1 /* 2 * refclock_nmea.c - clock driver for an NMEA GPS CLOCK 3 * Michael Petry Jun 20, 1994 4 * based on refclock_heathn.c 5 */ 6 #ifdef HAVE_CONFIG_H 7 #include <config.h> 8 #endif 9 10 #if defined(SYS_WINNT) 11 #undef close 12 #define close closesocket 13 #endif 14 15 #if defined(REFCLOCK) && defined(CLOCK_NMEA) 16 17 #include <stdio.h> 18 #include <ctype.h> 19 20 #include "ntpd.h" 21 #include "ntp_io.h" 22 #include "ntp_unixtime.h" 23 #include "ntp_refclock.h" 24 #include "ntp_stdlib.h" 25 26 #ifdef HAVE_PPSAPI 27 # include "ppsapi_timepps.h" 28 #endif /* HAVE_PPSAPI */ 29 30 /* 31 * This driver supports the NMEA GPS Receiver with 32 * 33 * Protype was refclock_trak.c, Thanks a lot. 34 * 35 * The receiver used spits out the NMEA sentences for boat navigation. 36 * And you thought it was an information superhighway. Try a raging river 37 * filled with rapids and whirlpools that rip away your data and warp time. 38 * 39 * If HAVE_PPSAPI is defined code to use the PPSAPI will be compiled in. 40 * On startup if initialization of the PPSAPI fails, it will fall back 41 * to the "normal" timestamps. 42 * 43 * The PPSAPI part of the driver understands fudge flag2 and flag3. If 44 * flag2 is set, it will use the clear edge of the pulse. If flag3 is 45 * set, kernel hardpps is enabled. 46 * 47 * GPS sentences other than RMC (the default) may be enabled by setting 48 * the relevent bits of 'mode' in the server configuration line 49 * server 127.127.20.x mode X 50 * 51 * bit 0 - enables RMC (1) 52 * bit 1 - enables GGA (2) 53 * bit 2 - enables GLL (4) 54 * multiple sentences may be selected 55 */ 56 57 /* 58 * Definitions 59 */ 60 #ifdef SYS_WINNT 61 # define DEVICE "COM%d:" /* COM 1 - 3 supported */ 62 #else 63 # define DEVICE "/dev/gps%d" /* name of radio device */ 64 #endif 65 #define SPEED232 B4800 /* uart speed (4800 bps) */ 66 #define PRECISION (-9) /* precision assumed (about 2 ms) */ 67 #define PPS_PRECISION (-20) /* precision assumed (about 1 us) */ 68 #define REFID "GPS\0" /* reference id */ 69 #define DESCRIPTION "NMEA GPS Clock" /* who we are */ 70 #define NANOSECOND 1000000000 /* one second (ns) */ 71 #define RANGEGATE 500000 /* range gate (ns) */ 72 73 #define LENNMEA 75 /* min timecode length */ 74 75 /* 76 * Tables to compute the ddd of year form icky dd/mm timecode. Viva la 77 * leap. 78 */ 79 static int day1tab[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; 80 static int day2tab[] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; 81 82 /* 83 * Unit control structure 84 */ 85 struct nmeaunit { 86 int pollcnt; /* poll message counter */ 87 int polled; /* Hand in a sample? */ 88 l_fp tstamp; /* timestamp of last poll */ 89 #ifdef HAVE_PPSAPI 90 struct timespec ts; /* last timestamp */ 91 pps_params_t pps_params; /* pps parameters */ 92 pps_info_t pps_info; /* last pps data */ 93 pps_handle_t handle; /* pps handlebars */ 94 #endif /* HAVE_PPSAPI */ 95 }; 96 97 /* 98 * Function prototypes 99 */ 100 static int nmea_start P((int, struct peer *)); 101 static void nmea_shutdown P((int, struct peer *)); 102 #ifdef HAVE_PPSAPI 103 static void nmea_control P((int, struct refclockstat *, struct 104 refclockstat *, struct peer *)); 