Why single ended signal line is not immune to noise in RS232?

Question

In single ended signal line the measurement is the difference between the signal and the ground. Isn't this way of calculation make it differential?

Almost all the resource I have read says that noise (unwanted signal contamination) is added only to the signal line (they don't even mention ground line picking up the same noise) because signal wires act as aerials, picking up environmental electrical activity making them sensitive to noise errors. Just like in the differential signaling the ground line here can also pick up the same noise as the signal wire and because the receiver is only looking at the difference it should make it immune to noise. But it doesn't why?

Answer 1

The ground connection is regarded as a big sponge for interference i.e. it soaks up that interference with barely any change in potential along its length. Therefore the single wire can be regarded as the most susceptible part of the transmission link.

Taking it one step further - interference hitting a long thin wire has a much more significant effect on the wire's end-to-end voltage than interference hitting a vast area of "fat" ground so, even if ground was (slightly) affected, the biggest point of vulnerability (by far) is the long thin wire.

And, because the long thin wire doesn't deal with noise as well as ground, you get a differential noise produced that cannot be cancelled by a differential receiver amplifier.

However, if the grounded signal source were connected to the far-away differential receiver by two wires (one being grounded at the source) AND the driving impedance of the source was very low (a few ohms max), then the two wires (of which one still represents ground) will likely receive the same interference signal and now, a differential amplifier will work. But, this only works if the wire that is remotely grounded at the source is handled electrically in the same way as the signal wire at the receiver. In other words, if you went and grounded that wire again you'd make a mess of things.

Here is a more general approach: -

Because the receiver is input impedance balanced AND the source is driving the signal line with the same impedance as seen in the grounded line then we have a balanced line driver circuit.

Answer 2

To achieve common mode rejection, differential lines need to also be balanced so the input impedance of both lines are the same. Being differential is isn't just a matter of opposite polarities. The circuit needs to experience the noise on both inputs identically to have them cancel out.

A ground line and signal line don't have the same impedance.

That's why a differential amp rejects common mode noise and a single-ended amp does not.

Answer 3

I think the answer's a lot simpler, OP is asking why interference induced on the ground conductor isn't a factor when the signal level is referenced to the "ground signal."

The ground conductor is connected to the chassis ground (most likely) on both ends of the cable. The chassis of equipment with rs-232 can probably be expected to be a significant sink for any charge.

(If you connected the serial out to the chassis ground on the other end, you wouldn't expect that the reference ground voltage would fluctuate significantly according to the signal being sent, you would expect hot wires and possible damage to the interface of the sending equipment)