Thicknet/10base5 Test Segment: The Cable is In!

[Paul Koning]

> On Jun 26, 2018, at 3:07 PM, systems_glitch via cctalk <cctalk at classiccmp.org> wrote:
> 
> I've confirmed that I now have N connector intrusive taps! These have a N
> female connector on each end, like the leftmost transceiver in this picture:
> 
> https://oelzant.priv.at/~aoe/images/galleries/hardware/802_3_transceivers/DSC_0927_med.jpg
> 
> I seem to recall the CCNA instructor telling us that you weren't really
> supposed to screw a 50 ohm terminator onto an intrusive tap; I don't know
> if there's good reason for it or if it was just a general practice. 

I'm not sure if I understood that correctly.  It sounds flat out wrong, and it presumably would not be general practice because an Ethernet segment with a missing terminator either doesn't work at all, or works very poorly.

An Ethernet segment must have a 50 ohm terminator at each end (and nowhere else).  Always. The spec is crystal clear about this, and so is elementary transmission line physics.

If you have an intrusive (connectorized) tap, installed at the very end of the cable, the other side of the tap must have a terminator since that's the end of the segment.

I once saw an article in a magazine ("DEC Professional" or "RSTS Professional", a US based rag for PDP11 users) that talked about thick and thin Ethernet.  It mentioned that you can connect the two -- which is correct.  But it showed the wrong way -- with a T connector and a terminator attached to one of the legs of the T.  In other words, a "terminator" in the MIDDLE of the segment.  The correct way, of course, is a BNC to N adapter, and you have to use the thin Ethernet length and station limits.

So it sounds like your instructor was just as ignorant as the writers of that magazine.

	paul