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I have a question, why is rs485 limited to less than 100 meters at 10mpbs ( mbaud ) and is 100Base-TX ethernet applicable on 100 meters with 100 mbps bitrate

i know that rs485 normally use a sort of NRZ encoding with 2 levels and ethernet uses a MLT-3 4B5B encoding with 3 levels

but i cannot understand why the difference between distance and baudrate of ethernet and rs485 is more than a factor 10 (ethernet has an actual baudrate of 125mbps due to 4b5b encoding)

and theoretical if 100base-tx would run at half speed would the usable distance then muliply with 2?

endgame for me would be to create an 2 wire (half-duplex) master slave (rs485-like )network with around 200 nodes , a length of 1000 meters and a speed of ~10 mbps.

or alternatively a half duplex P2P network like EtherCat but simpler like rs485 but with a usable distance of 200 meters and a speed of ~10mbps

-source of rs485 distance baudrate graph https://www.maximintegrated.com/en/images/appnotes/3884/3884Fig06.gif

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    \$\begingroup\$ Welcome to EE.SE. We don't expect every post to be perfect, but posts with correct spelling, punctuation, and grammar are easier to read. They also tend to get upvoted more frequently. Remember, you can always go back at any time and edit your answer to improve it. See Write to the best of your ability on the site's help pages. \$\endgroup\$ – Transistor Mar 21 at 20:21
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    \$\begingroup\$ Cost. Ethernet is more expensive (both financially and computationally than RS485). The mistake is thinking that it costs nothing to incorporate extra features into the hardware and software of a communication protocol. You would never make the same mistake with cars, right? We all COULD drive Ferraris to work, but we don't because Ferraris are expensive and we don't always need to burn rubber. \$\endgroup\$ – DKNguyen Mar 21 at 20:22
  • \$\begingroup\$ @Toor in my world, Ethernet is a consumer-grade technology and costs practically nothing, whereas RS485 endpoints typically are industrial grade technology and rather expensive. \$\endgroup\$ – Marcus Müller Mar 21 at 20:51
  • \$\begingroup\$ @MarcusMüller In my world, I have to build my hardware up from almost scratch so ethernet is definitely more challenging to build and support than RS-485. \$\endgroup\$ – DKNguyen Mar 21 at 20:53
  • \$\begingroup\$ @Toor fair enough! Reading OP's question, however, I duly hope they just plug together existing network hardware. \$\endgroup\$ – Marcus Müller Mar 21 at 20:55
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but i cannot understand why the difference between distance and baudrate of ethernet and rs485 is more than a factor 10 (ethernet has an actual baudrate of 125mbps due to 4b5b encoding)

Well, Ethernet (you mention rates specific to what is called Fast Ethernet, by the way, not just Ethernet) demands better cables, and the transceivers are technically way more complex.

It's like a high-speed train goes faster than a 1960's tractor, but then again, the high-speed train is a tiny bit more complex, and needs rails.

endgame for me would be to create an 2 wire (half-duplex) master slave (rs485-like )network with around 200 nodes , a length of 1000 meters and a speed of ~10 mbps.

Network. You say it yourself: Network. Use a networking standard that already solves all layer 2 (and above!) problems for you – RS485 really only defines how bits are exchanged between two ends of a cable. You need to come up with a working scheme to connect 200 nodes, and how to deal with collisions, and how to address nodes and …

Use ethernet. This is exactly what ethernet was built for: building medium-to-large sized networks. Switches are cheap, and reliable, and solve all problems of collisions. Ethernet offers tree-spanning, automatic discovery, switches do loop detection, support for it is available for every computer and many, many many microcontrollers…

100m work without any ado with standard Cat5 cabling (which is cheap as hell), and 1000m would be easy to build using cheap consumer switches as repeaters, or, professionally, with a media exchange to optical.

and theoretical if 100base-tx would run at half speed would the usable distance then muliply with 2?

No. That's not how things work: The distance is limited by drive strength; while that also defines speed, there's simply no "half-speed mode" you could use.

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