RS485 Bus I believe for everyone is not unfamiliar, I also used for many years, familiar with its circuit of the drawing, software program design methods, and accumulated a number of practical applications in engineering experience. But I always feel attached to the surface, "Why should I add a 120 ohms matching resistor?" "," where should the matching resistor be added? "," Why should I add bias resistors? "," How much of the bias resistor is appropriate "," how to properly pass EMC testing "and so on these problems have been bothering me for a long time, experience is the engineering of repeated experiments, it is time to sink the heart to make up their own theory, so close this article. This article hopes The RS485 can be elaborated from a new angle, and the theory and experience of RS485 are combined without involving too many complicated mathematical formulae ., seek to strengthen their own understanding, second to be able toshare and communicate technology with everyone.
Speaking of RS485, we firstThe need to consider is its transmission medium: twisted pair, this article starts from the characteristic impedance of the transmission line.
1. Transmission line
a transmission line is a signal communication circuit consisting of two components of a certain length, as shown in. One is marked as the signal path and the other is the return path. When we design the circuit, we want to: A, The return current is to be close to the signal current, B, the signal path at high frequency and the returnthe loop inductance of the path should be minimized, C, and as long as the condition permits, the return path will beThe volume is close to the signal path. when two lines are the same, such as the twisted pair used by the RS485, the signal path and the return path areThere is no strict distinction. We can specify any one as the signal path, and the otheris the return path. If the cross section of any one of the wires is the same, or the twisted pair we call ittransmission line is uniform transmission Line (note 1). Further, if the shape and size of the two conductors are the same,or what we call twisted-pair,We'll call thistransmission lines are symmetrical transmission lines. Well, we should know now. The twisted pair used on the RS485 should be a symmetrical uniform transmission line, or it can be called a symmetrical, controllable impedance transmission line (Note 1).
2. Transient impedance of transmission line
consider that we can think of the transmission line as (not a special effect) by a row of small capacitance model, its value is the above the villain Walk 1 steps of the capacitance. through the capacitance model, we can calculate the current on the transmission line. The current on the visible wire is only related to the capacitance of the unit length on the transmission line, the propagation speed of the signal and the voltage of the signal. so, every step forward in the transmission line, the signal is subjected to the same impedance asresistive loads. from this relationship, we can calculate the signal transmitted along the transmission lineto the transient impedance. The transient impedance equals the applied voltage and the power flowing through the device.ratio of flows: It can be seen that as long as the cross-section and material properties of the transmission line are unchanged, the two parametersthe transient impedance of a signal is a constant. since the speed of the signal depends on the material properties, we can draw the transmission linethe relationship between the unit length capacitance and the transient impedance. For example, if the dielectric constant is 4,unit length capacitance is 3.3/in, the transient impedance of the transmission line is:
3. Characteristic impedance
for uniform transmission lines, when the signal is propagated above, it is subjected to aThe transient impedance is the same. When the transient impedance is constant, wecalled the characteristic impedance. The characteristic impedance is equal to the transient impedance of the uniform transmission line in numerical value, it isthe inherent properties of the transmission line, and only with the material properties, dielectric constant and unit lengthCapacitance, regardless of the length of the transmission line. The formula is: The characteristic impedance of the transmission line is inversely proportional to the unit-length capacitance between the two conductors.System. If we increase the width of the two conductors, we increase the capacitance per unit length. if the distance between the two conductors is increased, the capacitance of the unit length is reduced.
The characteristic impedances of several common transmission lines are given below:
RG174 Cable: 50 ohm
RG58 Cable: Ohm
RG59Cable:Ohm
RG62Cable:Ohm
TV Antenna (television antenna):Ohm
Cable TV (Cable TV):Ohm
Twisted pairs (twisted pair): 100-120Ohm
This analyzes the RS485 used by the twisted pair is a symmetrical uniform transmission line, which has a 120 ohm characteristic impedance.
4. Image understanding of characteristic impedance: input impedance of transmission line
Suppose we use an ohmmeter to measure the characteristic impedance of an open-end transmission line, according to common sense we can tell that the value it displays is infinite. If we consider the infinite length of the transmission line, the signal is transmitted on the transmission line is a time, when the signal has not returned, we measured the resistance should be how much? The answer is the characteristic impedance of the transmission line.
So you can define the input impedance of the transmission line as follows: the impedance seen from one end of the transmission line changes over time. In the signalThe impedance measured is the characteristic impedance within the round-trip time. If the waiting time is long enough,the measured impedance will be an open circuit. so when the signal frequency increases, the characteristic impedance is revealed,for high-speed drives, when driving a transmissionline, the input impedance is equivalent to a pure power in the round-trip timeresistance.
5, Transmission line Drive
when the transmission line is driven by a high-speed drive, the input impedance of the transmission linein between, the size equals the characteristic impedance. , the establishment of floodingThe equivalent circuit model of the actuator and the transmission line to calculate the voltage added to the transmission line. in order to drive the transmission line, the voltage added to the transmission line is close to the source voltage,The output resistor of the drive is very small compared to the characteristic impedance of the transmission line. ExampleFor example, if the characteristic impedance of the transmission line is 50 ohms, the source resistor should be less than 10. if the output impedance is particularly low, such as 10 ohms or less, it is usually calledThey are linear drivers that can add most of the voltage to the transmission line. (Note 2)
6. Equivalent electrical model of transmission line
The transmission line model described above is an ideal physical model, and the transmission line electrical model shown is further approximate to the physical transmission line. in the simplest equivalent circuit model of the transmission line, every two small capacitors area small loop inductance is separated. The C in the figure represents the capacitance between the two conductors, and L indicatesthe loop inductance between the two bars. at this point the formula for the characteristic impedance becomes: With This formula we can further explain the relationship between the characteristic impedance and the frequency, and the loop inductance will decrease as the frequency increases . This shows that at low frequencies, the characteristic impedance is higher,as the frequency increases, the characteristic impedance drops to a constant value.
In this paper, the characteristic impedance of transmission line is described in detail, and the twisted pair used for RS485 transmission signal is symmetrical and uniform transmission line, which is usually selected as 120 ohms by specific characteristic impedance. So why add the same 120 ohm matching resistor to the RS485? I will be in the " RS485 Deep Analysis 2described in the ".
Note 1: Many other materials also known as the uniform transmission line is a controllable impedance transmission line.
NOTE 2: The model involved in this article is a single-ended output model, and RS485 is a two-terminal output, which I'll cover in a subsequent article.
Note 3: This article in the process of writing access to a large number of online information, the citation of the source could not be one by one marked, special apology.
Note 4: The main reference book from the "integrity of the signal," the United States Eric Bogatin, Li Yushan translation, electronic industry press 2005.4 published. I have uploaded the photocopy and the PPT document of each chapter to the space that I created in csdn, we can download it in http://download.csdn.net/detail/qingwufeiyang12346/8953423.
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RS485 Deep Analysis 1: Characteristic impedance of transmission lines
