Date: Thursday, 16 June 1988  16:55-MDT
From: attcan!utzoo!dciem!nrcaer!cognos!bruce at uunet.uu.net (Dwayne Bruce)
Re:   N.B.S. Time Service

Well kids, after reading about the NBS telephone time service, and
having read about an expensive commerical system from Precision
Standard Time, Inc., I just had to tell you about the system I cooked
up.

We've been keeping our VAXen in sync with a simple system that uses
the AFSK time code produced by CHU.  CHU is Canada's provider of
brodcast time, and it has a signal that can be heard regularly all
over the western hemisphere.  During seconds 31..39 of each minute,
they transmit a time code in 103-type AFSK.  I built a modem, plugged
it into an HF receiver, and wrote a piece of code to handle the
output. The code is enclosed.

If you want to know more about the hardware, consult the article I
wrote in the April, 1988 edition of "The Canadian Amateur" (published
by the Canadian Amateur Radio Federation).

Anyway, here's the code.  It runs under UNIX and VMS.

-------------------------
/* CHU      Marcus Leech  VE3MDL   Aug 1987 */
/* This program reads and understands the timecode format
*  produced by the CHU broadcast signal.  This signal provides
*  an atomic time standard to most places in North and South
*  America on 3.330 7.335 and 14.670 MHZ, mode A3. During
*  seconds 31 thru 39 the time is broadcast using Bell 103 AFSK.
*  The time code looks like this:
*  6.d|d.d|h.h|m.m|s.s|6.d|d.d|h.h|m.m|s.s
*  The time code is repeated twice for error detection. Each . represents
*    one nibble boundary.  The nibbles get transmitted reversed, so you
*    have to flip them to make sense of the time code.  Each nibble is a BCD
*    digit.
* It takes one argument, the input device, and some optional flags:
*  chu [-adst] <device>
*  -
*  a adjust  attempt to adjust the system clock to CHU time
*  d daemon  run continuously as a daemon
*  s show    show semi-raw timecode
*  t tune    show chars that wouldn't get considered for time code
*
* When used as a system-time-adjuster, you need to set the system time
*  to within 2mins of the correct time. The program will drag the
*  clock into exact synchronism.
*/
#include <stdio.h>
#ifndef VMS
#include <sys/types.h>
#include <sgtty.h>
#include <time.h>
#else VMS
#include <types.h>
#include <time.h>
#endif VMS
/* Macros to fetch individual nibbles. */
#define hinib(x) (((x) >> 4) & 0x0F)
#define lonib(x) ((x) & 0x0F)
/* Macro to restore correct ordering within a byte. */
#define byte(h,l) (((l) << 4)|(h))
#define ADJINT 27
#define TWEAK 50 + 12     /* Fudge factor in centiseconds. */
                         /* CHU code is skewed by 0.5 seconds. */
#define iabs(x) ((x < 0) ? -x : x)
char sample1[5];  /* The first time code. */
char sample2[5];  /* The second (error checking) time code. */
main (argc, argv)
int argc;
char **argv;
{
   char c, *p;
#ifndef VMS
   struct sgttyb sgb;            /* For fiddling with tty line params. */
#endif VMS
   int line;                     /* Fd for the tty line.*/
   int i;
   int adjcnt;                     /* Number of samples before we adjust.*/
   struct tm *localtime(), *ltp, *gmtime();  /* To break out the time. */
   time_t now, time();
#ifdef VMS
   long t0[2], t1[2], t2[2];
#endif VMS
   int amm, ass, mmss, diff;
   char *devstr;
   int adjust;
   int show;
   int tune;
   int daemon;

