NMEA(4) BSD Kernel Interfaces Manual NMEA(4)
nmea -- NMEA 0183 timedelta sensor
pseudo-device nmea [count]
This line discipline interfaces NMEA devices, such as GPS receivers
attached to a serial or USB port.
The line discipline is enabled by the following sequence:
int ldisc = NMEADISC, fildes; ...
ioctl(fildes, TIOCSETD, &ldisc);
The byte stream is unaltered by the line discipline which maintains
timedelta and position sensors using the NMEA data. The sensors will
appear as nmea* in the list. The timedelta (nanoseconds difference
between the received time information and the local time), and position
(calculated latitude and longitude in degrees) can be accessed through
the sysctl(8) interface.
The nmea line discipline decodes the following NMEA 0183 sentences:
GPRMC Recommended Minimum Specific GPS/TRANSIT Data. The time and
date information and position are extracted. The warning indi-
cation is used to provide the sensor status (see below). If the
attached device sends the GPRMC message in the 13-field format,
the operation mode of the GPS device is reported in the sensor
description. The sensor timestamp is copied from the tty time-
stamp if a device with PPS is being used and tty timestamping
has been turned on. Otherwise the sensor timestamp is taken
when the initial `$' character of a message block is received
from the NMEA device.
The quality of the timedelta is reported as the sensor status:
OK The time information and position are valid. The
timedelta is safe to use for applications like ntpd(8).
WARN The attached GPS receiver has been indicating a warning
condition for at least the last ten minutes. The
timedelta should be used with care.
CRITICAL tty timestamping has been turned on but there is no PPS
signal present or the GPS receiver indicated a warning
condition for at least the last twenty minutes. Check
your hardware. Some GPS units need PPS to be manually
The status of a second sensor is used to report the status of the device
OK The clock is synchronized, e.g. a GPS receiver has a
WARN The device issued a warning condition, e.g. a GPS
receiver has no fix.
tty(4), ldattach(8), ntpd(8), sysctl(8)
The nmea interface first appeared in OpenBSD 4.0.
The nmea line discipline was written by Marc Balmer
BSD January 15, 2015 BSD