Fred> Correct, this cable was usually on those ceilings, with taps Fred> being set up there, and then an AUI dropcable down the wall to Fred> the device, or sometimes another converter that ran BNC to Fred> small devices. That would have to be a repeater, since you cannot install side stubs on an Ethernet coax. That's why thin Ethernet had the BNC T right on the device. Some installations used wires that look like a single piece of coax but in fact is a double coax, running from the wall outlet box to the back of the computer. I once saw an article describing how to go from thick to thin coax -- it showed a T connector for that, with a terminator! Not good, terminators never go on the middle of a coax. The correct answer was simply an N to BNC barrel converter. Fred> For signal echo reasons, much like the requiement for Fred> terminators on both ends of the cable. If the devices are too Fred> close to eachother, there will be too *little* space (read: Fred> time) inbetween for them to check for any echo (collision). Fred> The transceiver will send out the first bit, and will then wait Fred> the minimal time (the time it takes to reach the first possible Fred> device on the cable) before it checks for collision. It then Fred> sends out the packet (if no coll.) and waits a full timeframe Fred> (meaning, the time maximally needed to reach a terminator, and Fred> then back) for a collision. If none, we have sent a frame. Fred> Otherwise, we have a collision. That's close but not quite. The spacing rule is to avoid having multiple taps that are integer multiples of a wavelength apart. Each tap unavoidably creates some disturbance of the cable impedance, which means electrical reflections. Those have to be kept low enough or the collision detect machinery breaks (it will mistake an excessive reflection for a collision). The maximum round trip delay on an Ethernet is 512 bit times, or 64 byte times, which is why the minimum packet size is 64 bytes. That ensures that when two stations transmit overlapping packets, both will see the collision. If shorter packets were used, it would be possible for packets to overlap at a station in the middle, yet the senders don't see it as a collision. Conversely, a "fragment" generated by a collision is always smaller than 64 bytes. There is no such thing as "send the first bit and wait to see if there is a collision". Instead, collision detection works by observing the voltage on the cable and comparing it with what you would get from a single transmitter. As the Ethernet spec puts it, "...the average DC signal on the coaxial cable implies a simultaneous transmit attempt by more than one station (a collision)..." (7.4.2.3.) paul

On Tue, Mar 09, 2004 at 05:22:10PM -0000, Antonio Carlini wrote: I have entertained thoughts of a Thickwire segment on my home network. I have a couple of vampire taps, at least one intrusive transceiver (N connectors rather than the tap), but no cable. I think I would also need to at least borrow one of the proper step drills of the day unless there's a workable kludge I could do. It seems like the perfect thing to hook my Unibus gear to. Ah... someday. -ethan -- Ethan Dicks, A-130-S Current South Pole Weather at 09-Mar-2004 18:59 Z South Pole Station PSC 468 Box 400 Temp -68.4 F (-55.8 C) Windchill -87.40 F (-66.3 C) APO AP 96598 Wind 5.6 kts Grid 078 Barometer 670.2 mb (10997. ft) Ethan.Dicks(a)amanda.spole.gov http://penguincentral.com/penguincentral.html

On Tue, 9 Mar 2004, Ethan Dicks wrote: I have the cable, the tape and whatnot, just not the time to do it. Still, it *would* be the ideal thing for the Unibus boxes I just aquired.. :) --f -- Fred N. van Kempen, DEC (Digital Equipment Corporation) Collector/Archivist Visit the VAXlab Project at http://VAXlab.pdp11.nl/ Visit the Archives at http://www.pdp11.nl/ Email: waltje(a)pdp11.nl BUSSUM, THE NETHERLANDS / Mountain View, CA, USA