Hello Rob,

Yes, I was able to get that time source working. It turned out to be easy. I just interfaced a 80C51 type IC to an external throw away modem which was hooked up to the telephone line. The 80C51 would dial up the telephone (ATDP "808-335-4721"), wait until carrier detect went active, wait until the below ASCII string was received three times. This was confirmed by searching for the OTM (*) character. Once that was received I would capture the ASCII string into ram, start a temporary timer, and hang up the phone. The rest of the program did some housekeeping separating the variables into their proper ram locations and resetting the real time clock (RTC). The program even calculated the error between the last time the RTC was last set and provided a correction factor which was incorporated at the end of each day.

This project provided the best accuracy of all the time methods because it incorporates the time it takes for the signal to be sent from the source to the destination.

There were two of my customer who needed that extra accuracy. One was an Oceanian going research vessel and the other was a seismic exploration company.

One day I will put the results online. Right now I am selling this as a product. It is being revised to handle cellular phones.

Regards,

Charles Bannister


JJJJJ YRMODA HH:MM:SS TT L DUT1 msADV UTC(NIST) OTM

where:

JJJJJ is the Modified Julian Date (MJD). The MJD is the last five digits of the Julian Date, which is the number of days since January 1, 4713 B.C. To get the Julian Date, add 2.4 million to the MJD.

YRMODA is the date. It shows the last two digits of the year, the month, and the current day of month.

HH:MM:SS is the time in hours, minutes, and seconds. The time is always sent as Coordinated Universal Time (UTC). An offset needs to be applied to UTC to obtain local time. For example, Mountain Time in the U. S. is 7 hours behind UTC during Standard Time, and 6 hours behind UTC during Daylight Saving Time.

TT is a two digit code (00 to 99) that indicates whether the United States is on Standard Time (ST) or Daylight Saving Time (DST). It also indicates when ST or DST is approaching. This code is set to 00 when ST is in effect, or to 50 when DST is in effect. On the day of the transition from DST to ST, the code is set to 01. On the day of the transition from ST to DST, the code is set to 51. The client software is responsible for implementing the change at 2 a.m. on the day of the transition. During the month of the transition, the code is decremented every day until the change occurs. For example, October is the month of the transition (in the United States) from DST to ST. On October 1, the number changes from 50 to the actual number of days until the time change. It will decrement by 1 every day, and reach 01 on the day of the transition. It will be set to 00 the day after the transition, and will remain there until the following April.

L is a one-digit code that indicates whether a leap second will be added or subtracted at midnight on the last day of the current month. If the code is 0, no leap second will occur this month. If the code is 1, a positive leap second will be added at the end of the month. This means that the last minute of the month will contain 61 seconds instead of 60. If the code is 2, a second will be deleted on the last day of the month. Leap seconds occur at a rate of about one per year. They are used to correct for irregularity in the earth's rotation.

DUT1 is a correction factor for converting UTC to an older form of universal time. It is always a number ranging from -0.8 to +0.8 seconds. This number is added to UTC to obtain UT1.

msADV is a five-digit code that displays the number of milliseconds that NIST advances the time code. It is originally set to 45.0 milliseconds. If you return the on-time marker (OTM) three consecutive times, it will change to reflect the actual one way line delay.

The label UTC(NIST) indicates that you are receiving Coordinated Universal Time (UTC) from the National Institute of Standards and Technology (NIST).

OTM (on-time marker) is an asterisk (*). The time values sent by the time code refer to the arrival time of the OTM. In other words, if the time code says it is 12:45:45, this means it is 12:45:45 when the OTM arrives.