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RS485 vs Ethernet
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ZeroResistance:
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.

Which got me thinking that even Ethernet uses differential signalling, and it uses 2 differential pairs to send and receive respectively, here I'm talking about 10 and 100Mbps speeds and not about gigabit ethernet which uses all 4 pairs?

Why then does this rule not apply to Ethernet, I mean I see that ethernet lan networks which work upto 100Mbps could use pretty long cables some times in 10's of meters, and if I remember correctly Ethernet uses a clock speed of 125Mhz for 10/100Mbps what does Ethernet do that cannot be done in RS485?

TIA
rrinker:
 Impedance for one. And Ethernet is a 1 to 1 connection, not multidrop like RS485. Even lower speed Ethernet requires twisted pairs to maintain impedance characteristics, while RS485 at fairly high speeds, at least for a serial bus, works fine with all sorts of wire. Really, the most they have in common is that they use differential signaling.

pwlps:

--- 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.
ZeroResistance:

--- Quote from: rrinker on November 05, 2019, 07:54:58 pm --- Impedance for one. And Ethernet is a 1 to 1 connection, not multidrop like RS485. Even lower speed Ethernet requires twisted pairs to maintain impedance characteristics, while RS485 at fairly high speeds, at least for a serial bus, works fine with all sorts of wire. Really, the most they have in common is that they use differential signaling.



--- End quote ---

So how does impedance change the game here?


--- 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 ---

Ok, So that is a software specific feature, correct? I was mainly interested in how does ethernet circumvent the cable length issue while still moving data around at high rates.
pwlps:

--- Quote from: ZeroResistance on November 05, 2019, 08:00:55 pm ---So how does impedance change the game here?

--- End quote ---

Precise impedance matching ensures that there is no significant reflected signal, you can send a data packet without worrying it will get corrupted by its echos.  Without impedance matching you have to wait after every bit until the echoes fade out (the echoes manifest as oscillations following each pulse edge).

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