Detailed explanation of network RJ45 interface

RJ45 brief introduction

chart 1 RJ45 modular
RJ45 Modules are used to implement PHY The interconnection between , Include PHY The chip is connected with RJ45 Interface connection , Pictured 1 Shown .
RJ45 The connector is composed of plug and socket ,RJ45 Plug is also called crystal head , Pictured 3-10 Shown . The connector composed of these two elements is connected between the wires , To achieve electrical continuity of the conductor .RJ45 The connector is the most important socket in the connector .RJ45 Sockets are divided into shielded and unshielded types .
RJ yes Registered Jack Abbreviation , intend “ Registered socket ”. stay FCC（ FCC standards and regulations ） The definition in is ,RJ Is an interface that describes a public telecommunications network , Commonly used RJ-11 and RJ-45, Computer network RJ-45 It's a standard. 8 Bit modular interface .

chart 2 RJ45 Sockets and plugs
RJ45 Interface support 10M/100M/1000M Three rates , With sending and receiving & Polarity automatically flips （ unreliable ）、 rate / Automatic negotiation function in duplex mode . Each rate mode has different implementation methods （ Corresponding to different agreements ）,10M Patterns include 10BASE-T、10BASE2、10BASE5;100M Patterns include 100BASE-TX、100BASE-F;1000M The rate mode includes 1000BASE-T、1000BASE-F. What is commonly used is 10BASE-T、100BASE-TX、1000BASE-T Three .
10BASE-T use Manchester code , Two level signal , When working, two pairs of differential lines, one receiving and one sending , Transmission rate 10Mbps.
100BASE-TX Mode adoption 4B/5B code , efficiency 80%, Three level signal , Pictured 3-11 Shown . When working, two pairs of differential lines, one receiving and one sending , Transmission rate 125 MT/s, Because of the adoption of 4B/5B code , The actual transmission rate is 125MT/s×80% = 100Mbps.

chart 3 100M Pattern RJ45 Signal eye diagram
1000M Mode adoption 4D-PAM5 code , Five level signal , Pictured 3-12 Shown . So-called 4D-PAM5 Refer to 4 dimension 5 Level coding technology ,4 Dimension refers to 4 Two differential line pairs ,5 Level refers to the transmission of one on each differential line pair 5 Binary data , Therefore, each symbol can represent 54=625 Status , Corresponding to binary number ,29=512, Each symbol can represent a 9bit Binary number of , One of them is used to control , in addition 8 Bits are valid data . Because the symbol rate is also 125MT/s, Working hours 4 Send and receive the differential line at the same time , port Rx Adopt hybrid circuit （Hybrid） Filter out the same port Tx The signal （ Similar principle ADSL Hybrid circuit , use Tx Reverse the phase and then stack to eliminate the received Tx）, So the actual The transmission rate is 125 MT/s×8bit = 1000Mbps.

chart 4 1000M Pattern RJ45 Signal eye diagram
Self negotiation technology
The earliest Ethernet networks were 10M Half duplex , So we need to CSMA/CD And a series of mechanisms to ensure the stability of the system . With the development of technology , There is full duplex , Then they appeared one after another 100M、1000M, The performance of Ethernet is greatly improved . But the question that follows is ： How to ensure the compatibility between the original Ethernet and the new Ethernet ？ therefore , Automatic negotiation technology came into being .
802．3 Section in the standard 28 This is how you define the auto negotiation function ： It allows a device to inform the device at the far end of the link of its working mode , And detect the corresponding operation mode of remote notification . The purpose of automatic negotiation is to provide a way for two devices sharing a link to exchange information , And automatically configure them to work under the optimal capacity . therefore , Automatic negotiation is a way to achieve the maximum possible transmission rate between two devices . It allows the device to “ Discuss ” Possible transmission rate , Then choose the best rate acceptable to both parties . They use what are called fast link pulses FLP Exchange announcements of their respective transmission capabilities .FLP The opposite end can know the transmission capacity of the source end .
for example ,A and B Negotiating automatically , also A have 10/100/1000 Full half duplex capability , however B Only 10/100 Full half duplex capability , In this way, the maximum link capacity shared by both parties is 100, full duplex . Once both parties conduct automatic negotiation , The link will run under the best capability that both sides can support .
In order to ensure that the peer can connect even if it cannot support self negotiation , Introduced what is called parallel detection （Parallel Detection） The mechanism of . Open self negotiation at one end , When the other end closes self negotiation , The establishment of the connection depends on the parallel detection function . For the end of closing self negotiation , The port will continuously send signals ,10M、100M、1000M The signals sent are different , The end that supports automatic negotiation after receiving the opposite end signal , You can know what mode the opposite end is working in , If you also support this mode , Then it will establish a connection with the opposite end . But this connection can only be half duplex , Because the parallel detection cannot detect the duplex mode of the opposite end , Therefore, half duplex connection is adopted by default .
therefore , In automatic negotiation , If both sides support automatic negotiation , The result of negotiation establishes a maximum rate and duplex mode supported by both ends ; If one end forces one end to negotiate , Then only one half duplex connection can be established , The connection rate is the rate of forced mode ; If both ends work in forced mode , Only when the duplex mode of two forced modes is exactly the same , To establish an effective connection . Pictured 5 Shown .