105 static int nmea_ppsapi P((struct peer *, int, int)); 106 static int nmea_pps P((struct nmeaunit *, l_fp *)); 107 #endif /* HAVE_PPSAPI */ 108 static void nmea_receive P((struct recvbuf *)); 109 static void nmea_poll P((int, struct peer *)); 110 static void gps_send P((int, const char *, struct peer *)); 111 static char *field_parse P((char *, int)); 112 113 /* 114 * Transfer vector 115 */ 116 struct refclock refclock_nmea = { 117 nmea_start, /* start up driver */ 118 nmea_shutdown, /* shut down driver */ 119 nmea_poll, /* transmit poll message */ 120 #ifdef HAVE_PPSAPI 121 nmea_control, /* fudge control */ 122 #else 123 noentry, /* fudge control */ 124 #endif /* HAVE_PPSAPI */ 125 noentry, /* initialize driver */ 126 noentry, /* buginfo */ 127 NOFLAGS /* not used */ 128 }; 129 130 /* 131 * nmea_start - open the GPS devices and initialize data for processing 132 */ 133 static int 134 nmea_start( 135 int unit, 136 struct peer *peer 137 ) 138 { 139 register struct nmeaunit *up; 140 struct refclockproc *pp; 141 int fd; 142 char device[20]; 143 144 /* 145 * Open serial port. Use CLK line discipline, if available. 146 */ 147 (void)sprintf(device, DEVICE, unit); 148 149 fd = refclock_open(device, SPEED232, LDISC_CLK); 150 if (fd <= 0) { 151 #ifdef HAVE_READLINK 152 /* nmead support added by Jon Miner (cp_n18@yahoo.com) 153 * 154 * See http://home.hiwaay.net/~taylorc/gps/nmea-server/ 155 * for information about nmead 156 * 157 * To use this, you need to create a link from /dev/gpsX to 158 * the server:port where nmead is running. Something like this: 159 * 160 * ln -s server:port /dev/gps1 161 */ 162 char buffer[80]; 163 char *nmea_host; 164 int nmea_port; 165 int len; 166 struct hostent *he; 167 struct protoent *p; 168 struct sockaddr_in so_addr; 169 170 if ((len = readlink(device,buffer,sizeof(buffer))) == -1) 171 return(0); 172 buffer[len] = 0; 173 174 if ((nmea_host = strtok(buffer,":")) == NULL) 175 return(0); 176 177 nmea_port = atoi(strtok(NULL,":")); 178 179 if ((he = gethostbyname(nmea_host)) == NULL) 180 return(0); 181 if ((p = getprotobyname("ip")) == NULL) 182 return(0); 183 so_addr.sin_family = AF_INET; 184 so_addr.sin_port = htons(nmea_port); 185 so_addr.sin_addr = *((struct in_addr *) he->h_addr); 186 187 if ((fd = socket(PF_INET,SOCK_STREAM,p->p_proto)) == -1) 188 return(0); 189 if (connect(fd,(struct sockaddr *)&so_addr,SOCKLEN(&so_addr)) == -1) { 190 close(fd); 191 return (0); 192 } 193 #else 194 return (0); 195 #endif 196 } 197 198 /* 199 * Allocate and initialize unit structure 200 */ 201 up = (struct nmeaunit *)emalloc(sizeof(struct nmeaunit)); 202 if (up == NULL) { 203 (void) close(fd); 204 return (0); 205 } 206 memset((char *)up, 0, sizeof(struct nmeaunit)); 207 pp = peer->procptr; 208 pp->io.clock_recv = nmea_receive; 209 pp->io.srcclock = (caddr_t)peer; 210 pp->io.datalen = 0; 211 pp->io.fd = fd; 212 if (!io_addclock(&pp->io)) { 213 (void) close(fd); 214 free(up); 215 return (0); 216 } 217 pp->unitptr = (caddr_t)up; 218 219 /* 220 * Initialize miscellaneous variables 221 */ 222 peer->precision = PRECISION; 223 pp->clockdesc = DESCRIPTION; 224 memcpy((char *)&pp->refid, REFID, 4); 225 up->pollcnt = 2; 226 gps_send(pp->io.fd,"$PMOTG,RMC,0000*1D\r\n", peer); 227 228 #ifdef HAVE_PPSAPI 229 /* 230 * Start the PPSAPI interface if it is there. Default to use 231 * the assert edge and do not enable the kernel hardpps. 