Difference between RS485 and Ethernet. Ground loops

[[email protected]]

Hi, im trying to get my head round ground loops and after some
verification of my limited understanding.

Looking at the driver of RS485, you measure the voltage between the
output_A and output_B. These go straight into another op-amp in the
diagram. I'm guessing the current from output_A cant flow back through
output_B becuase there are no means to do so (output_B goes into an
input)? What i dont understand is how it flows to the remote equipment
and back through the ground? We need to refernence a voltage point i
guess because a single line with some voltage on it is arbitary
correct? So they use ground, thats ok, but surely the ground is quite
resistive by the time you take into account the poor connections etc?

How does ethernet differ to the above? In diagrams i see output_A flow
through a transformer back to output_B... as it uses a transformer im
guessing the current can flow back through correct? Whats the name for
this type of input/output mechanisim?

Finally, how can there be a difference in potential between earth
points? I thought earth is earth??? Its obviously not becuase lots of
books mention ground loops i just dont fully understand yet.

Thanks in advance,

Chris

 

[Noway2]

Hi, im trying to get my head round ground loops and after some
verification of my limited understanding.

Looking at the driver of RS485, you measure the voltage between the
output_A and output_B. These go straight into another op-amp in the
diagram. I'm guessing the current from output_A cant flow back through
output_B becuase there are no means to do so (output_B goes into an
input)? What i dont understand is how it flows to the remote equipment
and back through the ground? We need to refernence a voltage point i
guess because a single line with some voltage on it is arbitary
correct? So they use ground, thats ok, but surely the ground is quite
resistive by the time you take into account the poor connections etc?

How does ethernet differ to the above? In diagrams i see output_A flow
through a transformer back to output_B... as it uses a transformer im
guessing the current can flow back through correct? Whats the name for
this type of input/output mechanisim?

Finally, how can there be a difference in potential between earth
points? I thought earth is earth??? Its obviously not becuase lots of
books mention ground loops i just dont fully understand yet.

Thanks in advance,

Chris

Both RS485 and ethernet use differential transmission. The
communication signal is defined, not with respect to ground, but
between the the two terminals, which you refered to as A and B.

You seem to understand the concept that a signal has to have a return
path and that this return path is equally as important as the signal
path, which is a lot more than I can say for many people. In the case
of differential transmission, however, the signal and return current
flow in the two 'signal' wires. These two wires are kept tightly
coupled, usually by twisting them. The twisting helps keep the pair in
close physical proximity and also helps prevent electromagnetic
interference.

Both RS485 and Ethernet employ electrical isolation. This is due to
the fact that 'ground' potentials are not always equal. Ground
potential is not an absolute reference and it does vary from location
to location, even within the same building. Another factor that
necessitates the use of differential transmission (not referenced to
ground) is the presenece of high 'common mode' voltages or ones that
are present with respect to 'ground' on both signal leads.

In the case of Ethernet, the transformer is used to provide isolation.
A transformer will block DC (bias or offset) voltages while passing AC
(signals).

 

[[email protected]]

Thanks for the reply. Ok now im really confused! I just found a circuit
cellar "The Art and Science of RS-485" and it says:

"Signals A and B are complementary, but this doesn't imply that one
signal is a current return for the other. RS-485 is not a current loop
The drivers and receivers must share a common ground. This is why the
two wire network is a misnomer when applied to RS-485".

It also talks about daisy chaining the earths between data modules to
give a good ground reference for return current?

Any ideas?

Cheers,

Chris

 

[petrus bitbyter]

Thanks for the reply. Ok now im really confused! I just found a circuit
cellar "The Art and Science of RS-485" and it says:

"Signals A and B are complementary, but this doesn't imply that one
signal is a current return for the other. RS-485 is not a current loop
The drivers and receivers must share a common ground. This is why the
two wire network is a misnomer when applied to RS-485".

It also talks about daisy chaining the earths between data modules to
give a good ground reference for return current?

Any ideas?

Cheers,

Chris

Circuit cellar is correct. I've seen the mistake before. As RS-458 is
balanced, the currents will be balanced as well, but as they are DC coupled
the common mode voltage needs to be kept within limits. Otherwise you'll
overdrive the receivers or even blow them. That's why the common ground is
required.

As ethernet is AC coupled using transformers you do not need a common
ground. Transformers will not let pass any common mode signals or
disturbances.

petrus bitbyter