   setbuf (stdout, NULL);
   adjust = show = tune = 0;
   devstr = "/dev/null";
   for (i = 1; i < argc; i++)
   {
       if (argv[i][0] == '-')
       {
           p = &argv[i][1];
           while (*p)
           {
               switch (*p)
               {
               case 'a':
                   adjust++;
                   break;
               case 't':
                   tune++;
                   break;
               case 's':
                   show++;
                   break;
               case 'd':
                   daemon++;
                   break;
               default:
                   fprintf (stdout, "unknown flag '%c'\n", *p);
                   exit (1);
               }
               *p++;
           }
       }
       else
       {
           strcpy (devstr, argv[i]);
       }
   }
   line = open (devstr, 0);
#ifndef VMS
   /* Set up 8-bit datapath, at 30CPS. */
   gtty (line, &sgb);
   sgb.sg_ispeed = sgb.sg_ospeed = B300;
   sgb.sg_flags |= RAW;
   stty (line, &sgb);
#endif VMS
   adjcnt = 0;
   if (!daemon)
   {
       adjcnt = 19;
   }
   /* Read forever, waiting for the synchronizing BCD 6 digit to appear. */
   for (;;)
   {
       read (line, &c, 1);
       /* We have a syncronizing digit. Grab two samples and compare. */
       if (lonib(c) == 6)
       {
           /* Get first sample. */
           sample1[0] = byte(hinib(c),lonib(c));
           for (i = 1; i < 5; i++)
           {
               read (line, &c, 1);
               sample1[i] = byte(hinib(c),lonib(c));
           }
           /* Get second sample. */
           for (i = 0; i < 5; i++)
           {
               read (line, &c, 1);
               sample2[i] = byte(hinib(c),lonib(c));
           }
           /* If samples match, we have a valid time code. */
           if (compare (sample1, sample2, 5) == 0)
           {
               /* Show the code (if -s).  The high-order nibble in the
                *  first byte is the synch digit, so it gets masked out
                *  for printing.
                */
               if (show)
               {
                   fprintf (stdout, "TC: ");
                   for (i = 0; i < 5; i++)
                   {
                       fprintf (stdout, "%02x", sample1[i]);
                   }
                   fprintf (stdout, "\n");
               }
               if (adjcnt++ >= ADJINT)
               {
                   adjcnt = 0;
                   /* Fetch UTC (GMT). */
                   time (&now);
                   ltp = gmtime (&now);
                   /* Convert time code minutes and seconds into
                    *   binary.
                    */
                   amm = (hinib(sample1[3]) * 10) + lonib(sample1[3]);
                   ass = (hinib(sample1[4]) * 10) + lonib(sample1[4]);
                   /* Convert minutes and seconds portion of system time. */
                   mmss = (ltp->tm_min * 60) + ltp->tm_sec;
                   /* Compute the difference. */
                   diff = ((amm * 60) + ass) - mmss;
                   /* Adjust the system time. */
                   now += (long)diff;
                   if (iabs(diff) > 120)
                   {
                       fprintf (stdout, "%02d-%02d-%02d %02d:%02d:%02d ",
                           ltp->tm_year, ltp->tm_mon + 1, ltp->tm_mday,
                           ltp->tm_hour, ltp->tm_min, ltp->tm_sec);
                       fprintf (stdout, "TERROR %d\n", diff);
                       if (!daemon)
                       {
                           break;
                       }
                       continue;
                   }
                   /* Only do it if there IS a (reasonable) difference. */
                   if ((diff != 0) && (adjust))
                   {
                       ltp = localtime (&now);
                       fprintf (stdout, "%02d-%02d-%02d %02d:%02d:%02d ",
                           ltp->tm_year, ltp->tm_mon + 1, ltp->tm_mday,
                           ltp->tm_hour, ltp->tm_min, ltp->tm_sec);
                       fprintf (stdout, "ADJUST %d\n", diff);
                       /* What we'd REALLY like here is a system call of
                        * the form:   stime (*time, ticks)
                        * that would allow you to set the system tick
                        * counter to <ticks> in centiseconds.  Too bad.
                        *-->stime (&now, TWEAK);<--
                        */
#ifndef VMS
#ifndef EUNICE
                       stime (&now);
#endif EUNICE
#else VMS
                       t1[0] = 100000 * ((diff * 100) + TWEAK);
                       t1[1] = 0;
                       if (diff < 0)
                       {
                           t1[1] = -1;
                       }
                       sys$gettim (t0);
                       lib$addx (t0, t1, t2);
                       sys$setime (t2);
#endif VMS
                   }
                   ltp = localtime (&now);
                   fprintf (stdout, "%02d-%02d-%02d %02d:%02d:%02d TVALID\n",
                       ltp->tm_year, ltp->tm_mon + 1, ltp->tm_mday,
                       ltp->tm_hour, ltp->tm_min, ltp->tm_sec);
                   if (!daemon)
                   {
                       break;
                   }
               }
           }
       }
       else if (tune)
       {
               fprintf (stdout, "FT: %c (%02x)\n", c, (unsigned)c);
       }
   }
}
/* Compare two byte-arrays (s1, s2) of length cnt. */
compare (s1, s2, cnt)
char *s1;
char *s2;
int cnt;
{
   int i;
   for (i = 0; i < cnt; i++)
   {
       if (*s1++ != *s2++)
       {
           return (1);
       }
   }
   return (0);
}
#ifdef VMS
struct tm *
gmtime (tod)
time_t *tod;
{
   int hh;
   int mm;
   char s;
   int secdiff, sgn;
   time_t utc;
   char *p, *getenv ();
   p = getenv ("UTCDISP");
   sscanf (p, "%c %02d:%02d", &s, &hh, &mm);
   utc = *tod;
   secdiff = (hh * 3600) + (mm * 60);
   sgn = (s == '+') ? 1 : -1;
   secdiff *= sgn;
   utc += secdiff;
   return (localtime (&utc));
}
#endif VMS

You are viewing proxied material from gopher.quux.org. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.