Garmin GPS 16x - Manual (Page 29)

190-00228-08 GPS 16x Technical Specifications Rev. B
Page 25
APPENDIX C: EPHEMERIS DATA DOWNLOAD (PROGRAMMING EXAMPLE)
Synopsis
This section describes, using an example, how to download ephemeris information from a Garmin 15, 16, 17 or 18
family GPS unit with the exception of the GPS 15-W and the GPS 15-F.
Garmin Binary Format Review
To download the ephemeris data, you must first command the unit to output information in Garmin Binary Format
(Garmin mode) instead of the default NMEA output format. To put the unit in Garmin mode, connect to the unit
using a terminal program and send the following NMEA sentence: $PGRMO,,G*hh<CR><LF>
The checksum *hh is used for parity checking data and generally is not required in normal PC environments, but is
recommended for use in environments containing high electromagnetic noise. When used, the parity bytes (hh) are
the ASCII representation of the exclusive-or (XOR) sum of all the characters between the “$” and “*” characters,
non-inclusive. Sentences may be truncated by <CR><LF> after any data field and valid fields up to that point will
be acted on by the GPS sensor. See
section 4 Software Interface. The unit will stay in Garmin mode until the next
power cycle.
Now that unit is in Garmin binary format, transmitted and received packets are structured as follows:
Byte Description Name Notes
Packet Delimiter DLE 0x10
Packet ID (type) ID Packet type
Data Size SIZE Number of bytes in data portion(not
including escaped DLEs. See below)
Data bytes DATA Not to exceed 256 bytes
. . .
. . .
. . .
Checksum CHKSUM 2’s complement of the arithmetic sum of
all the bytes from the Packet ID byte to
the last DATA byte(inclusive) not
counting escaped DLEs. See below
Packet Delimiter DLE 0x10
End of Packet ETX 0x03
The DLE (0x10) is a delimiter byte used in conjunction with the ETX byte to determine beginning and ending of a
packet. However, a 0x10 could appear in the data itself; if this occurs, the byte is escaped with another DLE byte
(sometimes referred to as DLE stuffing). In other words, if a DLE occurs in the data, another DLE is transmitted
immediately after to indicate that it is a data byte and it is not being used as a delimiter. Note that the size byte of the
packet does not count the second DLE byte in an escaped DLE pair in the data field. Since a DLE that is a part of
the data will have a second DLE to escape it, a single DLE followed by an ETX byte means that the end of a packet
has been reached.
In order to interpret these packets properly, one must remove the escaped DLE bytes. This can be achieved using an
algorithm similar to the Sample C Code fragment on the previous page.