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Ethernet line driver

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vlad777

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I know that VGA uses current drivers(sources) that at the other end (monitor) give
exact voltage on monitor input resistors(which are of course connected to ground).
So I was wondering about Ethernet ?
Voltage or current signal source?
(And of course if you are familiar with any information on modulation voltage/current levels
ar any low hardware level info please post.)
 
I am a bit rusty (so you will probably need to verify this) but I'm pretty sure Ethernet uses a 15V differential signal. I forget what the modulation scheme is called, but it uses transitions from high to low/low to high to represent 1's and 0's, not the voltage.

For example, instead of 15V being represented as 1, a transition from low to high (maybe the other way around) is represented as a 1. Something is telling me there is another small trick in there, but that is basically the gist of it.

The advantages to this is that it is much more noise immune.
 
Ethernet is a vague question, there are dozens of physical implementations. There are copper (single ended and differential), optical, wireless, etc.

Perhaps you are asking about 10base t (or 100base t) over copper, which is quite common. The voltage on the 10base t cable is 0V or +/- 2.5V. 10base t and 100base t are point to point, so it can be (and is) impedance matched at both ends. Since it is impedance matched, it is (neither or both) a current source or a voltage source. It can be described either as +/-5V with 100 ohms in series or +/-50mA with 100 ohms in parallel. 100base t uses +/-1V, same cable.

10base t uses manchester encoding where the position of the rising and falling edges determine a 0 or 1. 100base t uses an encoding method called "8b10b" where 10-bit values are transmitted that correspond to all possible 8-bit symbols. The permissible 10-bit codes all have five ones and five zeros, allowing AC coupling. Manchester encoding requires 16 transitions per 8 bits, but 8b10b is more efficient in terms of bandwidth, requiring only 10 transitions per 8 bits.

I know of no implementation that used 15V, not even back in the early 1980s when the networks were still experimental.
 
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Are you sure mneary? According to what I found (quick Wikipedia search) 100BTX uses 4b5 encoding not 8b10. I thought 100BTX was still just manchester encoding that's why I looked it up.

An important thing to note, is that one of the primary requirements of twisted pair Ethernet standards of all types is that they require transformer isolation on both ends to meet the physical interface standards.
 
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