[G4] G4 MDD HD ?

[Harry Freeman]

Greetings ( + )!( + )

On Jul 18, 2005, at 12:14 PM, John Baltutis wrote:

> On 07/18/05, Harry Freeman <harry at gifutiger.com> wrote:
>> On Jul 17, 2005, at 5:58 PM, Daniel Brieck Jr. wrote:
>>
>>> No james you are exactly right the G4 MDD supports auto sensing ports
>>> I researched this before hand to say what I did originally to make
>>> things less confusing... So no gotchas there John...
>>>
>>> http://docs.info.apple.com/article.html?artnum=42717
>>
>> How do they do that?
>
> The document answers:
>
> "Many later Macintosh computers are able to use either a 
> straight-through
> Ethernet cable or a crossover Ethernet cable automatically through the 
> use
> of Auto-Medium Dependent Interface Crossover (Auto-MDIX). Other 
> Macintosh
> computers require the use of an Ethernet crossover cable because they 
> only
> work with the Medium Dependent Interface (MDI)
>
> Auto-MDIX automatically switches between MDI and MDI-X, which allows 
> either
> type of cable to be connected regardless of whether you are connecting 
> two
> computers directly, or you are connecting to a hub or switch. MDI 
> devices
> connected to Auto-MDIX devices do not require a crossover Ethernet 
> cable."
> _______________________________________________

Thanks John ( + )!( + )

That's got me started so I guess that I can find out more than I ever 
wanted to know. I've got a background in ethernet (been retired for 
about 6 years now) and what I was looking for is "How do they do that" 
What I remember is (well as best that I can remember with out diggin 
out the old books) the every device on the ethernet interface holds 
it's transmit line at a low-level until it wants to transmit, at which 
time it then raises the level on the transmit pair to the high-state. 
Then there is the guard timer (to avoid collision) during which time 
the device requesting to send waits to make sure that no other device 
on the network wants to send. If during this interval the requesting 
device receive line pair  goes high it means that another device is 
wanting to send also. At which time (since both devices are wanting to 
send (a collision)) they both access an random number generator which 
is used as a timer, and then again start the request to send scenario. 
Since both devices more than likely have not accessed the same random 
number then one or the other will be granted permission to send. That 
is of course unless another device has started the request to sent 
process, and if there is collision again then the who process is 
repeated.

Sort of a simplistic explanation, I hope that everyone understands.

So my question is "How Do They Do That?" I'll do a some research 
because now I know that it is called Auto-MDIX

I'm guessing that the device that has Auto-MDIX  senses that it's 
receive pair never goes' low or that there is an receiver on both of 
the pairs and which ever one senses the ethernet address broadcast the 
receive pair and the other the transmit pair.

It looks like HP holds the patent, I guess that I'll have to search the 
IEEE web page to see if there is a "White Paper" on how the Auto-MDIX 
is implemented.



Best Regards, /\*_*/\

Harry (*^_^*)
I know, when people see a cat's litter box, they always say, "Oh, have 
you got a cat?"  Just once I want to say, "No, it's for company!"