Transmission lines Was: UNIBUS extension card/cable sets

Paul Koning paulkoning at
Thu Jun 25 13:05:16 CDT 2015

> On Jun 25, 2015, at 1:49 PM, tony duell <ard at> wrote:
>>>> ...
>> I still have a photo copied out of the 1980s magazine RSTS Professional, which claimed to show how to convert 
>> thick to thin Ethernet.  The simple answer is “with a coax connector adapter” since both are 50 ohm coax.  The 
> Err, yes :-). The BNC-N adapter is very useful :-). More seriously, I've seen thickwire transceivers that
> had a pair of N connectors (not a beesting tap) fitted with BNC-N adapters and ues on thinwire. Technically
> that is wrong, there is a minor difference in the transceiver spec (I forget what, but the data sheet for at 
> least one of the transceiver ICs pointed it out), but in will work.

I see a slight difference in the input current spec of the receiver part of the transceiver.  250 uA instead of 25 uA.  That makes some small amount of sense given the smaller station count.

>> article instead used a thinwire T connector, with the terminator still on it.  As Tony points out, terminators go at 
> ARGH! Ethernet is more touchhy than most as IIRC the transmitter is a current source, the receiver effectively 
> senses the voltage across the terminator. A collision is too high a voltage. So ethernet can't work with
> incorrect termination.
> That's why DEC had you put 2 terminators on a T piece on the ethernet BNC connector of a VAXstation (or
> whatever) to get it to pass diagnostics. WIth a network that short you are not going to get detectable 
> reflection problems, but if you only had one terminator, every transmission would be a collision.

Good point, collisions are not reflections, though reflections will be interpreted as collisions.

Your comment about measuring voltages reminds me of a network monitoring device DEC built, and almost turned into a product.  This was mainly for thickwire, though it would work on thinwire too.  The intent was to map stations by their physical position on the cable.  The approach was to put voltage measuring devices at both terminators, and record the observed voltage levels for packets from a given MAC address.  The voltage ratios at the two ends would tell you where the station is, provided the coax is reasonably uniform.  And the ranking of those ratios would pretty accurately show you the station order even if the cable isn’t all that uniform.

I’m not sure why this wasn’t shipped.  Perhaps it was around the time that structured wiring and star-based wiring with lots of repeaters started to come out, displacing the long bus topologies of the original Ethernet.


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