143

The correct answer is because the ethernet specification requires it. Although you didn't ask, others may wonder why this method of connection was chosen for that type of ethernet. Keep in mind that this applies only to the point-to-point ethernet varieties, like 10base-T and 100base-T, not to the original ethernet or to ThinLan ethernet. The problem is ...


48

If you just ignore the POE 48 Volts in the image below, you can see Ethernet uses transformers on both sides. This way there is no need for common ground as long as the common mode voltage stays below 1500V generally. The isolation specification of the transformers. And as a bonus you now also know how POE works. (802.3at) However, CAT6A often has a ...


39

Isolation. So if the cable is shorted to a high voltage, your board won't blow up. It is needed since the other end may have a different ground. That's a specific case of isolation, but it is also required in normal operation.


38

40G Ethernet is really four physical 10G links running in parallel. Modern FPGAs have SERDES hardware that can run at well over 10 Gbps, and it's common to use four 32-bit buses running at 312.5 MHz inside the FPGA. That gives you a data rate of exactly 40.000 Gbps.


28

Long before USB there was the Etherkiller. And yes, it can fry your equipment. But unless you have bargain-basement (and I mean real sh*t equipment, not just inexpensive) stuff it won't affect any further devices connected to it.


27

Why is Ethernet not grounded? There are two reasons: 1. It would create a ground loop between devices 2. The device would also be more susceptible to ESD which is prevalent in cables that are being moved or handled (from triboelectric charging of the cable) The reason Ethernet is more susceptible to a ground loop is because: The loops could be much ...


25

Most of the current is the quiescent current of the transmit driver. If you read the datasheet, it explains that the output driver is sinking current through both sides of the transmit transformer's primary all the time. The two 49.9Ω termination resistors account for 66 mA of that all by themselves. When active, the transmit driver must launch a ...


25

Chasing this answer down took a few different links, but it appears to boil down to this: 1. 4 differential pairs (8 wires total, but only 4 lanes). 2. 800 Mega Symbols a second. 3. Using PAM16, 16 symbols are used which translates into 4 bits per baud per lane. Given that information you come up with 4 bits*800 Mhz*4 lanes which results in 12800 Mb/s or ...


23

There are several ways how you can make a data link faster: make more transmissions per second send more bits per transmission run several links in parallel 40G Ethernet does all of this: according to Wikipedia, it uses 4 channels, running at 1.6GHz each and transmitting 6.25 bits per clock cycle, which results in 40Gbit/s of total bandwidth. Here's a ...


23

That "transformer" is a common mode choke. It's used to suppress EMI (either being induced onto the line and affecting the circuit or being transmitted from the circuit out over the line). It's called "common mode" because it's very effective in suppressing HF currents that are common to both lines.


22

CAN sounds the most applicable in this case. The distances inside a house can be handled by CAN at 500 kbits/s, which sounds like plenty of bandwidth for your needs. The last node can be a off the shelf USB to CAN interface. That allows software in the computer to send CAN messages and see all the messages on the bus. The rest is software if you want to ...


21

A PHY chip or layer converts data between a "clean" clocked digital form which is only suitable for very-short-distance (i.e. inches) communication, and an analogue form which is suitable for longer range transmission. It has no particular clue as to what any of the bits "mean", nor how they should be interpreted or assembled. The MAC chip or layer ...


19

A even number of twists is better, but I am not aware of practical cable situations where this is worth the trouble: there are other sources of interference which are probably more important that the small difference it would make. Another way to look at it: the amount of magnetic interference is proportional to the area between the two wires. With a ...


18

There are actually three terms you want to know about Bandwidth Bandwidth is measured in Hz. It describes the frequency band that a communication channel is able to transmit with low loss. Typically we talk about a 3-dB bandwidth, meaning the range of frequencies a channel can transmit with less than 3 dB of loss. For a baseband system, the bandwidth ...


