[Beowulf] Jury rigged ethernet?

Ed Karns edkarns at firewirestuff.com
Mon Jun 27 11:42:15 PDT 2005

... below ...

On Monday, June 27, 2005, at 10:22  AM, Reuti wrote:

> Hi,
> point to mention is the most likely unknown wave resistance of the  
> existing wires. You have
> 150 Ohm for IBM Type 1 .... possible to use.
> 100 (sometimes 105?) Ohm for CAT3/5 twisted pair ... The best  
> candidates ... if you have enough pairs between this cabling and above  
> ...


These cables below are not recommended. Using coax will only make your  
connections unreliable unless you can make or use a breakout box with  
the proper connection converters (RG type coax conversion to RJ type  
connectors = bad news)  ...

> 50 Ohm for coax 10Base2 (RG58)
> 93 Ohm for coax of the IBM 3278 Terminal connection (RG59)
> ...
> There are adapters (called baluns) around, to convert it at both ends  
> of the wire.

DO NOT incorporate any impedance matching devices in your connections  
if possible. (Do not use these "baluns" as they will only add  
complications, increasing line losses and decrease reliability.)


> I would first try to get it working without any balun using 10BaseT  
> connections. At this distance it might work already. If not, there are  
> long distance adapters from e.g. Allied Telesyn around which will  
> convert 10BaseT to a CAT 1/2/3 connection with RJ11 connectors.
> If you must use coax because it's on the machines, you can try a balun  
> first, or convert 10Base2 to 10BaseT by using an used hub with these  
> type of connecters (or another type of converter from the above  
> mentioned company).

You will find that the impedance matching capabilities of most modern  
ethernet adapters' chip sets will overcome any irregularities in the  
actual cabling over short lengths. As a point of reference, two wire  
coat hangers in parallel have approximately 150 Ohms impedance ... so  
using the available (unshielded) twisted pairs of the cables available  
will produce the more reliable results when attached to modern ethernet  
adapters. (Using the center conductors of the coax would probably work,  
but then you would also have to deal with termination of the coax  

FYI: You may be better off disregarding the cables available and simply  
using one of the "plug in the wall" power line network adapters  
instead. This would be done by using one of the three phases (" A  
single existing cable provides 3 phase power, ...")


> Cheers - Reuti
> Ed Karns wrote:
>> Not a good idea, although it may actually work. ... not necessarily   
>> doomed to failure, but not industry standard practice.
>> Test before you commit.
>> Ed
>> FireWireStuff.com
>> On Sunday, June 26, 2005, at 12:04  PM, beowulf-request at beowulf.org   
>> wrote:
>>> Today's Topics:
>>>    1. Jury rigged ethernet? (Huntress Gary B NPRI)
>>>  .....
>>> --------------------------------------------------------------------- 
>>> -
>>> Message: 1
>>> Date: Wed, 22 Jun 2005 22:37:29 -0400
>>> From: Huntress Gary B NPRI <HuntressGB at Npt.NUWC.Navy.Mil>
>>> Subject: [Beowulf] Jury rigged ethernet?
>>> To: "'beowulf at beowulf.org'" <beowulf at beowulf.org>
>>> Message-ID:
>>> <A506359028CAD21191C10008C75D895C0C6D5248 at NPRI54EXC14.NPT.NUWC.NAVY.M 
>>> IL >
>>>     Content-Type: text/plain
>>> This is more than a little off topic but if the answer is  
>>> encouraging  it will strengthen my case to purchase a small-ish  
>>> cluster  significantly.
>>> Basically I am exploring the idea of placing a cluster in a  
>>> dedicated  "non-traditional" physical location where space is at a  
>>> dramatic  premium.  This particular space has existing cabling that  
>>> I cannot  change and it is my only electrical access.  A single  
>>> existing cable  provides 3 phase power, a few DC discrete signal  
>>> pairs, and a few  twisted shielded pairs that were used for serial  
>>> communication.   The  cable is approximately 30 feet long.  I would  
>>> like to adapt two  twisted pairs for ethernet.
>>> What are the chances that I can coax 10Mbps out of this connection   
>>> with modern (hopefully tolerant) ethernet cards at either end?  Is   
>>> this doomed to failure?
>>> Thanks,
>>> Gary H.
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