[ntp:questions] ntp server specs
David J Taylor
david-taylor at blueyonder.co.uk.invalid
Wed Jun 15 05:30:16 UTC 2011
"C BlacK" <rblak at non.net> wrote in message
news:G6KdnapTj_XCfWrQnZ2dnUVZ_oKdnZ2d at supernews.com...
> For our discussion, lets say there are many machine but the distance is
> not going to me more that 100ft.
Then some top-of-the head sums might be:
100 machines - RS-232 input resistance, guess, 4K, all 100 in parallel 40
ohms.
Voltage needed, +/- 5V, so let's guess on a 50-ohm driver source. Hence
the drive voltage needs to be a little above +/- 10V, say 12V (still
within RS-232 levels).
Strawman suggestion, based slightly on thin-wire Ethernet: find a driver
which will supply (nearly) +/-12V into an open circuit, with a 50-ohm
source impedance (i.e. it supplies +/-6V into a 50-ohm load). Drive that
signal into a piece of 50-ohm cable, with T-pieces used to tap off the PPS
signal as required. Terminate the far end of the 50-ohm cable with the
resistance required to approximately halve the open-circuit voltage of the
source, thus providing an approximately correct termination to reduce any
reflections. Take a look at the waveform to ensure that it's reasonably
"clean".
Doubtless others will come up with improvements on this scheme!
Cheers,
David
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