/* * refclock_nmea.c - clock driver for an NMEA GPS CLOCK * Michael Petry Jun 20, 1994 * based on refclock_heathn.c * * Updated to add support for Accord GPS Clock * Venu Gopal Dec 05, 2007 * neo.venu@gmail.com, venugopal_d@pgad.gov.in * * Updated to process 'time1' fudge factor * Venu Gopal May 05, 2008 * * Converted to common PPSAPI code, separate PPS fudge time1 * from serial timecode fudge time2. * Dave Hart July 1, 2009 * hart@ntp.org, davehart@davehart.com */ #ifdef HAVE_CONFIG_H #include #endif #if defined(REFCLOCK) && defined(CLOCK_NMEA) #include #include #include #include "ntpd.h" #include "ntp_io.h" #include "ntp_unixtime.h" #include "ntp_refclock.h" #include "ntp_stdlib.h" #ifdef HAVE_PPSAPI # include "ppsapi_timepps.h" #include "refclock_atom.h" #endif /* HAVE_PPSAPI */ #ifdef SYS_WINNT #undef write /* ports/winnt/include/config.h: #define write _write */ extern int async_write(int, const void *, unsigned int); #define write(fd, data, octets) async_write(fd, data, octets) #endif /* * This driver supports NMEA-compatible GPS receivers * * Prototype was refclock_trak.c, Thanks a lot. * * The receiver used spits out the NMEA sentences for boat navigation. * And you thought it was an information superhighway. Try a raging river * filled with rapids and whirlpools that rip away your data and warp time. * * If HAVE_PPSAPI is defined code to use the PPSAPI will be compiled in. * On startup if initialization of the PPSAPI fails, it will fall back * to the "normal" timestamps. * * The PPSAPI part of the driver understands fudge flag2 and flag3. If * flag2 is set, it will use the clear edge of the pulse. If flag3 is * set, kernel hardpps is enabled. * * GPS sentences other than RMC (the default) may be enabled by setting * the relevent bits of 'mode' in the server configuration line * server 127.127.20.x mode X * * bit 0 - enables RMC (1) * bit 1 - enables GGA (2) * bit 2 - enables GLL (4) * bit 3 - enables ZDA (8) - Standard Time & Date * bit 3 - enables ZDG (8) - Accord GPS Clock's custom sentence with GPS time * very close to standard ZDA * * Multiple sentences may be selected except when ZDG/ZDA is selected. * * bit 4/5/6 - selects the baudrate for serial port : * 0 for 4800 (default) * 1 for 9600 * 2 for 19200 * 3 for 38400 * 4 for 57600 * 5 for 115200 */ #define NMEA_MESSAGE_MASK_OLD 0x07 #define NMEA_MESSAGE_MASK_SINGLE 0x08 #define NMEA_MESSAGE_MASK (NMEA_MESSAGE_MASK_OLD | NMEA_MESSAGE_MASK_SINGLE) #define NMEA_BAUDRATE_MASK 0x70 #define NMEA_BAUDRATE_SHIFT 4 /* * Definitions */ #define DEVICE "/dev/gps%d" /* GPS serial device */ #define PPSDEV "/dev/gpspps%d" /* PPSAPI device override */ #define SPEED232 B4800 /* uart speed (4800 bps) */ #define PRECISION (-9) /* precision assumed (about 2 ms) */ #define PPS_PRECISION (-20) /* precision assumed (about 1 us) */ #define REFID "GPS\0" /* reference id */ #define DESCRIPTION "NMEA GPS Clock" /* who we are */ #define NANOSECOND 1000000000 /* one second (ns) */ #define RANGEGATE 500000 /* range gate (ns) */ #ifndef O_NOCTTY #define M_NOCTTY 0 #else #define M_NOCTTY O_NOCTTY #endif #ifndef O_NONBLOCK #define M_NONBLOCK 0 #else #define M_NONBLOCK O_NONBLOCK #endif #define PPSOPENMODE (O_RDWR | M_NOCTTY | M_NONBLOCK) /* * Unit control structure */ struct nmeaunit { #ifdef HAVE_PPSAPI struct refclock_atom atom; /* PPSAPI structure */ int ppsapi_tried; /* attempt PPSAPI once */ int ppsapi_lit; /* time_pps_create() worked */ int ppsapi_fd; /* fd used with PPSAPI */ int tcount; /* timecode sample counter */ int pcount; /* PPS sample counter */ #endif /* HAVE_PPSAPI */ l_fp tstamp; /* timestamp of last poll */ int gps_time; /* 0 UTC, 1 GPS time */ }; /* * Function prototypes */ static int nmea_start (int, struct peer *); static void nmea_shutdown (int, struct peer *); static void nmea_receive (struct recvbuf *); static void nmea_poll (int, struct peer *); #ifdef HAVE_PPSAPI static void nmea_control (int, struct refclockstat *, struct refclockstat *, struct peer *); static void nmea_timer (int, struct peer *); #define NMEA_CONTROL nmea_control #define NMEA_TIMER nmea_timer #else #define NMEA_CONTROL noentry #define NMEA_TIMER noentry #endif /* HAVE_PPSAPI */ static void gps_send (int, const char *, struct peer *); static char * field_parse (char *, int); static int nmea_checksum_ok(const char *); /* * Transfer vector */ struct refclock refclock_nmea = { nmea_start, /* start up driver */ nmea_shutdown, /* shut down driver */ nmea_poll, /* transmit poll message */ NMEA_CONTROL, /* fudge control */ noentry, /* initialize driver */ noentry, /* buginfo */ NMEA_TIMER /* called once per second */ }; /* * nmea_start - open the GPS devices and initialize data for processing */ static int nmea_start( int unit, struct peer *peer ) { register struct nmeaunit *up; struct refclockproc *pp; int fd; char device[20]; int baudrate; char *baudtext; pp = peer->procptr; /* * Open serial port. Use CLK line discipline, if available. */ snprintf(device, sizeof(device), DEVICE, unit); /* * Opening the serial port with appropriate baudrate * based on the value of bit 4/5/6 */ switch ((peer->ttl & NMEA_BAUDRATE_MASK) >> NMEA_BAUDRATE_SHIFT) { case 0: case 6: case 7: default: baudrate = SPEED232; baudtext = "4800"; break; case 1: baudrate = B9600; baudtext = "9600"; break; case 2: baudrate = B19200; baudtext = "19200"; break; case 3: baudrate = B38400; baudtext = "38400"; break; #ifdef B57600 case 4: baudrate = B57600; baudtext = "57600"; break; #endif #ifdef B115200 case 5: baudrate = B115200; baudtext = "115200"; break; #endif } fd = refclock_open(device, baudrate, LDISC_CLK); if (fd <= 0) { #ifdef HAVE_READLINK /* nmead support added by Jon Miner (cp_n18@yahoo.com) * * See http://home.hiwaay.net/~taylorc/gps/nmea-server/ * for information about nmead * * To use this, you need to create a link from /dev/gpsX to * the server:port where nmead is running. Something like this: * * ln -s server:port /dev/gps1 */ char buffer[80]; char *nmea_host, *nmea_tail; int nmea_port; int len; struct hostent *he; struct protoent *p; struct sockaddr_in so_addr; if ((len = readlink(device,buffer,sizeof(buffer))) == -1) return(0); buffer[len] = 0; if ((nmea_host = strtok(buffer,":")) == NULL) return(0); if ((nmea_tail = strtok(NULL,":")) == NULL) return(0); nmea_port = atoi(nmea_tail); if ((he = gethostbyname(nmea_host)) == NULL) return(0); if ((p = getprotobyname("ip")) == NULL) return(0); memset(&so_addr, 0, sizeof(so_addr)); so_addr.sin_family = AF_INET; so_addr.sin_port = htons(nmea_port); so_addr.sin_addr = *((struct in_addr *) he->h_addr); if ((fd = socket(PF_INET,SOCK_STREAM,p->p_proto)) == -1) return(0); if (connect(fd,(struct sockaddr *)&so_addr, sizeof(so_addr)) == -1) { close(fd); return (0); } #else pp->io.fd = -1; return (0); #endif } msyslog(LOG_NOTICE, "%s serial %s open at %s bps", refnumtoa(&peer->srcadr), device, baudtext); /* * Allocate and initialize unit structure */ up = emalloc(sizeof(*up)); memset(up, 