Electronics > Beginners
RS485 vs Ethernet
mansaxel:
--- Quote from: dom0 on November 06, 2019, 09:06:21 pm ---
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).
--- End quote ---
But most every electrical Ethernet port retains the ability, at least at 10Mbit/s. It of course for all practical purposes is obsolete now, but it still is there. I know we ran 10/half duplex as standard on some client systems as a crude but efficient speed regulator up to about 2010. A strongly contributing factor was the complete and utter failure that was the first 10 years of 100Mbit/s Ethernet autonegotiating devices. While 100 Mbit/s frequently required manual intervention and carefully controlled configuration to work at all, 10Mbit/s half-duplex worked, and reliably so.
Gigabit Ethernet over 4 pairs copper is, in comparison, a very good protocol, and the implementations are with very few exceptions interoperable. They even work reasonably well towards 100Mbit/s devices, and in auto-negotiation at that.
ZeroResistance:
--- Quote from: Berni on November 05, 2019, 08:54:47 pm ---
For transferring a large amount of data over long distances at high speeds Ethernet is the way to do it. You don't even need to implement any of the real protocols to use it. You only need it when you want to connect to a real network with real routers and switches. You can still just connect two Ethernet PHYs together with a network cable and use it like a really fast serial cable between two MCUs using your own proprietary protocol.
--- End quote ---
How would a Ethernet PHY compare to a RS422 transceiver. So lets say we take a CAT5 cable and and use 2 pairs, one setup with a RS422 transceiver and another with only a Ethernet PHY and run them both at say 1Mbps, wouldn't these 2 setups be fundamentaly the same, I mean give similar performance?
Also how do we interface the MCU with the PHY, I see that most PHY's are MII / RMII interface, is it easy to create such an interface on a MCU, the MCU's with bulit RMII generally have MAC's embedded in them.
ZeroResistance:
If I use only a PHY and a MCU, am I still limited to send data in Ethernet frames / packet size? Or can I send arbitrary length data?
ZeroResistance:
--- Quote from: GeorgeOfTheJungle on November 08, 2019, 08:57:58 am ---You can send whatever you want, but if you do so the ethernet switch won't know what to do with your packets and very likely will just drop them I'm afraid.
--- End quote ---
Are there know techniques of connecting a PHY to MCU, most PHY's have a MII/ RMII port and MCU's don't seem to have this by default and if the do have one they have an embedded MAC as well?
borjam:
--- Quote from: paulca on November 05, 2019, 08:26:38 pm ---
--- Quote from: rrinker on November 05, 2019, 07:54:58 pm ---And Ethernet is a 1 to 1 connection
--- End quote ---
Only modern Ethernet. Unless I miss understood. Ethernet was traditionally a "bus" network with everyone broadcasting on the same wire/medium. Wifi still is. Carrier-sense multiple access with collision detection (CSMA/CD). "Connections" are not implemented at the Ethernet level, Ethernet is fire and forget, connectionless.
--- End quote ---
Indeed, you are right.
Ethernet has gone through several iterations. The first one was a bus based on coaxial cabling. Bring a bus, collision detection and avoidance was mandatory of course.
The first star cabling schemes using twisted pair were still a bus, with the hub being a multi port repeater. So it was still half duplex and it needed collision detection.
However, once switches (which are multi port bridges instead of repeaters) became popular they made full duplex Ethernet possible. And collisions do not exist on full duplex Ethernet.
Hubs were somewhat common during the first years of twisted pair 10 Mbps and 100 Mbps Ethernet until switches became more affordable. On GbE I have never seen a hub.
If you are on a twisted pair Ethernet network with switches and you have more than zero collisions on a port probably you have a negotiation mismatch (unless the Ethernet card of the affected node is really ancient), which was quite common on Fast Ethernet (100 Mbps). Probably one of the sides of the is in full duplex while the other is in half duplex. I have seen such mismatches reducing effective throughput to values as low as 64 Kbps.
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