232 */ 233 if (time_pps_create(fd, &up->handle) < 0) { 234 up->handle = 0; 235 msyslog(LOG_ERR, 236 "refclock_nmea: time_pps_create failed: %m"); 237 return (1); 238 } 239 return(nmea_ppsapi(peer, 0, 0)); 240 #else 241 return (1); 242 #endif /* HAVE_PPSAPI */ 243 } 244 245 /* 246 * nmea_shutdown - shut down a GPS clock 247 */ 248 static void 249 nmea_shutdown( 250 int unit, 251 struct peer *peer 252 ) 253 { 254 register struct nmeaunit *up; 255 struct refclockproc *pp; 256 257 pp = peer->procptr; 258 up = (struct nmeaunit *)pp->unitptr; 259 #ifdef HAVE_PPSAPI 260 if (up->handle != 0) 261 time_pps_destroy(up->handle); 262 #endif /* HAVE_PPSAPI */ 263 io_closeclock(&pp->io); 264 free(up); 265 } 266 267 #ifdef HAVE_PPSAPI 268 /* 269 * nmea_control - fudge control 270 */ 271 static void 272 nmea_control( 273 int unit, /* unit (not used */ 274 struct refclockstat *in, /* input parameters (not uded) */ 275 struct refclockstat *out, /* output parameters (not used) */ 276 struct peer *peer /* peer structure pointer */ 277 ) 278 { 279 struct refclockproc *pp; 280 281 pp = peer->procptr; 282 nmea_ppsapi(peer, pp->sloppyclockflag & CLK_FLAG2, 283 pp->sloppyclockflag & CLK_FLAG3); 284 } 285 286 287 /* 288 * Initialize PPSAPI 289 */ 290 int 291 nmea_ppsapi( 292 struct peer *peer, /* peer structure pointer */ 293 int enb_clear, /* clear enable */ 294 int enb_hardpps /* hardpps enable */ 295 ) 296 { 297 struct refclockproc *pp; 298 struct nmeaunit *up; 299 int capability; 300 301 pp = peer->procptr; 302 up = (struct nmeaunit *)pp->unitptr; 303 if (time_pps_getcap(up->handle, &capability) < 0) { 304 msyslog(LOG_ERR, 305 "refclock_nmea: time_pps_getcap failed: %m"); 306 return (0); 307 } 308 memset(&up->pps_params, 0, sizeof(pps_params_t)); 309 if (enb_clear) 310 up->pps_params.mode = capability & PPS_CAPTURECLEAR; 311 else 312 up->pps_params.mode = capability & PPS_CAPTUREASSERT; 313 if (!up->pps_params.mode) { 314 msyslog(LOG_ERR, 315 "refclock_nmea: invalid capture edge %d", 316 !enb_clear); 317 return (0); 318 } 319 up->pps_params.mode |= PPS_TSFMT_TSPEC; 320 if (time_pps_setparams(up->handle, &up->pps_params) < 0) { 321 msyslog(LOG_ERR, 322 "refclock_nmea: time_pps_setparams failed: %m"); 323 return (0); 324 } 325 if (enb_hardpps) { 326 if (time_pps_kcbind(up->handle, PPS_KC_HARDPPS, 327 up->pps_params.mode & ~PPS_TSFMT_TSPEC, 328 PPS_TSFMT_TSPEC) < 0) { 329 msyslog(LOG_ERR, 330 "refclock_nmea: time_pps_kcbind failed: %m"); 331 return (0); 332 } 333 pps_enable = 1; 334 } 335 peer->precision = PPS_PRECISION; 336 337 #if DEBUG 338 if (debug) { 339 time_pps_getparams(up->handle, &up->pps_params); 340 printf( 341 "refclock_ppsapi: capability 0x%x version %d mode 0x%x kern %d\n", 342 capability, up->pps_params.api_version, 343 up->pps_params.mode, enb_hardpps); 344 } 345 #endif 346 347 return (1); 348 } 349 350 /* 351 * Get PPSAPI timestamps. 352 * 353 * Return 0 on failure and 1 on success. 