0, sizeof(*up)); pp->io.clock_recv = nmea_receive; pp->io.srcclock = (caddr_t)peer; pp->io.datalen = 0; pp->io.fd = fd; if (!io_addclock(&pp->io)) { pp->io.fd = -1; close(fd); free(up); return (0); } pp->unitptr = (caddr_t)up; /* * Initialize miscellaneous variables */ peer->precision = PRECISION; pp->clockdesc = DESCRIPTION; memcpy(&pp->refid, REFID, 4); gps_send(fd,"$PMOTG,RMC,0000*1D\r\n", peer); return (1); } /* * nmea_shutdown - shut down a GPS clock * * NOTE this routine is called after nmea_start() returns failure, * as well as during a normal shutdown due to ntpq :config unpeer. */ static void nmea_shutdown( int unit, struct peer *peer ) { register struct nmeaunit *up; struct refclockproc *pp; UNUSED_ARG(unit); pp = peer->procptr; up = (struct nmeaunit *)pp->unitptr; if (up != NULL) { #ifdef HAVE_PPSAPI if (up->ppsapi_lit) { time_pps_destroy(up->atom.handle); if (up->ppsapi_fd != pp->io.fd) close(up->ppsapi_fd); } #endif free(up); } if (-1 != pp->io.fd) io_closeclock(&pp->io); } /* * nmea_control - configure fudge params */ #ifdef HAVE_PPSAPI static void nmea_control( int unit, struct refclockstat *in_st, struct refclockstat *out_st, struct peer *peer ) { char device[32]; register struct nmeaunit *up; struct refclockproc *pp; int pps_fd; UNUSED_ARG(in_st); UNUSED_ARG(out_st); pp = peer->procptr; up = (struct nmeaunit *)pp->unitptr; if (!(CLK_FLAG1 & pp->sloppyclockflag)) { if (!up->ppsapi_tried) return; up->ppsapi_tried = 0; if (!up->ppsapi_lit) return; peer->flags &= ~FLAG_PPS; peer->precision = PRECISION; time_pps_destroy(up->atom.handle); if (up->ppsapi_fd != pp->io.fd) close(up->ppsapi_fd); up->atom.handle = 0; up->ppsapi_lit = 0; up->ppsapi_fd = -1; return; } if (up->ppsapi_tried) return; /* * Light up the PPSAPI interface. */ up->ppsapi_tried = 1; /* * if /dev/gpspps$UNIT can be opened that will be used for * PPSAPI. Otherwise, the GPS serial device /dev/gps$UNIT * already opened is used for PPSAPI as well. */ snprintf(device, sizeof(device), PPSDEV, unit); pps_fd = open(device, PPSOPENMODE, S_IRUSR | S_IWUSR); if (-1 == pps_fd) pps_fd = pp->io.fd; if (refclock_ppsapi(pps_fd, &up->atom)) { up->ppsapi_lit = 1; up->ppsapi_fd = pps_fd; return; } NLOG(NLOG_CLOCKINFO) msyslog(LOG_WARNING, "%s flag1 1 but PPSAPI fails", refnumtoa(&peer->srcadr)); } #endif /* HAVE_PPSAPI */ /* * nmea_timer - called once per second, fetches PPS * timestamp and stuffs in median filter. */ #ifdef HAVE_PPSAPI static void nmea_timer( int unit, struct peer * peer ) { struct nmeaunit *up; struct refclockproc *pp; UNUSED_ARG(unit); pp = peer->procptr; up = (struct nmeaunit *)pp->unitptr; if (up->ppsapi_lit && refclock_pps(peer, &up->atom, pp->sloppyclockflag) > 0) { up->pcount++, peer->flags |= FLAG_PPS; peer->precision = PPS_PRECISION; } } #endif /* HAVE_PPSAPI */ /* * nmea_receive - receive data from the serial interface */ static void nmea_receive( struct recvbuf *rbufp ) { register struct nmeaunit *up; struct refclockproc *pp; struct peer *peer; int month, day; char *cp, *dp, *msg; int cmdtype; int cmdtypezdg = 0; /* Use these variables to hold data until we decide its worth keeping */ char rd_lastcode[BMAX]; l_fp rd_timestamp; int rd_lencode; /* * Initialize pointers and read the timecode and timestamp */ peer = rbufp->recv_peer; pp = peer->procptr; up = (struct nmeaunit *)pp->unitptr; rd_lencode = refclock_gtlin( rbufp, rd_lastcode, sizeof(rd_lastcode), &rd_timestamp); /* * There is a case that a gives back a "blank" line */ if (rd_lencode == 0) return; DPRINTF(1, ("nmea: gpsread %d %s\n", rd_lencode, rd_lastcode)); /* * We check the timecode format and decode its contents. The * we only care about a few of them. The most important being * the $GPRMC format * $GPRMC,hhmmss,a,fddmm.xx,n,dddmmm.xx,w,zz.z,yyy.,ddmmyy,dd,v*CC * mode (0,1,2,3) selects sentence ANY/ALL, RMC, GGA, GLL, ZDA * $GPGLL,3513.8385,S,14900.7851,E,232420.594,A*21 * $GPGGA,232420.59,3513.8385,S,14900.7851,E,1,05,3.4,00519,M,,,,*3F * $GPRMC,232418.19,A,3513.8386,S,14900.7853,E,00.0,000.0,121199,12.,E*77 * * Defining GPZDA to support Standard Time & Date * sentence. The sentence has the following format * * $--ZDA,HHMMSS.SS,DD,MM,YYYY,TH,TM,*CS * * Apart from the familiar fields, * 'TH' Time zone Hours * 'TM' Time zone Minutes * * Defining GPZDG to support Accord GPS Clock's custom NMEA * sentence. The sentence has the following format * * $GPZDG,HHMMSS.S,DD,MM,YYYY,AA.BB,V*CS * * It contains the GPS timestamp valid for next PPS pulse. * Apart from the familiar fields, * 'AA.BB' denotes the signal strength( should be < 05.00 ) * 'V' denotes the GPS sync status : * '0' indicates INVALID time, * '1' indicates accuracy of +/-20 ms * '2' indicates accuracy of +/-100 ns */ #define GPXXX 0 /* any/all */ #define GPRMC 1 #define GPGGA 2 #define GPGLL 4 #define GPZDG_ZDA 8 cp = rd_lastcode; cmdtype=0; if (cp[0] == '$') { /* Allow for GLGGA and GPGGA etc. */ msg = cp + 3; if (strncmp(msg, "RMC", 3) == 0) cmdtype = GPRMC; else if (strncmp(msg, "GGA", 3) == 0) cmdtype = GPGGA; else if (strncmp(msg, "GLL", 3) == 0) cmdtype = GPGLL; else if (strncmp(msg, "ZD", 2) == 0) { cmdtype = GPZDG_ZDA; if ('G' == msg[2]) cmdtypezdg = 1; else if ('A' != msg[2]) return; } else return; } else return; /* See if I want to process this message type */ if (peer->ttl && !(cmdtype & (peer->ttl & NMEA_MESSAGE_MASK))) return; /* * $GPZDG provides GPS time not UTC, and the two mix poorly. * Once have processed a $GPZDG, do not process any further * UTC sentences (all but $GPZDG currently). */ if (up->gps_time && !cmdtypezdg) return; /* make sure it came in clean */ if (!nmea_checksum_ok(rd_lastcode)) { refclock_report(peer, CEVNT_BADREPLY); return; } pp->lencode = (u_short) rd_lencode; memcpy(pp->a_lastcode, rd_lastcode, pp->lencode + 1); cp = pp->a_lastcode; up->tstamp = rd_timestamp; pp->lastrec = up->tstamp; DPRINTF(1, ("nmea: timecode %d %s\n", pp->lencode, pp->a_lastcode)); /* Grab field depending on clock string type */ switch (cmdtype) { case GPRMC: /* * Test for synchronization. Check for quality byte. */ dp = field_parse(cp, 2); if (dp[0] != 'A') pp->leap = LEAP_NOTINSYNC; else pp->leap = LEAP_NOWARNING; /* Now point at the time field */ dp = field_parse(cp, 1); break; case GPGGA: /* * Test for synchronization. Check for quality byte. */ dp = field_parse(cp, 6); if (dp[0] == '0') pp->leap = LEAP_NOTINSYNC; else pp->leap = LEAP_NOWARNING; /* Now point at the time field */ dp = field_parse(cp, 1); break; case GPGLL: /* * Test for synchronization. Check for quality byte. */ dp = field_parse(cp, 6); if (dp[0] != 'A') pp->leap = LEAP_NOTINSYNC; else pp->leap = LEAP_NOWARNING; /* Now point at the time field */ dp = field_parse(cp, 5); break; case GPZDG_ZDA: /* * Test for synchronization. For $GPZDG check for validity of GPS time. */ if (cmdtypezdg) { dp = field_parse(cp, 6); if (dp[0] == '0') pp->leap = LEAP_NOTINSYNC; else pp->leap = LEAP_NOWARNING; } else pp->leap = LEAP_NOWARNING; /* Now