OBSERVATIONS OF THE GARMIN-GARMIN PROTOCOL
last update: July 4, 1996
author: william.soley@sun.com, werme@zk3.dec.com
URL: http://playground.sun.com/pub/soley/garmin.txt
(this work has nothing to do with Sun Microsystems)
(nor Digital Equipment Corp.)
This is a description of the GARMIN-GARMIN protocol as spoken by the
Garmin GPS-75 GPS receiver and the Garmin PCX5 MS-DOS software. The
information here has been determined by observing the communication
between the two units while sending "chosen plaintext". This spec is
at best, incomplete, and at worst, incorrect. Lots of assumptions
were made based on the observed behavior of the protocol. It is also
unknown how much of this protocol is common to other Garmin products.
But, unless Garmin decides to be cooperative, this is about the best
we can do. Use it at your own risk!
All references to GPS-75 behavior came from William Soley. A major
update to this was made in July, 1996 by Eric Werme, studying the
behavior of his GPS-45. There is much in common between the protocols
used by the two models, but there are differences. Much of Werme's study
focused on protocol commands not used by PCX5, all of which are subject
to change, even between revisions of firmware within a model! If you
must rely on some part of the protocol, it should be part that is used
by Garmin's PCX5 software.
Layer 1
-------
Data is standard Async.
9600 bits per second.
8 data bits.
no parity.
1 stop bit.
Layer 2
-------
The data stream consists of frames each having the following format:
DataLinkEscape (0x10)
RecordType (one byte) $$
Length (one byte) ** $$
DataField ("length" bytes) ** $$
CheckSum (one byte) **
DataLinkEscape (0x10)
EndTransmission (0x03)
** - fields indicated by "**" are subject to escape. Any occurrence of
a DataLinkEscape character (0x10) in these fields is preceded by
another DataLinkEscape (0x10) resulting in a pair. Escape bytes added
in this way are not counted in the record length (i.e. bytes are counted
before the escapes are added).
$$ - the bytes comprising fields indicated by "$$" are summed and the
low-order 8 bits are then negated (2's complement) to form the
CheckSum. Any added DataLinkEscapes are not included in the sum.
Software processing this protocol layer on input can discard the initial
DataLinkEscape, the checksum and later bytes, and compress the double
DataLinkEscapes into one. The remaining bytes constitute Layer 3 protocol
frames.
Layer 3
-------
When a non-ACK/NAK or asynchronous frame is received an ACK or NAK is sent
depending on the checksum validity of the received frame. ACK/NAK frames are
the same format as a regular frame:
RecordType = 0x06 (ACK) or 0x15 (NAK)
Length = 2
DataField = RecordType of record being acknowledged, 0x00
When a non-ACK/NAK frame is sent, the sender waits for an ACK/NAK. If
a NAK is received, or if no ACK is received after about 1 second, the
sender retransmits the frame.
Power Up
--------
When the GPS-75 is powered on, it transmits an 0x5A character.
When the GPS-45 is powered on, it transmits 0xFE and 0X5A characters.
Holding the button while pressing to turn the unit on
will erase the unit's memory and result in the messages:
Stored Data Lost
Searching the Sky
Holding the button while pressing to turn the unit on will put the
unit in TEST mode. The GPS-75 a will display a series of concentric
rectangles will be displayed and animated to give the illusion of passing
through a tunnel. Pressing will toggle between the tunnel display and a
display showing:
Testing ...
Sgnl Amplitude 2168
TCX0 Drift -19.978
While in TEST mode, a continuous stream of RecordType=0x00, 0x01,
and 0x0d messages will be sent to the serial port.
Also, while in TEST mode the "" may be used to adjust
display contrast, but the adjustment does not appear to be saved.
The GPS-45 behaves somewhat differently. The status screen is the first to
be displayed and includes a keypad test test feature. In addition to the
box display is an all black display. The GPS will send a stream of
RecordTypes 0x00, 0x27, 0x28.
Data Presentation
-----------------
Numeric data within the Garmin protocol is in binary, and several formats
are used. Those are described here. Skimming this section now will make it
easier to understand the protocol's commands.
short integer - length = 2 bytes, in order of increasing significance.
Negative numbers are expressed as 2's complement.
Examples:
00 00 = 0x0 = 0
01 00 = 0x1 = 1
10 00 = 0x10 = 16
00 01 = 0x100 = 256
00 10 = 0x1000 = 4096
ff ff = 0xffff = -1
f0 ff = 0xfff0 = -16
long integer - length = 4 bytes, in order of increasing significance.
Negative numbers are expressed as 2's complement.
Examples:
00 00 00 00 = 0x0 = 0
01 00 00 00 = 0x1 = 1
00 01 00 00 = 0x100 = 256
00 00 01 00 = 0x10000 = 65536
00 00 00 01 = 0x1000000 = 16777216
ff ff ff ff = 0xffffffff = -1
f0 ff ff ff = 0xfffffff0 = -16
float - length = 4 bytes, IEEE single precision floating point.
Least significant byte first. This format consists of:
1 bit - sign
8 bits - exponent, excess 127
23 bits - mantissa, implied high-order 1
double - length = 8 bytes, IEEE double precision floating point.
Least significant byte first. This format consists of:
1 bit - sign
11 bits - exponent, excess 1023
52 bits - mantissa, implied high-order 1
ASCII string - series of ASCII bytes, usually padded with blanks.
Example:
47 50 53 20 37 35 20 20 32 2e 32 31 20 = "GPS 75 2.21"
date/time - length = 4 bytes, long unsigned integer encoded as
86400 + number of seconds since midnight, Jan 1 1990.
Note: unix_time = garmin_time + 631065600
Examples:
00 00 00 00 = 0x0 = undefined
80 51 01 00 = 0x15180 = Jan 1 1990 00:00:00
7d f0 ef 2d = 0x2deff07d = Jun 4 1994 03:09:49
long lat/lon - length = 4 bytes, long signed integer encoded as
11930464.7111111111 * lat/lon in degrees, or
683565275.576431632 * lat/lon in radians.
The former factor as a ratio is 2**30/90. As a C
expression, it is ((1L
of packets" message.
----------------------------------------------------------------
RecordType = 0x0d Event
Length = 4
DataField =
2 bytes - EventType
05 00 Satellite acquired
07 00 Navigation quality
0c 00 Keyboard event (TEST mode only)
10 00 Screen message displayed
14 00 Power on sequence message
2 bytes - SubType
This is a function of the EventType. Each known EventType
is described below with a list of the SubTypes.
4 bytes - GPS clock time
This is a time that is initialized to 0 at startup and
incremented 255750 per second. This is 1/4 of the
1.023 Mhz bit rate of the CA code transmitted by GPS
satellites. (Rumors that GPS satellites are powered by
Apple ][ computers are totally untrue!) Some events may
use the low one or two bytes for additional data.
Description:
Sent asynchronously while in TEST mode or when enabled
by the RecordType=0x1C command, with mask 0x02. There
are other possible values of EventType but we haven't
figured them out, yet. Those seen from the GPS-45 but
not understood are listed at the end of this document.
SubType list:
EventType 05 00 Satellite acquired
This is sent when a satellite is acquired. In diagnostic
mode (Enter held down while powered on) or on the GPS-38
This coincides with the satellite signal strength bar changing
from white to black. The SubType field is the satellite ID
number - 1.
EventType 07 00 Navigation quality
The subtype is either 2 or 3 for 2D or 3D navigation.
EventType 0c 00 Keyboard event (TEST mode only)
GPS-75 codes:
01 00 down
02 00 down + 1 sec
03 00 down + 2 sec
04 00
05 00
08 00
