission rate is 20KB / s.
2. RS485: the maximum wireless transmission distance of RS485 is 1200 meters. The maximum transmission rate is 10Mbps, and the maximum communication distance can be achieved at the transmission rate of 100kb / s.
The special cable with impedance matching and low attenuation can reach 1800 meters! If it is more than 1200 meters, repeaters (up to 8) can be added, so the transmission distance is close to 10km.
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4、 Can multi-point communication be supported
RS232: only one transceiver is allowed to be connected to the RS232 interface on the bus, which can not support the multi station receiving and transmitting ability, so it can only communicate point-to-point without supporting multi-point communication.
RS485: RS485 interface is allowed to connect up to 128 transceivers on the bus. That is, it has the ability of multi station communication, so that users can easily set up the equipment network by using a single RS485 interface.
5、 Differences in communication lines
RS232: three core twisted pair and three core shielded wire can be used.
RS485: two core twisted pair and two core shielded wire can be used. Common twisted pair cable can be used in low-speed, short-distance and non-interference situations. On the contrary, in high-speed and long-term transmission, RS485 special cable (stp-120 Ω (for RS485 &amp; can) with impedance matching (generally 120 Ω) must be used, and armored twisted pair shielded cable (astp-120 Ω (for RS485) should also be used in harsh interference environment&amp; Can) one pair of 18awg).
Add: even if the RS232 transmission distance is only 15 meters, what is the effect?
In fact, it is widely used, can be connected to a variety of devices, such as monitoring, other equipment upgrades or debugging may need to use it. The function is close to USB. With the increasing popularity of USB port, there will be more devices to convert USB to RS-232 or other interfaces.
More RS-232 devices can be connected through USB interface, which can not only achieve higher transmission speed and realize real plug and play, but also solve the problem that USB interface can not transmit long distance (USB communication distance is within 5 meters).
Several common communication protocols
Before we start, we should understand a problem that I've been confused about.
The difference between RS485 and MODBUS: RS485 is a physical interface, simply speaking, hardware; MODBUS is an international standard communication protocol, which is used to exchange data between different manufacturers (generally for industrial purposes); the so-called protocol can also be understood as the "language" mentioned above, which is simply software.
In general, two devices transmit data through Modbus protocol: RS232 is used as hardware interface at first, and RS422 is also used, and RS485 is commonly used. This interface has a long transmission distance and is widely used in general industrial fields.
Modbus protocol is divided into Modbus RTU, MODBUS ASCII and Modbus TCP.
The physical hardware interfaces of the first two (Modbus RTU, MODBUS ASCII) are serial communication (RS232, RS422, RS485). Modbus TCP is to comply with the development trend of today's world. Everything can be connected and transmitted by Ethernet or Internet. Therefore, Modbus TCP mode is generated. The hardware interface of this mode is Ethernet port, which is the network port commonly used on our computer.
Then we can understand several communication protocols supported by S7-200 PLC through a picture.
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1、 PPI communication
It is a communication protocol specially developed by Siemens for S7-200 series PLC. Built in S7-200 CPU. PPI Protocol is physically based on RS-485 port, and PPI communication can be realized through shielded twisted pair. PPI is a master-slave protocol. The master station equipment sends the request to the slave station equipment, and the slave station equipment responds, and the slave station can not send information actively. The master station communicates with the slave station by sharing connection managed by PPI Protocol. PPI Protocol does not limit the number of master stations communicating with any slave station, but in a network, the number of master stations cannot exceed 32. The basic purpose of PPI Protocol is to enable Siemens STEP7 micro / win programming software to upload and download programs and Siemens human-computer interface to communicate with PC.
2、 MPI communication
MPI (multipoint interface) is the interface of SIMATIC S7 multipoint communication. It is a kind of network suitable for the communication between a few sites, which is mostly used for connecting