point at the time field */ dp = field_parse(cp, 1); break; default: return; } /* * Check time code format of NMEA */ if (!isdigit((int)dp[0]) || !isdigit((int)dp[1]) || !isdigit((int)dp[2]) || !isdigit((int)dp[3]) || !isdigit((int)dp[4]) || !isdigit((int)dp[5])) { DPRINTF(1, ("NMEA time code %c%c%c%c%c%c non-numeric", dp[0], dp[1], dp[2], dp[3], dp[4], dp[5])); refclock_report(peer, CEVNT_BADTIME); return; } /* * Convert time and check values. */ pp->hour = ((dp[0] - '0') * 10) + dp[1] - '0'; pp->minute = ((dp[2] - '0') * 10) + dp[3] - '0'; pp->second = ((dp[4] - '0') * 10) + dp[5] - '0'; /* * Default to 0 milliseconds, if decimal convert milliseconds in * one, two or three digits */ pp->nsec = 0; if (dp[6] == '.') { if (isdigit((int)dp[7])) { pp->nsec = (dp[7] - '0') * 100000000; if (isdigit((int)dp[8])) { pp->nsec += (dp[8] - '0') * 10000000; if (isdigit((int)dp[9])) { pp->nsec += (dp[9] - '0') * 1000000; } } } } /* * Manipulating GPS timestamp in GPZDG as the seconds field * is valid for next PPS tick. Just rolling back the second, * minute and hour fields appopriately */ if (cmdtypezdg) { if (pp->second == 0) { pp->second = 59; if (pp->minute == 0) { pp->minute = 59; if (pp->hour == 0) pp->hour = 23; } } else pp->second -= 1; } if (pp->hour > 23 || pp->minute > 59 || pp->second > 59 || pp->nsec > 1000000000) { DPRINTF(1, ("NMEA hour/min/sec/nsec range %02d:%02d:%02d.%09ld\n", pp->hour, pp->minute, pp->second, pp->nsec)); refclock_report(peer, CEVNT_BADTIME); return; } /* * Convert date and check values. */ if (GPRMC == cmdtype) { dp = field_parse(cp,9); day = dp[0] - '0'; day = (day * 10) + dp[1] - '0'; month = dp[2] - '0'; month = (month * 10) + dp[3] - '0'; pp->year = dp[4] - '0'; pp->year = (pp->year * 10) + dp[5] - '0'; } else if (GPZDG_ZDA == cmdtype) { dp = field_parse(cp, 2); day = 10 * (dp[0] - '0') + (dp[1] - '0'); dp = field_parse(cp, 3); month = 10 * (dp[0] - '0') + (dp[1] - '0'); dp = field_parse(cp, 4); pp->year = /* 1000 * (dp[0] - '0') + 100 * (dp[1] - '0') + */ 10 * (dp[2] - '0') + (dp[3] - '0'); } else { /* only time */ time_t tt = time(NULL); struct tm * t = gmtime(&tt); day = t->tm_mday; month = t->tm_mon + 1; pp->year= t->tm_year + 1900; } if (month < 1 || month > 12 || day < 1) { refclock_report(peer, CEVNT_BADDATE); return; } /* pp->year will be 2 or 4 digits if read from GPS, 4 from gmtime */ if (pp->year < 100) { if (pp->year < 9) /* year of our line of code is 2009 */ pp->year += 2100; else pp->year += 2000; } /* pp->year now 4 digits as ymd2yd requires */ day = ymd2yd(pp->year, month, day); if (-1 == day) { refclock_report(peer, CEVNT_BADDATE); return; } pp->day = day; /* * If "fudge 127.127.20.__ flag4 1" is configured in ntp.conf, * remove the location and checksum from the NMEA sentence * recorded as the last timecode and visible to remote users * with: * * ntpq -c clockvar * * Note that this also removes the location from the clockstats * log (if it is enabled). Some NTP operators monitor their * NMEA GPS using the change in location in clockstats over * time as as a proxy for the quality of GPS reception and * thereby time reported. */ if (CLK_FLAG4 & pp->sloppyclockflag) { /* * Start by pointing cp and dp at the fields with * longitude and latitude in the last timecode. */ switch (cmdtype) { case GPGLL: cp = field_parse(pp->a_lastcode, 1); dp = field_parse(cp, 2); break; case GPGGA: cp = field_parse(pp->a_lastcode, 2); dp = field_parse(cp, 2); break; case GPRMC: cp = field_parse(pp->a_lastcode, 3); dp = field_parse(cp, 2); break; case GPZDG_ZDA: default: cp = dp = NULL; } /* Blank the entire latitude & longitude. */ while (cp) { while (',' != *cp) { if ('.' != *cp) *cp = '_'; cp++; } /* Longitude at cp then latitude at dp */ if (cp < dp) cp = dp; else cp = NULL; } /* Blank the checksum, the last two characters */ if (dp) { cp = pp->a_lastcode + pp->lencode - 2; if (0 == cp[2]) cp[0] = cp[1] = '_'; } } /* * Note if we're only using GPS timescale from now on. */ if (cmdtypezdg && !up->gps_time) { up->gps_time = 1; NLOG(NLOG_CLOCKINFO) msyslog(LOG_INFO, "%s using only $GPZDG", refnumtoa(&peer->srcadr)); } /* * Process the new sample in the median filter and determine the * timecode timestamp, but only if the PPS is not in control. */ #ifdef HAVE_PPSAPI up->tcount++; if (peer->flags & FLAG_PPS) return; #endif /* HAVE_PPSAPI */ if (!refclock_process_f(pp, pp->fudgetime2)) refclock_report(peer, CEVNT_BADTIME); } /* * nmea_poll - called by the transmit procedure * * We go to great pains to avoid changing state here, since there may be * more than one eavesdropper receiving the same timecode. */ static void nmea_poll( int unit, struct peer *peer ) { register struct nmeaunit *up; struct refclockproc *pp; pp = peer->procptr; up = (struct nmeaunit *)pp->unitptr; /* * Process median filter samples. If none received, declare a * timeout and keep going. */ #ifdef HAVE_PPSAPI if (up->pcount == 0) { peer->flags &= ~FLAG_PPS; peer->precision = PRECISION; } if (up->tcount == 0) { pp->coderecv = pp->codeproc; refclock_report(peer, CEVNT_TIMEOUT); return; } up->pcount = up->tcount = 0; #else /* HAVE_PPSAPI */ if (pp->coderecv == pp->codeproc) { refclock_report(peer, CEVNT_TIMEOUT); return; } #endif /* HAVE_PPSAPI */ pp->polls++; pp->lastref = pp->lastrec; refclock_receive(peer); record_clock_stats(&peer->srcadr, pp->a_lastcode); /* * usually nmea_receive can get a timestamp every second, * but at least one Motorola unit needs prompting each * time. */ gps_send(pp->io.fd,"$PMOTG,RMC,0000*1D\r\n", peer); } /* * * gps_send(fd,cmd, peer) Sends a command to the GPS receiver. * as gps_send(fd,"rqts,u\r", peer); * * We don't currently send any data, but would like to send * RTCM SC104 messages for differential positioning. It should * also give us better time. Without a PPS output, we're * Just fooling ourselves because of the serial code paths * */ static void gps_send( int fd, const char *cmd, struct peer *peer ) { if (write(fd, cmd, strlen(cmd)) == -1) { refclock_report(peer, CEVNT_FAULT); } } static char * field_parse( char *cp, int fn ) { char *tp; int i = fn; for (tp = cp; i && *tp; tp++) if (*tp == ',') i--; return tp; } /* * nmea_checksum_ok verifies 8-bit XOR checksum is correct then returns 1 * * format is $XXXXX,1,2,3,4*ML * * 8-bit XOR of characters between $ and * noninclusive is transmitted * in last two chars M and L holding most and least significant nibbles * in hex representation such as: * * $GPGLL,5057.970,N,00146.110,E,142451,A*27 * $GPVTG,089.0,T,,,15.2,N,,*7F */ int nmea_checksum_ok( const char *sentence ) { u_char my_cs; u_long input_cs; const char *p; my_cs = 0; p = sentence; if ('$' != *p++) return 0; for ( ; *p && '*' != *p; p++) { my_cs ^= *p; } if ('*' != *p++) return 0; if (0 == p[0] || 0 == p[1] || 0 != p[2]) return 0; if (0 == hextoint(p, &input_cs)) return 0; if (my_cs != input_cs) return 0; return 1; } #else int refclock_nmea_bs; #endif /* REFCLOCK && CLOCK_NMEA */