Do I need a Hub for my application?

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I would like to develop an embedded device that has an Ethernet
interface connected to a hone or corporate LAN. One potential usage is
to bundle several device close together and work as a bunch of
"sensors", and pass data to the server at a remote office. For exampe,
3 of them can be connected together one by one with almost no distance
in between, then connected to LAN ith just one cable.

Traditionally to connect 3 PCs at one location to LAN, we will need a
hub, or a switch. However, my devices will work close together, and
Ethernet by its definition support Bus Topology, meaning that all
devices or hosts on the network use the same shared communication
line. So can I physically use one cable to connect 3 devices to LAN
directly without a hub? It will look like a stick, one end is
connected to a ethernet jack on the wall (to LAN),  the other end is
split into 3 tails, connecting to each device. Will it work?


Re: Do I need a Hub for my application?
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Of the commonly implemented types of Ethernet, only 10-base-5 and 10-
base-2 can be arranged in a physical bus.  10-base-T is simply not
electrically compatible with that sort of thing (specifically the TX
lines from one side connect to the RX lines on the other, and the
sender doesn't disconnect, unlike the coaxial PHYs where the sender is
only really driving the (single) bus when it's transmitting a packet).

Re: Do I need a Hub for my application?
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The twisted-pair Ethernet does not work that way. You'll need
hubs or switches.

The original coaxial-cable Ethernet worked that way, if the
branching and tapping rules were obeyed.

--

Tauno Voipio
tauno voipio (at) iki fi

Re: Do I need a Hub for my application?
Well, my application needs to support both coaxial-cable and twisted-
pair, 10-base-2 and 10-base-5 and 10-base-T. I am thinking about
integrating a switch into my device, say the broadcom switch for
example, then connect the devices by "daisy-chain". Will it work? Do I
need to modify the firmware or everthing will be taken care by the
switch automatically?

Re: Do I need a Hub for my application?
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WTF? Are you working on a method of sending this device through a time
machine into 1992? If you are asking this type of question, I highly
doubt you are building a device to retrofit into legacy military/
banking operations, and I can think of no other first-world entity
likely to have such ancient technology in active service.

Re: Do I need a Hub for my application?
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Right. The moon is too high to reach. Well, I am willing to drop off
the support for coaxial-cable, let's leave just the twisted-pair here.



Re: Do I need a Hub for my application?
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Ignoring physical connections for the moments.  Your devices still
need to have distinct IPs or the very least distinct tcp ports on the
same IP.

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Probably not,  without more work.

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Most likely.


Probably not, without DHCP, IP NAT and companies.


Re: Do I need a Hub for my application?
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Not any more it doesn't.  Ethernet is logically a shared medium, but
hasn't physically been a bus since effectively forever.

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No.


No.  For that to work, that "stick" must be, for all intents and
purposes, a hub.  Or something even more unlike a simple cable than that.

Re: Do I need a Hub for my application?
On Thu, 4 Nov 2010 22:48:14 -0700 (PDT), Like2Learn

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Why not use RS-485 sensors and connect the sensors to a RS-485 serial
cable and that to some ethernet/RS-485 converter. Of course, there can
be only one master (at a time) on the LAN side to poll the sensors one
at the time.

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Just install an extra double port ethernet interface cards on each PC
and use point to point ethernet cables between each machine, with
plenty of throughput :-).


Re: Do I need a Hub for my application?
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That depends on how smart the RS485-Ethernet converter is.  You can
get converters that understand Modbus and will handle multiple masters
on the Ethernet side.

--
Grant

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