Forums

Ethernet daisy chaining

Started by Al Williams May 28, 2007
I read this in Wikipedia near the end of the RMII docs:
http://en.wikipedia.org/wiki/RMII

"Since the RMII standard neglected to stipulate that TX_EN should only be 
sampled on alternate clock cycles, it is not symmetric with CRS_DV and two 
RMII PHY devices cannot be connected back to back to form a repeater; this 
is possible, however, with the National DP83848 which supplies the decoded 
RX_DV as a supplemental signal in RMII mode [3]."

It seems to imply you can create a simple repeater with two PHY's "back to 
back" in MII mode.  This would be useful for a new device we are working on 
that needs to daisy chain its Ethernet in and out.  I can't find much info 
on how to do this, anyone done this and if so, how?

Thanks.

-- 
Allan Williams




On Mon, 28 May 2007 21:27:36 GMT, "Al Williams"
<donotreplydirect@usenewsgroup.com> wrote:

>I read this in Wikipedia near the end of the RMII docs: >http://en.wikipedia.org/wiki/RMII > >"Since the RMII standard neglected to stipulate that TX_EN should only be >sampled on alternate clock cycles, it is not symmetric with CRS_DV and two >RMII PHY devices cannot be connected back to back to form a repeater; this >is possible, however, with the National DP83848 which supplies the decoded >RX_DV as a supplemental signal in RMII mode [3]." > >It seems to imply you can create a simple repeater with two PHY's "back to >back" in MII mode. This would be useful for a new device we are working on >that needs to daisy chain its Ethernet in and out. I can't find much info >on how to do this, anyone done this and if so, how? > >Thanks.
AFAIK it should be possible - from what I can tell that is what dumb ethernet hubs do anyway. See http://www.zen22142.zen.co.uk/Circuits/Interface/pethhub.htm for a simple 3 port ethernet hub schematic. -- Brendan Gillatt www.brendangillatt.co.uk GPG: 0x6E265E61
Thanks for the input.  Not sure that would apply to full duplex and 100 BT.

Looking at it further I think I've misread the wiki article - I think what 
they are referring to is a inline repeater used to double the segment length 
of a connection:

PC ---------------------- max length cable -------------------- [MII PHY - 
MII PHY] --------------------- max length 
cable ----------------------------- PC

This isn't what I need.  What I want is my device to provide a repeater port 
for a local PC so it doesn't need its own LAN connection:

LAN ------------------------------------ my 
device ------------------------------ PC (PC sees the LAN through my device 
port)

I think I'll need to goto an Ethernet switch like the Micrel KS8993 unless 
someone has another idea how to do it discretely.

-- 
Allan Williams



"Brendan Gillatt" <brendan@brendanREMOVETHISgillatt.co.uk> wrote in message 
news:cslm53l8kca7mcta2fa6scbm2cblp7mql4@4ax.com...
> On Mon, 28 May 2007 21:27:36 GMT, "Al Williams" > <donotreplydirect@usenewsgroup.com> wrote: > >>I read this in Wikipedia near the end of the RMII docs: >>http://en.wikipedia.org/wiki/RMII >> >>"Since the RMII standard neglected to stipulate that TX_EN should only be >>sampled on alternate clock cycles, it is not symmetric with CRS_DV and two >>RMII PHY devices cannot be connected back to back to form a repeater; this >>is possible, however, with the National DP83848 which supplies the decoded >>RX_DV as a supplemental signal in RMII mode [3]." >> >>It seems to imply you can create a simple repeater with two PHY's "back to >>back" in MII mode. This would be useful for a new device we are working >>on >>that needs to daisy chain its Ethernet in and out. I can't find much info >>on how to do this, anyone done this and if so, how? >> >>Thanks. > > AFAIK it should be possible - from what I can tell that is what dumb > ethernet hubs do anyway. > See http://www.zen22142.zen.co.uk/Circuits/Interface/pethhub.htm for a > simple 3 port ethernet hub schematic. > -- > Brendan Gillatt > www.brendangillatt.co.uk > GPG: 0x6E265E61