I have a few questions about serial communication?

乱世煮酒论天下

I have a few questions about serial communication? [Copy link]

I would like to ask the experts in the forum about several questions about common serial communication.
1. Common serial communication CAN, DP, PN, ETHER IP/TCP, ETHERCAT, etc. What are the terminal resistances of these serial protocols? Moreover, whether the terminal resistance needs to be added is related to the communication distance?
2. These terminal resistances are for impedance matching. When do you need to add these terminal resistances? Do you need to add them in one-to-one communication? Sometimes it may be better not to add them. Is there any theoretical proof for this?
3. How to add terminal resistance in one-to-many communication? Common bus connection methods include daisy connection, star connection, bus connection, etc. How to connect the terminal resistance for such connection methods?
4. I have seen some methods of using terminal resistance before. The first end is connected in series in the circuit, and the end is connected in parallel in the circuit. These should only be applicable to differential signal transmission, that is, two-wire transmission. What if it is single-wire transmission? Do you need terminal resistance too? Is
there any information recommended for the above?

tagetage

You should first search for answers to the above questions on the Internet. If you encounter specific problems that you don’t understand, then post a question.

qwqwqw2088

According to the ISO 11898-2 standard, the CAN segment resistance is required to be between 50 and 65 ohms.

In practical applications, a 120-ohm terminal resistor is generally added to the farthest ends of the CAN bus because the characteristic impedance of the twisted pair cable is 120 ohms.

qwqwqw2088

It is recommended to refer to the standards of the relevant protocols and the specifications of the equipment in the several communications listed on the first floor. This requires checking the standards of each protocol.

Generally speaking, in actual applications, the value of the terminal resistor and whether it needs to be added, as well as the length of the communication distance, all depend on the specific serial communication protocol and application scenarios. This is all I can say.

chunyang

The original poster's basic concepts are not clear, and the questions he asked are not valid.

First of all, CAN and other standards include protocols and hardware. It is impossible to say what the terminal resistance of the protocol is. The protocol has nothing to do with the terminal resistance. Serial communication does not necessarily include protocols, such as RS232, RS485, etc., or in other words, standards that do not include protocols can build any required protocols.

The terminal resistor is used to match the impedance of the signal transmission line. Without the terminal resistor, as long as the length of the signal line is greater than 1/20 of the signal wavelength, the reflection of the signal at the terminal of the transmission line will have an impact on the signal waveform that cannot be ignored. Therefore, as long as the length of the signal line is greater than 1/20 of the shortest wavelength of the transmission signal, a terminal resistor must be added to suppress signal reflection in order to maintain the signal waveform.

Terminal resistance must be added at both ends of the signal transmission line, and the signal line cannot be connected in a T-type terminal, that is, it cannot be connected in a star shape, but must be connected in an L-type, that is, a daisy chain connection. That is to say, there cannot be branches, which will cause signal reflection and degrade the waveform.

As can be seen above, whether a terminal resistor is needed has nothing to do with differential or single-ended signals, but only with the signal wavelength and the length of the transmission line. However, in practice, differential signal transmission must be used for long-distance communication, and single-ended voltage signals cannot be transmitted over long distances. The problem of induced noise is difficult to solve under single-ended voltage transmission. Therefore, the common situations where terminal resistors are needed are basically those under differential signal situations.

However, the situation is different for current signals. The serial port can have current transmission type, such as industrial 4-20mA current loop. The current loop does not require any terminal resistor, but it must be connected to a resistor, which is used for current-voltage conversion, but it is only arranged at the receiving end.

hjl2832

The so-called terminal resistance refers to the equivalent resistance of the AC signal, so the coaxial cable is 50 ohms and the twisted pair is 120 ohms. This refers to the AC (signal) impedance matching. We all know that under impedance matching conditions, the attenuation of signal transmission is minimal. In circuit simulation analysis, it can be found that when the impedance is matched, the signal integrity is also better.

乱世煮酒论天下

chunyang posted on 2024-4-5 23:56 The author's basic concepts are not clear, and the questions he asked are not valid. First of all, CAN and the like are standards, including protocols and hardware parts, and we cannot say that the protocol...

May I ask which forum did you search for a standard protocol hardware circuit for communication? I found some that were not comprehensive or even one-sided, and the explanations were not clear enough.

damiaa

The original poster has too many and too complicated questions. It is recommended to subdivide the questions. Ask one type of question in one post. Otherwise, you will not get an answer in the end.

idgyf

You should first search for answers to the above questions on the Internet. If you encounter specific problems that you don’t understand, then post a question.

langtuodianzi

For the RS-485 bus, the terminal resistor is mainly used to match the characteristic impedance of the communication line, prevent signal reflection, and improve signal quality.

It is recommended not to add terminal resistance when the communication speed is low or the communication distance is short.

When the communication distance is long and the communication speed is fast, and the signal quality requirement is high, the terminal resistance can be increased.

chunyang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-4-6 15:36 May I ask which forum I searched for a standard communication protocol hardware circuit? I found some that were not comprehensive or even one-sided, and the explanations were also...

This is not a forum search, but a standard document search, such as the official website of IEEE. However, in practice, it is mainly through mastering relevant theoretical knowledge and then gradually accumulating it in practice. It cannot be understood by simply reading an article. In fact, your question is still invalid and should not be asked at all.