354 */ 355 static int 356 nmea_pps( 357 struct nmeaunit *up, 358 l_fp *tsptr 359 ) 360 { 361 pps_info_t pps_info; 362 struct timespec timeout, ts; 363 double dtemp; 364 l_fp tstmp; 365 366 /* 367 * Convert the timespec nanoseconds field to ntp l_fp units. 368 */ 369 if (up->handle == 0) 370 return (0); 371 timeout.tv_sec = 0; 372 timeout.tv_nsec = 0; 373 memcpy(&pps_info, &up->pps_info, sizeof(pps_info_t)); 374 if (time_pps_fetch(up->handle, PPS_TSFMT_TSPEC, &up->pps_info, 375 &timeout) < 0) 376 return (0); 377 if (up->pps_params.mode & PPS_CAPTUREASSERT) { 378 if (pps_info.assert_sequence == 379 up->pps_info.assert_sequence) 380 return (0); 381 ts = up->pps_info.assert_timestamp; 382 } else if (up->pps_params.mode & PPS_CAPTURECLEAR) { 383 if (pps_info.clear_sequence == 384 up->pps_info.clear_sequence) 385 return (0); 386 ts = up->pps_info.clear_timestamp; 387 } else { 388 return (0); 389 } 390 if ((up->ts.tv_sec == ts.tv_sec) && (up->ts.tv_nsec == ts.tv_nsec)) 391 return (0); 392 up->ts = ts; 393 394 tstmp.l_ui = ts.tv_sec + JAN_1970; 395 dtemp = ts.tv_nsec * FRAC / 1e9; 396 tstmp.l_uf = (u_int32)dtemp; 397 *tsptr = tstmp; 398 return (1); 399 } 400 #endif /* HAVE_PPSAPI */ 401 402 /* 403 * nmea_receive - receive data from the serial interface 404 */ 405 static void 406 nmea_receive( 407 struct recvbuf *rbufp 408 ) 409 { 410 register struct nmeaunit *up; 411 struct refclockproc *pp; 412 struct peer *peer; 413 int month, day; 414 int i; 415 char *cp, *dp; 416 int cmdtype; 417 /* Use these variables to hold data until we decide its worth keeping */ 418 char rd_lastcode[BMAX]; 419 l_fp rd_tmp; 420 u_short rd_lencode; 421 422 /* 423 * Initialize pointers and read the timecode and timestamp 424 */ 425 peer = (struct peer *)rbufp->recv_srcclock; 426 pp = peer->procptr; 427 up = (struct nmeaunit *)pp->unitptr; 428 rd_lencode = (u_short)refclock_gtlin(rbufp, rd_lastcode, BMAX, &rd_tmp); 429 430 /* 431 * There is a case that a <CR><LF> gives back a "blank" line 432 */ 433 if (rd_lencode == 0) 434 return; 435 436 #ifdef DEBUG 437 if (debug) 438 printf("nmea: gpsread %d %s\n", rd_lencode, 439 rd_lastcode); 440 #endif 441 442 /* 443 * We check the timecode format and decode its contents. The 444 * we only care about a few of them. The most important being 445 * the $GPRMC format 446 * $GPRMC,hhmmss,a,fddmm.xx,n,dddmmm.xx,w,zz.z,yyy.,ddmmyy,dd,v*CC 447 * For Magellan (ColorTrak) GLL probably datum (order of sentences) 448 * also mode (0,1,2,3) select sentence ANY/ALL, RMC, GGA, GLL 449 * $GPGLL,3513.8385,S,14900.7851,E,232420.594,A*21 450 * $GPGGA,232420.59,3513.8385,S,14900.7851,E,1,05,3.4,00519,M,,,,*3F 451 * $GPRMB,... 452 * $GPRMC,232418.19,A,3513.8386,S,14900.7853,E,00.0,000.0,121199,12.,E*77 453 * $GPAPB,... 454 * $GPGSA,... 455 * $GPGSV,... 456 * $GPGSV,... 457 */ 458 #define GPXXX 0 459 #define GPRMC 1 460 #define GPGGA 2 461 #define GPGLL 4 462 cp = rd_lastcode; 463 cmdtype=0; 464 if(strncmp(cp,"$GPRMC",6)==0) { 465 cmdtype=GPRMC; 466 } 467 else if(strncmp(cp,"$GPGGA",6)==0) { 468 cmdtype=GPGGA; 469 } 470 else if(strncmp(cp,"$GPGLL",6)==0) { 471 cmdtype=GPGLL; 472 } 473 else if(strncmp(cp,"$GPXXX",6)==0) { 474 cmdtype=GPXXX; 475 } 476 else 477 return; 478 479 480 /* See if I want to process this message type */ 481 if ( ((peer->ttl == 0) && (cmdtype != GPRMC)) 482 || ((peer->ttl != 0) && !