chart 5 Three automatic negotiation results
RJ45 Circuit design
1、RJ45 Line sequence of interface
From the signal transformer of Ethernet port TX-/TX+、RX-/RX+ Signals and RJ45 The connection must conform to RJ45 The interface specification , Pictured 3-14、 chart 3-15 Shown . The specific requirements are as follows ：
1） The polarity should be correct , It must not be reversed , Although some chips support polarity reversal , But when the chips of different schemes cooperate , Sometimes things go wrong .
2） Send and receive correctly , You must not reverse , Although the current chip design plays down the position of the transceiver pair , But in order not to take risks , Unless cabling cannot be achieved , Or do not reverse the wiring according to the regulations .

chart 6 10BASE-T/100BASE-TX Linear sequence
Detailed polarity and transceiver connection regulations are as follows ：
100M Card terminals （ Corresponding to general routers WAN mouth ）RJ45 Interface ：1-TX+、2-TX-;3-RX+、6-RX-;
100M Switch （ Corresponding to general routers LAN mouth ）RJ45 Interface ：1-RX+、2-RX-;3-TX+、6-TX-;
1000M RJ45 Interface ：1-DA+、2-DA-;3-DB+、6-DB-;4-DC+、5-DC-;7-DD+、8-DD-; or ：1-DB+、2-DB-;3-DB+、6-DB-;4-DD+、5-DD-;7-DC+、8-DC-;

chart 6 1000BASE-T Linear sequence
2、PCB Layout matters needing attention
PHY To the signal transformer is differential wiring , Difference is needed 100 Ohmic impedance control . The routing requires that the differential pair of two lines should be as long as possible , The difference line is equidistant , Try to complete the package .
RJ45 Network cable
Internationally recognized network cable standards are divided into seven categories , At present, five categories are commonly used 、 CAT5 and CAT6 twisted pair , Pictured 3-19 Shown . All three are composed of four pairs of twisted pairs , The difference lies in , The super category 5 network cable increases the winding density compared with category 5 network cable , Thus, the crosstalk is smaller , The transmission distance is longer , Larger transmission bandwidth . And category 6 network cable increases the diameter of cable on super category 5 network cable , And an insulating cross frame is added in the middle of the network cable , Place four pairs of twisted pairs on the four grooves of the cross frame , Further improve the signal-to-noise ratio .
Category 5 network cable is suitable for 100 Gigabit Ethernet , Cat 5 and Cat 6 network cables are suitable for Gigabit Ethernet . Of course, in short distance transmission , There will be no problem using category 5 network cables for Gigabit Ethernet connections .

The production standards of network cable mainly include T568-A and T568-B Two kinds of ,T568-A The line order of is ： Green and white 、 green 、 Orange white 、 blue 、 Bluish white 、 orange 、 Brown and white 、 Brown .T-568B The line order of is ： White orange 、 orange 、 white green 、 blue 、 White and blue 、 green 、 White brown 、 Brown , Pictured 7 Shown .
At present, it is widely used T568-B Standard line sequence .

chart 7 RJ45 Plug and socket wire sequence
There are two types of network cable connection: straight through and cross , Straight line means that the line sequence at both ends of the network line is the same , A fellow T568A Standard or T568B standard , The straight line is used to connect two interfaces with different properties , Such as PC to Switch/Hub,Router to Switch/Hub etc. ; Cross line means that the line sequence at both ends of the network line is different , At one end T568A standard , The other end is T568B standard , The cross line is used to connect two ports with the same properties, such as Switch to Switch,Switch to Hub, Hub to Hub, Host to Host,Host to Router etc. . It's generally understood that ： Use cross wire connection between the same type of equipment , Different types of equipment are connected by straight lines , Routers and PC Belong to DTE（ Data terminal equipment ） Type of equipment , Switch and HUB Belong to DCE（ Data communication equipment ） Type of equipment .