Electronics > Beginners
RS485 vs Ethernet
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paulca:
As an ill-informed IT person I thought the cable length for Ethernet was determined by timing, not signal strength.
Berni:

--- Quote from: paulca on November 05, 2019, 09:13:37 pm ---As an ill-informed IT person I thought the cable length for Ethernet was determined by timing, not signal strength.

--- End quote ---

The RX and TX data path are not really coupled together, its much like two separate tubes going in each direction. Not that much in the way of smarts in the Ethernet PHY, its only smart enough to detect if more than one device is talking on the network and report a collision so that the Ethernet controller can do something about that (Since you can still use old school hubs on modern Ethernet ports). I think it can also signal a error in the encoding pattern but not completely sure about that one.

An example of a bus that is limited in length by timing is the CAN bus.

To completely avoid collisions all together the CAN bus uses a special arbitration method where all devices on the bus can play a role in. Because of the way it works it requires devices to respond correctly within 1 bit period, so if the length of cable is long enough to delay the signal by more than that amount the arbitration will start failing and everyone will start trying to resend data and constantly fail again and all hell breaks loose. Because of this you will get a very sudden cutoff point in the longest possible cable as only a meter or two of extra cable might cause a previously working bus to stop working all together. Getting better quality cables won't really do much (Apart from getting higher velocity factor cable but that can only go so far before you start nearing the speed of light)

EDIT:
Actually there is one case where propagation delay can be problematic in Ethernet. Its in that case of having multiple devices on the same line where two devices transmit a packet on the line at the same time but are far enough for there packets not to collide due to propagation delay, but a device sitting somewhere in the middle will have the two packets collide there (Like two trains running towards each other from the two devices on opposite ends of the line). So the devices on the end would both think there was no collision and not re transmit the packet, but the device in the middle seen just a garbled mess of both packets slammed together.
mansaxel:

--- Quote from: paulca on November 05, 2019, 09:13:37 pm ---As an ill-informed IT person I thought the cable length for Ethernet was determined by timing, not signal strength.

--- End quote ---

That is somewhat true for the 10Base-2 (and the old 10mm yellow banana cable as well) network where you have a multidrop network made from coax and T-pieces.

For the other physical modes where you as discussed upthread have switches or run back-to-back between computers, distance is limited by attenuation. Electrical 10/100/1000Mbit Ethernet is -- by standard -- limited to 5+5+90 metres of cable (think switch -> patch cord -> patch panel -> fixed installed cable -> outlet -> patch cord -> computer) but in practice, and with fewer splices, can be run up to about 200 metres on good cable. 

Optical has no practical limit, but you usually need an amplifier after ~110km of single mode cable, providing you have high-power transceivers. (People suggesting multimode should be taken outside and reeducated, mostly because multimode is an echo chamber where signals exit the Hall Radius after 550 metres at 1000Mbit/s. This is bad for you, and today is as expensive as singlemode.)

I have gotten 1000Mbit/s to work on 125 km, without intermediate amplifiers, using a long-range transceiver. We run a couple of un-amplified ~30 km links at 100Gbit (using 100G-BASE-LR4 which is a 4-channel DWDM around 1310nm) across town, and I'm just now preparing for a job where we'll be renting 100Gbit links over 600km spans. Those are handed off to us using 100G-BASE-LR4 as well.

Society evolution through technology tip: If you want your country to flourish, make sure that dark fibre is available and cheap. It is an amazing force multiplier.
ZeroResistance:

--- Quote from: GeorgeOfTheJungle on November 05, 2019, 08:46:53 pm ---
--- Quote from: ZeroResistance on November 05, 2019, 07:05:16 pm ---I recently read regarding RS485 the following statement

The multiplication of the data rate in Mbps and length of cable in Meters should not be more than 10

this is some kind of rule of thumb for length of cable and data rate over a RS485 network.
So a data rate of 10Mbps would limit the distance to 1m.

--- End quote ---

From https://en.wikipedia.org/wiki/RS-485 :

--- Quote ---As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108
--- End quote ---

That would be 10e8/10e6 => 100m @ 10Mbps.

--- End quote ---
That's some amazing information, I probably didn't register that raised to 8, but that pretty much nails this question.
dom0:

--- Quote from: pwlps on November 05, 2019, 07:58:01 pm ---
--- Quote from: ZeroResistance on December 26, 1974, 01:54:31 pm ---...what does Ethernet do that cannot be done in RS485?

--- End quote ---

The Ethernet protocol features a collision detection mechanism.

--- End quote ---

No Ethernet link made in this millenium uses CDMA/CD, because it uses full-duplex. Gigabit Ethernet uses echo cancellation to run full duplex over the same pair (transmitting while receiving on the same channel, conceptually like a telephone).
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