RE: [ethmac] FW: exchanging data trough the Ethernet card.

["NewsGroup" <nbase@nbase.com>]

24-Aug-00

   Hi,

        Both question depend on your card.

Most likely the ful duplex can be change by software as you most likely have
a register in the card that you write to it telling the card wether to work
in full duplex or half duplex.

now days I belive everyone make the Ethernet interface to work in both mode
just as all new 100 are actually 10/100 simple because it is almost the same
effort.

If I understand your second question you would like to send short packet
with are shorter than what the Ethernet standart allow.

in general this can sure be done just as you can shronk the preamble in the
"network" you will have the "big" question is will your card support it !.

many card will filter all packet that are short than 64 and simple reject
them and rais an error flag, and on the other direction in many cases the
card will add padding to get the minimum size.

so if I understand right what you want to do , you will need to find a card
that will forward to the cpu even fragment packets as well as "agree" to
send short packet to the line. and if you don't find such card I guess you
will have to design one.

have a nice day

   Illan



-----Original Message-----
From: owner-ethmac@opencores.org [mailto:owner-ethmac@opencores.org]On
Behalf Of Andrea Aizza
Sent: Wednesday, August 23, 2000 8:13 AM
To: Jean-François Larin
Cc: ethmac@opencores.org
Subject: Re: [ethmac] FW: exchanging data trough the Ethernet card.


Dear Jean-Francois,

thank you for your help. I have some further comments:

On 23-Aug-00 Jean-François Larin wrote:
>> Dear OPENCORE friends,
>>
>> I have never worked with Ethernet protocol but I need about 15Mbps from
an
>> FPGA
>> device and a PC. One idea I thought today to implement a `null modem`
based
>> connection from the PC to the device (just two devices - no hubs or
friends)
>> I
>> have just 2 questions:
>>
>> 1) Assuming they claim 10/100Mbps on those Eth cards, can you really
reach
>> those speeds?
>
> Ethernet works in half duplex and in full duplex.  If you are working in
half
> duplex, then only one device can speak on the wire at the same time, so
you
> get
> 10/100 Mbps.  But when 2 device start to transmit on the wire on the same
> time,
> there is a collision and both have to retry.  If there is litte traffic,
> there is
> little collisions.  When more traffic gets in you get more collision.
I've
> heard
> that you reach at peak at about 30% of utilisation (3/30 Mbps)  If you try
to
> use
> more bandwidth, you get too much collision and the actual bandwidth
decrease.
>
> But since you only use 2 devices, you can use full duplex (if you use a
> unshielded
> twistet pairs (similar to telephone jack in north america).  Full duplex
is
> not
> supporter on co-axial cable (afaik).  Then you get a real 100Mbs.
>

I understood that, is the full duplex mode something you set by software? In
fact hat i have to do is a hardware part and a software one.

> In fact, you never get 100 percent of the 100Mbs.  Every packet has a 12
> bytes of
> addresses, 2 bytes for type and 4 bytes for CRC.  The total packet lenght
> must be
> between 64 bytes or 1518 bytes.  between each packet, there is some
overhead
> equivalent to 20 bytes (12 for inter-packet gap and 8 for pre-amble).  So
if
> you use
> large packets (because you actually have lot of information to send), the
> overhead
> (12+2+4+20) is negligible compared to 1518 bytes, so you get almost 100Mbs
of
> true
> bandwidth.  If you use full duplex, you get all this bandwith 'guaranteed'
> because
> there is no collision.  that is as far as Ethernet is concerned.  You Eth
> Card must
> be of good design.  Maybe the cheap (old) cards use I/O access to transfer
> the
> packets.  Then your CPU is going to be the limitations.

no problem with the card, we can buy a new one if our is not enought good.
What
I do not understood here is: if I attach the ethernet rj45 cable to my FPGA
(just thos etwo devices) and I write some HDL code in such a way that it
sends
on that serial ethernet link a packet of lets say 1000 bytes, well formatted
(using the right system info - header, addresses..., and obviuosly the right
frequency) is it there the way to read it by writing some software on the
PC?
And on the other way, if I write some software, can I send just a packet on
the
ethernet in such a way that I can read it in my FPGA?

I think this is what I need to do!

Thank you again for your help.

Have a great week,

Andrea.

>
> Note also that Ethernet is not like a null modem. You have to send data in
> packets.
> You can't just send a single caracter on the wire.
>
> good luck
>
>
> --
>
> ************************************************************
> Jean-Francois Larin ing. stag.
> FPGA design
> Network Products Group
> Matrox Electronic Systems
> 1055 St. Regis Blvd.
> Dorval, Quebec, Canada
> H9P 2T4
> Tel:(514)685-7230 ext. 2041
> Fax:(514)822-6272
> E-Mail: JeanFrancois.Larin@matrox.com
> ************************************************************
>

----------------------------------------
Andrea AIZZA
----------------------------------------

Swiss Federal Institute of Technology
Department of Electrical Engineering
Integrated Systems Center (LSI)

EPFL-DE-LSI Office: ELB-137
Ecublens, CH-1015 Lausanne (CH)
tel. +41 21 693 69 73
fax. +41 21 693 46 63
email  Andrea.Aizza@epfl.ch
http://lsiwww.epfl.ch/

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