Technical reference manual for NIST automated computer time service (ACTS)
Judah Levine, Michael A. Lombardi, L.M. Nelson, Victor S. Zhang · 2001
Error Handling and Log Files Handling Leap Seconds The ACTSM REF CLK Uploading Files to an FTP Server Appendix A. Low-Speed ACTS System Appendix B. Summary of Server Commands Appendix C. Summary of Monitor Commands Appendix D. Description of Monitor Operation Status /V a technical reference for the server and monitor.V Chapter 1 -Overview of ACTS Chapter 1 Overview of ACTSThe poor performance of computer clocks is well documented, and causes problems in many areas of business, government, and industry.Many computer applications require that time be kept to the nearest second or better, and existing computer hardware is simply not up to the task.For example, the computers in a financial institution must keep very accurate records of when transactions were completed, for legal or other reasons.Computer systems that make physical measurements and acquire scientific data need to know precisely when the measurements were made.Software used on a manufacturing floor may need to turn a piece of equipment on or off at a specified time.Also, any system involved with synchronous communications must keep accurate time.For example, radio and TV stations may need computers that can switch feeds or link up with remotes at the right time.In response to these problems, NIST began the Automated Computer Time Service (ACTS) in 1988.This service allows any user with the appropriate client software and modem to synchronize their computer clock using an ordinary telephone line.This manual documents the dial-up time server and monitoring system used by ACTS.This chapter provides an overview of what ACTS is, and how ACTS works Let's begin our discussion by describing how a typical computer keeps time.How a Personal Computer Keeps Time Since the introduction of the IBM-AT personal computer in 1984, all PC-compatible computers have kept time the same way.Each PC built using the AT architecture contains two clocks, regardless of whether it uses the 286, 386, 486, or Pentium microprocessor (or a derivative) These clocks go by several different names, but for simplicity, we'll call them the software and hardware clocks.The software clock keeps time while the computer is turned on, but stops when the computer isturnedoff.The hardware clock has its own power supply (battery) and runs even while the computer is turned off.The software clock is generated by an Intel 8254 timer-counter (or a functionally equivalent device).This timer-counter generates an interrupt every 54.936 ms, or about 18.2 times per second.The computer's BIOS (Basic Input Output System) contains a software routine that counts the interrupt requests and generates a time-of-day clock that can be read or set byUsing ACTS Using ACTS requires only a computer, a modem, and some simple software.When a computer connects to ACTS by telephone, it receives an ASCII time code.The information in this time code is then used to set the computer clock to the correct time.ACTS is usable at modem speeds up to 9600 baud with 8 data bits, 1 stop bit, and no parity.To receive the full time code, you must connect at a speed of at least 1200 baud.The full time code is transmitted every second and contains more information than the 300 baud time code, which is transmitted once every 2 s.The table describes the full ACTS time code: I JJJJJ YR-MO-DA HH:MM:SS TT L UT1 msADV UTC(NIST) | I c5 I JJJJJ is the Modified Julian Date (MJD), The MJD is the last five digits of the Julian Date, which is simply a | !count of the number of days since January 1, 4713 B.C To get the Julian Date, add 2 4 million to the MJD I : YR-MO-DA 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