(cmdtype & peer->ttl)) ) 483 return; 484 485 pp->lencode = rd_lencode; 486 strcpy(pp->a_lastcode,rd_lastcode); 487 cp = pp->a_lastcode; 488 489 pp->lastrec = up->tstamp = rd_tmp; 490 up->pollcnt = 2; 491 492 #ifdef DEBUG 493 if (debug) 494 printf("nmea: timecode %d %s\n", pp->lencode, 495 pp->a_lastcode); 496 #endif 497 498 499 /* Grab field depending on clock string type */ 500 switch( cmdtype ) { 501 case GPRMC: 502 /* 503 * Test for synchronization. Check for quality byte. 504 */ 505 dp = field_parse(cp,2); 506 if( dp[0] != 'A') 507 pp->leap = LEAP_NOTINSYNC; 508 else 509 pp->leap = LEAP_NOWARNING; 510 511 /* Now point at the time field */ 512 dp = field_parse(cp,1); 513 break; 514 515 516 case GPGGA: 517 /* 518 * Test for synchronization. Check for quality byte. 519 */ 520 dp = field_parse(cp,6); 521 if( dp[0] == '0') 522 pp->leap = LEAP_NOTINSYNC; 523 else 524 pp->leap = LEAP_NOWARNING; 525 526 /* Now point at the time field */ 527 dp = field_parse(cp,1); 528 break; 529 530 531 case GPGLL: 532 /* 533 * Test for synchronization. Check for quality byte. 534 */ 535 dp = field_parse(cp,6); 536 if( dp[0] != 'A') 537 pp->leap = LEAP_NOTINSYNC; 538 else 539 pp->leap = LEAP_NOWARNING; 540 541 /* Now point at the time field */ 542 dp = field_parse(cp,5); 543 break; 544 545 546 case GPXXX: 547 return; 548 default: 549 return; 550 551 } 552 553 /* 554 * Check time code format of NMEA 555 */ 556 557 if( !isdigit((int)dp[0]) || 558 !isdigit((int)dp[1]) || 559 !isdigit((int)dp[2]) || 560 !isdigit((int)dp[3]) || 561 !isdigit((int)dp[4]) || 562 !isdigit((int)dp[5]) 563 ) { 564 refclock_report(peer, CEVNT_BADREPLY); 565 return; 566 } 567 568 569 /* 570 * Convert time and check values. 571 */ 572 pp->hour = ((dp[0] - '0') * 10) + dp[1] - '0'; 573 pp->minute = ((dp[2] - '0') * 10) + dp[3] - '0'; 574 pp->second = ((dp[4] - '0') * 10) + dp[5] - '0'; 575 /* Default to 0 milliseconds, if decimal convert milliseconds in 576 one, two or three digits 577 */ 578 pp->nsec = 0; 579 if (dp[6] == '.') { 580 if (isdigit((int)dp[7])) { 581 pp->nsec = (dp[7] - '0') * 100000000; 582 if (isdigit((int)dp[8])) { 583 pp->nsec += (dp[8] - '0') * 10000000; 584 if (isdigit((int)dp[9])) { 585 pp->nsec += (dp[9] - '0') * 1000000; 586 } 587 } 588 } 589 } 590 591 if (pp->hour > 23 || pp->minute > 59 || pp->second > 59 592 || pp->nsec > 1000000000) { 593 refclock_report(peer, CEVNT_BADTIME); 594 return; 595 } 596 597 598 /* 599 * Convert date and check values. 600 */ 601 if (cmdtype==GPRMC) { 602 dp = field_parse(cp,9); 603 day = dp[0] - '0'; 604 day = (day * 10) + dp[1] - '0'; 605 month = dp[2] - '0'; 606 month = (month * 10) + dp[3] - '0'; 607 pp->year = dp[4] - '0'; 608 pp->year = (pp->year * 10) + dp[5] - '0'; 609 } 610 else { 611 /* only time */ 612 time_t tt = time(NULL); 613 struct tm * t = gmtime(&tt); 614 day = t->tm_mday; 615 month = t->tm_mon + 1; 616 pp->year= t->tm_year; 617 } 618 619 if (month < 1 || month > 12 || day < 1) { 620 refclock_report(peer, CEVNT_BADTIME); 621 return; 622 } 623 624 /* Hmmmm this will be a nono for 2100,2200,2300 but I don't think I'll be here */ 625 /* good thing that 2000 is