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Anyone out there other than me old enough to have run a 10base2 network in their home? My first computers 2 computers when I was in high school I connected together with a 10base2 with 50 ohm terminators and all that. My mom wasnt happy as I literally knocked a hole in her wall without asking her.
This was back when the internet was still fairly new so you would get on with 1200 baud modems to a BBS that would give you a piggyback onto the internet which you might be lucky to get access to for 30 - 60 minutes a day.
10base2 network card attached for prosperity.
#History #Computers #CS #Engineering #ComputerEngineering #ComputerScience #Science @Science
@freemo @Science What is the shield connected to? I vaguely remember breaking a network card's transceiver by dropping scissors in a way that shorted the shield of the BNC connector to the computer's chassis (the card that broke was, weirdly enough, the one where that happened and not the other one on the network). I expect that the shield cannot be connected to ground via low-impedance anything, because this would create a ground loop. So, is it galvanically isolated from the rest of the computer or is it connected to something?
At that time the house in question was essentially wired as TN-C, with the E-N split happening in outlets. I don't think there was a way to get different PE potentials in different places, unless one was totally disconnected (which I doubt -- I think we actually measured PE-N resistance as part of figuring out how everything is wired up). So, dunno what could have been wrong.
Maybe the adapter was weird, but if someone cheaped out, I'd expect them to do less galvanic insulation and just ground everything that's vaguely ground-shaped.
Also: how was this supposed to work when computers it connected just had different ground potentials? Coax had something like 100m max distance it was supposed to be used for 10Mbps connections over, and that can easily span buildings, which could then be on TT or be on TN but on different power substations.
I just saw an application note for 10base{2,5} transceivers and it shows transceivers with complete galvanic insulation (well, apart from a 1MOhm resistor between shield and ground): http://bitsavers.trailing-edge.com/components/national/_appNotes/AN-0442.pdf
It also states that the 1MOhm resistor is mandated by spec to discharge static electricity, so I infer that spec requires galvanic insulation.
Not sure if you misread or mistyped: it was 1 megaohm.
I really don't see how you could avoid having a ground loop through the network cable otherwise, at least for runs that span buildings.
Re safety: do you mean that one of the transceivers could be broken and connect the shield to something?
@robryka
I mistyped.
Why do you think a ground loop would occur when spanning buildings? The neutral line in a power line is grounded at many points in each building, usually this doesnt cause a problem.
Yes the safety issue largely has to do with shorts to the shield.
Why would it need to be the same PE line? If the buildings are on TN-S (which is rare), then their ground/PE is connected to grounding rods, water pipes, and nothing else. If they are on TN-C, then their ground/PE is connected to the neutral line from the grid: but they might be on different substations. In both the TN-S and TN-C with different substations case, PE of both buildings is grounded (hopefully with some reasonably low resistance to ground[1] on the order of tens of ohms), but the only way they are connected with each other is via the soil, so the resistance between PE of one and the other building is of the same order of magnitude as the resistance-to-ground of either. This is way higher than the resistance of the shield in a coax cable, so if the actual ground potential under the two buildings is different, you'll get some current through the shield.
Am I mistaken in the way I imagine this somewhere?
[1] I use a model where we have some "true" ground and grounding rods have a connection to it via some resistance. I don't know when this model breaks down.
Attached you will see a picture of a transformer which could represent a substation. Notice how all the grounds are electrically connected and they even add an additional bonding wire (this is because neutral can be disconnected via fuse and you dont want this to disconnect your bonding).
@robryk
Ideally the shield should be connected to ground as in any coax configuration. However even if it werent connected to ground it would work, so its possible you had some sort of a cheap card that was wired weird. Also possible there was a bias-t hooked up to inject power into the coax, in which case the shield should have still been counded but could possibly explain the problem you had if it was wired wrong.
You may have had a ground loop going on, but that should only happen if, again, your home or computer is wrired incorrectly.
@Science