<<< EISNER::DUA0:[NOTES$LIBRARY]SITE_MANAGEMENT.NOTE;1 >>> -< Site Management >- ================================================================================ Note 55.9 NBS Time Synch "on-line" 9 of 9 EISNER::KOZAM 151 lines 24-MAR-1990 22:58 -< NBS Time Modem Information >- -------------------------------------------------------------------------------- The National Bureau of Standards has two phone numbers you can use (maybe more). They are, in Colorado, 303-494-4774, and in Washington (or thereabouts) 202-653-0351. Of the two numbers, only the Colorado one provides the information described below. The Washington number provides a subset of this information. Hope this is helpful. *********************************************************************** ATZ OK ATDT4944774 CONNECT 1200 ? = HELP National Bureau of Standards Telephone Time Service D L D MJD YR MO DA H M S ST S UT1 msADV OTM 47774 89-09-05 05:09:38 50 0 -.4 045.0 UTC(NBS) * DESCRIPTION OF THE AUTOMATED COMPUTER TELEPHONE SERVICE (ACTS) The following is transmitted (at 1200 baud) after completion of the telephone connection. ? = HELP National Bureau of Standards Telephone Time Service D L D MJD YR MO DA H M S ST S UT1 msADV OTM 47222 88-03-02 21:39:15 83 0 +.3 045.0 UTC(NBS) * 47222 88-03-02 21:39:16 83 0 +.3 045.0 UTC(NBS) * 47222 88-03-02 21:39:17 83 0 +.3 045.0 UTC(NBS) * 47222 88-03-02 21:39:18 83 0 +.3 045.0 UTC(NBS) * 47222 88-03-02 21:39:19 83 0 +.3 037.6 UTC(NBS) # 47222 88-03-02 21:39:20 83 0 +.3 037.6 UTC(NBS) # etc..etc...etc....... UTC = Universal Time Coordinated, the official world time referred to the zero meridian. _________________________________________________________________________ DST = Daylight savings time characters, valid for the continental U.S., are set as follows: 00 = We are on standard time (ST). 50 = We are on DST. 99 to 51 = Now on ST, go to DST when your local time is 2:00 am and the count is 51. The count is decremented daily at 00 (UTC). 49 to 01 = Now on DST, go to ST when your local time is 2:00 am and the count is 01. The count is decremented daily at 00 (UTC). The two DST characters provide up to 48 days advance notice of a change in time. The count remains at 00 or 50 at other times. _________________________________________________________________________ LS = Leap second flag is set to "1" to indicate that a leap second is to be added as 23:59:60 (UTC) on the last day of the current UTC month. The LS flag will be reset to "0" starting with 23:59:60 (UTC). The flag will remain on for the entire month before the second is added. Leap seconds are added as needed at the end of any month. Usually June and/or December are chosen. The leap second flag will be set to a "2" to indicate that a leap second is to be deleted at 23:59:58--00:00:00 on the last day of the current month. (This latter provision is included per international recommendation however it is not likely to be required in the near future.) __________________________________________________________________________ DUT1 = Approximate difference between earth rotation time (UT1) and UTC, in steps of 0.1 second. DUT1 = UT1 - UTC ___________________________________________________________________________ MJD = Modified Julian Date, often used to tag certain scientific data. ___________________________________________________________________________ The full time format is sent at 1200 baud, 8 bit, 1 stop, no parity. The HH:MM:SS msADV time format at 300 baud is also 8 bit, 1 stop, no parity. ___________________________________________________________________________ Maximum on line time will be 55 seconds. If all lines are busy at any time, the oldest call will be terminated if it has been on line more than 15 seconds, else, the call that first reaches 15 seconds will be terminated. ___________________________________________________________________________ Current time is valid at the "on-time" marker (OTM), either "*" or "#". The nominal on-time marker (*) will be transmitted 45 ms early to account for the 8 ms required to send 1 character at 1200 baud, plus an additional 7 ms for delay from NBS to the user, and approximately 30 ms "scrambler" delay inherent in 1200 baud modems. If the caller echoes all characters, NBS will measure the round trip delay and advance the on-time marker so that the midpoint of the stop bit arrives at the user on time. The amount of msADV will reflect the actual required advance in milliseconds and the OTM will be a "#". The NBS system requires 4 consecutive delay measurements which are consistent before switching from "*" to "#". If the user has a 1200 baud modem with the same internal delay as that used by NBS, then the "#" OTM should arrive at the user within +-2 ms of the correct time. However, NBS has studied different brands of 1200 baud modems and found internal delays from 24 ms to 40 ms and offsets of the "#" OTM of +-10 ms. For many computer users, +-10 ms accuracy should be more than adequate since many computer internal clocks can only be set with granularity of 20 to 50 ms. In any case, the repeatability of the offset for the "#" OTM should be within +-2 ms, if the dial-up path is reciprocal and the user doesn't change the brand or model of modem used. This should be true even if the dial-up path on one day is a land-line of less than 40 ms (one way) and on the next day is a satellite link of 260 to 300 ms. In the rare event that the path is one way by satellite and the other way by land line with a round trip measurement in the range of 90 to 260 ms, the OTM will remain a "*" indicating 45 ms advance. For the user who wants the best possible accuracy at the OTM, NBS offers an alternate 300 baud service with only HH:MM:SS MSADV and OTM. To use the alternate service, simply call at 300 baud. Because of the simple FSK modulation scheme used at 300 baud, all modems tested had the same delay within about 1 ms. ___________________________________________________________________________ For user comments write: NBS-ACTS Time and Frequency Division Mail Stop 524 325 Broadway Boulder, CO 80303 Software for setting (PC)DOS compatible machines is available on a 360-kbyte diskette for $35.00 from: NBS Office of Standard Reference Materials B311-Chemistry Bldg, NBS, Gaithersburg, MD, 20899, (301) 975-6776 -------------------------------------------------------------------------- -----TIME CODE GENERATOR B----- sd^s NO CARRIER *************************************************************************** I now have a short C program that automatically sets the system time NBS once the modem connections have been completed (use VAXnet or other dialer software for that). It probably isn't worth the trouble to echo the text in order to get the # on-time mark, although I do it. As an interesting aside, as was posted elsewhere, the VAX hardware clock is FAR more accurate than the system clock used to display the time of day. The command SET TIME resets the system clock from the hardware clock. My VAXStation's time was off by 15 seconds a day! I don't reboot often, so ofer the course of a month, I could be FAR off. A SET TIME a day keeps things on track.