On 2012-03-21, David Lord <sn...@lordynet.org> wrote: > unruh wrote: >>>>> The MAX232 datasheet gives maximum for output slew as 30V/us. >>>>> The MC1488 circuit assumes a capacitor at the output and has >>>>> a graph of slew rate vs capacitance. I've used values between >>>>> 100pF and 330pF. >>>> No, it gives that as the maximum over all samples. Unless your >>>> particular one was cherry picked, the typical is more like 5. 5V/us slew >>>> rate means a transition time of 2us for -5V to 5V (even 30V/us is a >>>> transition time of .3us) That then goes along a long (or short) wire, >>>> which introduces noise) into an rs232 chip on the computer motherboard. >>>> Computer motherboards are horribly noisy places. That on board chip then >>>> choses some place along that slow ramp to trigger and interrupt. That >>>> place can vary as the temperature varies and as the noise from the >>>> motherboard adds or subtracts from the ramp voltage. >>>> >>>> Furthermore, that positive feedback is also positive feedback for noise, >>>> including noise reflected back to the output of the chip by the wires >>>> connecting to the computer. >>> >>> Hi >>> >>> seems that my datasheet must be wrong then. >>> >>> The lack of noise must be due to my scope being faulty. >>> >>> I'll continue using the MAX232 feeding the pc via about >>> 60ft of cable with no problems so far. >>> >>> Last dozen summary lines >>> mean=0.000 all same >>> rms=0.004 avg of 12 between 0.003 and 0.006 >>> max= between 0.027 to 0.039 >> >> Not at all sure what those lines mean. The mean of what? What are the >> units of those (ntp usiually works either in seconds of ms). >> mean of what? rms of what? >> >>> This Sure has been online since about April last year >>> and the Garmin 18x-LVC used through 2010 gave similar >>> offsets. >> >> offsets from what? >> >> Recall that I was talking about ns resolution, not ms. > > I forgot that you know nothing about ntpd
What has ntpd got to do with it? You quoted some number of completely unkown provinence. > > My oscilloscope is supposedly 30Mhz bandwidth so can just > about display 10s of nanosecond variations. My diy scope > from around 1970 could do much better. I agree that the Sure PPS signal has about a 10ns rise time. But that is before sending it through the rs232 level converter. After that it has about a 1/2 to 1us rise time. > > Ntpd can be set to log stats including peerstats. Yes, I do it all the time. > > Ntpd distribution comes with a set of scripts that can > process the stats files and output loop_summary, > peer_summary etc. Yes, and you could write your own as well. I have written my own for example. > > The units are milliseconds so 0.001 represents 1000ns. Thank you. It is always good to give units when you quote a number. (and yes, I know how many ns there are in a ms.) > > > David > >> >> >>> >>> David >>> >>> _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