a leap year */ 626 /* pp->year will be 00-99 if read from GPS, 00-> (years since 1900) from tm_year */ 627 if (pp->year % 4) { 628 if (day > day1tab[month - 1]) { 629 refclock_report(peer, CEVNT_BADTIME); 630 return; 631 } 632 for (i = 0; i < month - 1; i++) 633 day += day1tab[i]; 634 } else { 635 if (day > day2tab[month - 1]) { 636 refclock_report(peer, CEVNT_BADTIME); 637 return; 638 } 639 for (i = 0; i < month - 1; i++) 640 day += day2tab[i]; 641 } 642 pp->day = day; 643 644 645 #ifdef HAVE_PPSAPI 646 /* 647 * If the PPSAPI is working, rather use its timestamps. 648 * assume that the PPS occurs on the second so blow any msec 649 */ 650 if (nmea_pps(up, &rd_tmp) == 1) { 651 pp->lastrec = up->tstamp = rd_tmp; 652 pp->nsec = 0; 653 } 654 #endif /* HAVE_PPSAPI */ 655 656 /* 657 * Process the new sample in the median filter and determine the 658 * reference clock offset and dispersion. We use lastrec as both 659 * the reference time and receive time, in order to avoid being 660 * cute, like setting the reference time later than the receive 661 * time, which may cause a paranoid protocol module to chuck out 662 * the data. 663 */ 664 665 if (!refclock_process(pp)) { 666 refclock_report(peer, CEVNT_BADTIME); 667 return; 668 } 669 670 671 672 /* 673 * Only go on if we had been polled. 674 */ 675 if (!up->polled) 676 return; 677 up->polled = 0; 678 pp->lastref = pp->lastrec; 679 refclock_receive(peer); 680 681 /* If we get here - what we got from the clock is OK, so say so */ 682 refclock_report(peer, CEVNT_NOMINAL); 683 684 record_clock_stats(&peer->srcadr, pp->a_lastcode); 685 686 } 687 688 /* 689 * nmea_poll - called by the transmit procedure 690 * 691 * We go to great pains to avoid changing state here, since there may be 692 * more than one eavesdropper receiving the same timecode. 693 */ 694 static void 695 nmea_poll( 696 int unit, 697 struct peer *peer 698 ) 699 { 700 register struct nmeaunit *up; 701 struct refclockproc *pp; 702 703 pp = peer->procptr; 704 up = (struct nmeaunit *)pp->unitptr; 705 if (up->pollcnt == 0) 706 refclock_report(peer, CEVNT_TIMEOUT); 707 else 708 up->pollcnt--; 709 pp->polls++; 710 up->polled = 1; 711 712 /* 713 * usually nmea_receive can get a timestamp every second 714 */ 715 716 gps_send(pp->io.fd,"$PMOTG,RMC,0000*1D\r\n", peer); 717 } 718 719 /* 720 * 721 * gps_send(fd,cmd, peer) Sends a command to the GPS receiver. 722 * as gps_send(fd,"rqts,u\r", peer); 723 * 724 * We don't currently send any data, but would like to send 725 * RTCM SC104 messages for differential positioning. It should 726 * also give us better time. Without a PPS output, we're 727 * Just fooling ourselves because of the serial code paths 728 * 729 */ 730 static void 731 gps_send( 732 int fd, 733 const char *cmd, 734 struct peer *peer 735 ) 736 { 737 738 if (write(fd, cmd, strlen(cmd)) == -1) { 739 refclock_report(peer, CEVNT_FAULT); 740 } 741 } 742 743 static char * 744 field_parse( 745 char *cp, 746 int fn 747 ) 748 { 749 char *tp; 750 int i = fn; 751 752 for (tp = cp; *tp != '\0'; tp++) { 753 if (*tp == ',') 754 i--; 755 if (i == 0) 756 break; 757 } 758 return (++tp); 759 } 760 #else 761 int refclock_nmea_bs; 762 #endif /* REFCLOCK */ 763