RS232,RS422,RS485 and CAN bus are common communication methods in IoT applications and can be used for data transmission and communication between different devices. Different communication lines have different transmission rates and transmission distances. How can we enable them to achieve longer-distance transmission?
RS-232 Overview
RS-232 (also known as EIA RS-232) is one of the commonly used serial communication interface standards and is an older standard. Full-duplex communication requires three wires: ground wire, sending wire and receiving wire. RS-232 is limited to point-to-point communication between the PC serial port and the device, and its anti-interference ability is poor.
RS-232 usually uses a DB9 or DB25 connector and requires parity checking. The standard transmission level of RS-232 is -12V~12V, but in fact it rarely reaches this range.
RS-232 in industrial control generally uses only three lines: RXD, TXD, and GND.
RS-232 is suitable for communication between local devices, such as connecting mice, printers, and industrial instruments. It is also used for driver and wiring improvements. In actual applications, the transmission length or speed of RS-232 often exceeds the standard value.
RS-485 Overview
RS-485 is also a common serial communication interface. Supports multi-point and long-distance transmission, using half-duplex communication method. It increases the number of devices and defines the electrical characteristics under the maximum number of devices to ensure sufficient signal voltage. RS-485 uses balanced transmission and differential reception, with high data transmission rate, good anti-interference ability, excellent noise immunity and multi-device capability. The signal can be transmitted for thousands of meters, which makes up for the short communication distance problem of RS-232.
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RS-422 Overview
RS-422 (EIA RS-422-A Standard) is the serial port connection standard for Apple's Macintosh computers. Using full-duplex communication mode, there are 4 signal lines (equivalent to dividing the RX and TX of 232 into two lines respectively). The receiving and transmitting are separated and can be received and transmitted at the same time. RS-422 uses differential signals and unbalanced signals referenced to ground. Differential transmission uses two wires to send and receive signals. Compared with RS-232, it has better noise immunity and longer transmission distance, which is especially suitable in industrial environments.
Level standard
According to the different level standards used in communication, serial communication can be divided into TTL standard and RS-232 standard.
TTL level standards are generally used in common electronic circuits. Under ideal conditions, 5V is used to represent binary logic 1, and 0V is used to represent logic 0. In order to increase the long-distance transmission and anti-interference capabilities of serial communication, RS-232 uses - 15V means logic 1, +15V means logic 0.
Because controllers generally use the TTL level standard, the MA3232 chip is often used to convert TTL and RS-232 level signals to each other.
CAN bus Overview
CAN bus (Controller Area Network) is a serial data communication protocol developed by BOSCH in Germany in the early 1980s to solve the problem of data exchange between many control and test systems in modern automobiles. It can use twisted pair, A multi-master bus using coaxial cable or optical fiber as the communication medium, with a communication rate of up to 1Mbps. Compared with many RS-485 distributed control systems based on R lines, the distributed control system based on CAN bus has stronger real-time data communication between nodes in the network.
Transmission distance comparison: RS232 vs RS485 vs CAN
RS-232 data transmission rate is low, and the standard transmission rate range is 110bps~115200bps. RS-232 is suitable for short-distance transmission, and the distance is mostly within 15m.
RS-485 uses differential signal transmission, with high data transmission rate, up to 10Mbps, and can be extended to a distance of more than 1,200 meters.
The maximum transmission rate of the RS-422 interface is 10Mbps, and the maximum transmission distance is about 1200 meters.
High-speed CAN communication standard: the communication rate is 125Kbps~1Mbps, and the bus length is ≤40 meters; low-speed CAN communication standard: the communication rate is 10~125Kbps, and when the transmission rate is 40kbps, the bus length can reach 1,000 meters.
For the above communication methods, under normal circumstances, the communication distance is: RS-232 ＜ CAN ＜ RS-422 ＜ RS-485
However, the communication distance is affected by many factors such as communication speed, connection lines, external interference, etc., and there will be a certain difference between the actual transmission distance and the theoretical one.
1.Baud rate
For serial communication, usually the higher the baud rate, the faster the transmission rate, the shorter the transmission distance, and the worse the anti-interference ability. Generally, the devices at both ends of the communication must be set to the same baud rate, but some devices can also adapt to the baud rate. The commonly used baud rate for serial ports is 9600 bps. At the 9600 baud rate, RS-232 recommends that the communication distance be within 13 meters.
2.Connecting cable
When transmitting long-distance signals, using twisted pairs for data transmission can physically reduce the coupling of noise to the signal transmission process. Shielded twisted pairs can also avoid interference from strong electromagnetic environments. The wiring must be strict, and it is best to ground it if it needs to be grounded. Some 485 communications also need to consider connecting terminal resistors to match; if it is RS-232 communication, try not to make the line too long. The length of a balanced twisted pair is inversely proportional to the transmission rate. For example, RS-485 can achieve the specified longest transmission distance only at a rate below 100Kbps; the highest transmission rate can only be obtained at a very short distance. Generally, the maximum transmission rate of a 100-meter twisted pair is only 1Mbps.
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3. External interference
Staying away from interference sources is one of the most commonly used methods on site. In actual wiring, it is necessary to ensure that strong current and weak current are separated and as far away from each other as possible; if it is impossible to avoid them, it is best to cross vertical wiring. For example, one of the reasons for the short transmission distance of RS-232 is that 232 is a single-ended signal transmission, which has problems such as common ground noise and the inability to suppress common mode interference, so it is generally used for communications within 20m.
Therefore, in order to ensure stable signal transmission and increase the actual transmission distance, it is necessary to select an appropriate baud rate or transmission rate as much as possible, add shielding measures, and stay away from interference sources. If you want to achieve longer-distance transmission, it is recommended to choose a wireless transmission solution.