18

Ignoring that the article referred to a 'field effect resistor,' the drive is purely a hardware attack, not a software one. Really, there is nothing to stop the user from wiring main voltage to any output port either. It is generally agreed in security that if an attacker has physical access, you are out of options, the same could be said for physical damage ...


17

PHY chips handle the physical layer (Layer 1 of the OSI model), while MAC chips handle the data link layer (Layer 2 of the OSI model).


17

Differential signaling means that there is no need for a common ground as a reference point. Also, negates the need for shielding, which is usually grounded. No DC power transfer again removes the need for a common ground and makes point #3 possible. Galvanic separation makes grounding counterproductive. Specs put considerable effort into making devices at ...


16

You cannot really send the data without modification (0V - 0, some positive voltage - 1) over long lines noise can disrupt it and also, if the data contains a lot of "0" or "1" in sequence, the receiving device can lose count (was it 1000 zeros or 1001?), so line codes are used. RS232, for example, just sends the data (also called non-return-to-zero (NRZ) ...


16

Keep in mind that the type of ethernet that uses magnetics requires the transformer coupling in the spec. The transformers do some nice things for you, like completely decoupling common mode signals, providing significant voltage isolation, and in some cases projecting a different impedance to the electronics than what is natively on the cable. If your ...


15

Profibus is really low speed compared to ethernet and hanging components on the line to convert its energy levels to be compatible with intrinsically safe installations is quite trivial. I'm talking about a thing called a zener barrier - this consists of a fuse, 2 or 3 zeners (hazardous zone dependant) and a resistor. The fuse protects the zeners, the ...


15

The method is called echo cancellation, and it requires a bit of signal processing. Basically, the idea is since you know what you're sending out, then you can separate the signal you just sent from what is coming in from the far end of the link. The way the circuitry is set up, the transmit and receive signals are superimposed on top of each other, more ...


14

At least three reasons, probably: In the era when RS-232 ports were common, it so by far the most common means of bit-at-a-time communication, so much so that the term "serial port" became synonymous with "RS-232 port"; using the term "serial" in connection with anything else would add confusion. Note that USB avoids such confusion mainly because it seldom ...


14

You are describing using a concept called "Energy Harvesting," but you are trying to use the data pairs of the ethernet port as your energy source. Update: Well, let's qualify this a bit... While extremely interesting (I did my masters work in this area), what you are describing simply will not work well in practice for a number of reasons: All versions ...


14

There's two things you might call speed: bandwidth and latency. Latency is the duration of time needed for a signal at one node of the network to reach another node of the network. Processing time for the electronics to packetize the signal and place it on the wire often dominates the latency, but the physical medium does also affect it. As far as the ...


14

A 100BASE-TX device transmits on 1 pair (2 wires) and receives on another pair (2 more wires). Because of the 4B5B encoding -- for each 4 data bits, it transmits a series of 5 symbols -- 100BASE-TX has a 125 MHz symbol rate (sometimes called the baud rate) on each pair. Each symbol carries slightly less than 1 bit of information. 1000BASE-T -- Gigabit ...


14

In the old days of DIPs, sometimes entire sides of chips were designed as heatsinks, so it made sense to connect them with a big blob of solder too. But this heat-sinking technique is extremely uncommon for SOPs, which usually use pads underneath the package when moderate heat-sinking is needed. (Before someone says that using pins as heat-sinks is ...


14

There are a few bad things that the system was designed to mitigate, one is ESD, noise and ground loops. The first transformer (up down one) is an isolation transformer, it passes the high frequency signals that represent the bits of ethernet, but rejects DC. The second transformer is a common mode choke, it blocks signals that are common to both terminals ...


14

10G ethernet (as described by other answers) does not do signal transitions at 10 GHz, it uses multiple level encoding spread across 4 pairs to achieve 10 Gb/s. However, 10+ gigabit serial transceivers are quite common on high speed chips. For instance PCIe, USB3.1, thunderbolt, and similar protocols all use 10 gbit/s serial rate on individual pairs. You ...


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