Loosing Ethernet Connectivity

[Doug Hardie]

On Jan 21, 2007, at 21:59, David Schulz wrote:

> hey, sure, of course i have checked the cat5 first, but it is  
> clearly not the cable. id say it is as   ted has written. what i  
> would like to know now is how exactly happens this "hardware  
> incompatibility"?

The interface chips use a very low level "protocol" to identify the  
rates and modes being used by the other end.  Those are dependent on  
voltage thresholds which sometimes are not as accurate as one would  
like.  Components age and tolerances change which can cause the two  
ends to get out of sync with each other.  The interface  
specifications also tend to change a bit over time.  I don't have the  
exact specs for ethernet, but the same issue arose many years ago  
with RS-232 devices.  The original specification had a threshold  
voltage of around 20 volts.  For line drivers with 25-28 volt sources  
it worked great.  But, 25 volts is somewhat difficult in many  
situations and people started fudging using 12 V sources which would  
work with many of the drivers that actually used a 10 V threshold.   
Those devices would interface with some, but not all of the older  
devices.  Ethernet has undergone a number of changes from the  
original RG-8 cabling to today.