Introduction
This article shows how to use virtual serial (COM) port to read and convert GPS data stream from one format to another allowing older legacy GPS applications to work with the newer GPS receivers.
Background
I have HPC CASIO Cassiopeia A-10 with Windows CE 2.00 that I'm using exclusively for GPS navigation. I'm using TeleType application for WinCE 2.00 with Delorme Tripmate GPS. Recently I upgraded my GPS with Pharos GPS-500 SiRF III and the TeleType application didn't work with it. It turns out that the application is working with an older version of the NMEA protocol than the GPS-500. The application is using NMEA 0183 v2.0 and the GPS-500 NMEA 0183 v2.3.
The application is actually using only two sentences from the NMEA protocol - GGA and RMC:
GGA —Global Positioning System Fixed Data
$GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M,,,,0000*18
| Name | Example | Units | Description |
| Message ID | $GPGGA | GGA protocol header | |
| UTC Time | 161229.487 | hhmmss.sss | |
| Latitude | 3723.2475 | ddmm.mmmm | |
| N/S Indicator | N | N=north or S=south | |
| Longitude | 12158.3416 | dddmm.mmmm | |
| E/W Indicator | W | E=east or W=west | |
| Position Fix Indicator | 1 | See Table 1-4 | |
| Satellites Used | 7 | Range 0 to 12 | |
| HDOP | 1 | Horizontal Dilution of Precision | |
| MSL Altitude | 9 | meters | |
| Units | M | meters | |
| Geoid Separation | meters | ||
| Units | M | meters | |
| Age of Diff. Corr. | second | Null fields when DGPS is not used | |
| Diff. Ref. Station ID | 0000 | ||
| Checksum | *18 | ||
| <CR> <LF> | End of message termination |
Note that there are no differences in GGA between NMEA v2.0 and v2.3.
RMC—Recommended Minimum Specific GNSS Data
$GPRMC,161229.487,A,3723.2475,N,12158.3416,W,0.13,309.62,120598,,A*10
| Name | Example | Units | Description |
| Message ID | $GPRMC | RMC protocol header | |
| UTC Time | 161229.487 | hhmmss.sss | |
| Status | A | A=data valid or V=data not valid | |
| Latitude | 3723.2475 | ddmm.mmmm | |
| N/S Indicator | N | N=north or S=south | |
| Longitude | 12158.3416 | dddmm.mmmm | |
| E/W Indicator | W | E=east or W=west | |
| Speed Over Ground | 0.13 | knots | |
| Course Over Ground | 309.62 | degrees | True |
| Date | 120598 | ddmmyy | |
| Magnetic Variation | degrees | E=east or W=west | |
| Mode | A | A=Autonomous, D=DGPS, E= DR (missing in v2.0) | |
| Checksum | *10 | ||
| <CR> <LF> | End of message termination |
The last field "Mode" does not exists in NMEA v2.0. In order for my application to work, it needs to translate the RMC from v2.3 (the GPS-500) to v2.0 (the application). In the translation, it needs to remove the last field from the RMC data and made it look like this:
$GPRMC,161229.487,A,3723.2475,N,12158.3416,W,0.13,309.62,120598,*20
Using the code
To build virtual serial (COM) port I used the excellent example provided in the following article DemoDriver.asp. (Big thanks to the author.)
The virtual port serves as a connection between the actual port and the application. In the process of reading the data from the GPS and passing it to the application, the virtual port translates the GPS data format from NMEA 2.3 to NMEA 2.0 for the RMC sentences.
The following code shows the virtual port "read" function:
DWORD COM_Read( DWORD hOpenContext, LPVOID pBuffer, DWORD Count )
{
if (hOpenContext != OPEN_CONTEXT)
return 0;
// read from the actual port
DWORD dwBytes = 0;
ReadFile(hComm, pBuffer, Count, &dwBytes, NULL);
// convert the data before it's returned
Convert( pBuffer, dwBytes );
return dwBytes;
}
The function "Convert" is doing the work of locating and translating the RMC sentences:
Collapse
char * atoh = "0123456789ABCDEF";
// " 0 1 2 3 4 5 6 7 8 9 : ; < = > ? @ A B C D E F"
BYTE htoa1[] = {0x00,0x10,0x20,0x30,0x40,0x50,0x60,0x70,0x80,0x90,0,0,0,0,0,0,0,0xA0,0xB0,0xC0,0xD0,0xE0,0xF0};
BYTE htoa2[] = {0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0,0,0,0,0,0,0,0x0A,0x0B,0x0C,0x0D,0x0E,0x0F};
void Convert( LPVOID pBuffer, DWORD Count )
{
char* pCh = (char*)pBuffer;
char ch, crc1, crc2; DWORD nCRC; BYTE state = 10;
for (DWORD i=0;i<Count; i++)
{
ch = pCh[i];
if (ch == '$')
state = 11;
else
{
switch ( state )
{
case 11: state = ch == 'G'? 12:10; break;
case 12: state = ch == 'P'? 13:10; break;
case 13: state = ch == 'R'? 14:10; break;
case 14: state = ch == 'M'? 15:10; break;
case 15: state = ch == 'C'? 16:10; break;
case 16: state = ch == ','? 20:10; break;
case 20: if (ch == 'A') state = 21; break;
case 21: state = ch == '*'? 22:20; break;
case 22:
crc1 = ch; state = 23; break;
case 23:
{
crc2 = ch;
// get the crc
nCRC = htoa1[crc1 - '0'] + htoa2[crc2 - '0'];
// remove the chars we are deleting
nCRC -= 'A';
nCRC -= ',';
// ... ,,A*72\n
// ^ current pos
// ^ write over pos
pCh[i-4]='*';
pCh[i-3]=atoh[(nCRC & 0xF0) >> 4];
pCh[i-2]=atoh[ nCRC & 0x0F ];
pCh[i-1]=13;
pCh[i-0]=10;
state = 10;
}
break;
}
}
}
}
The function goes through the input buffer pBuffer and locates any RMC sentences. For each RMC it checks for the string "A*" at the end and removes it. It also calculates and updates the CRC check sum at the end.
To install the virtual port driver we need to copy the driver dll file to \windows folder at the device and add these entries in the registry:
[HKEY_LOCAL_MACHINE\Drivers\BuiltIn\Serial6]
"ManagePort"=dword:00000002
"DeviceType"=dword:00000001
"Index"=dword:00000006
"DevConfig"=hex: 10,00,00,00,05,00,00,00,10,01,00,00,00,4b,00,00,00,00,08,00,00,00,00,00,00
"Tsp"="Unimodem.dll"
"Order"=dword:00000006
"Prefix"="COM"
"FriendlyName"="GPS Virtual Driver"
"Dll"="GpsVirtualDriver.dll"
"ManagePort" is the port number where the GPS device is connected to. In my case the GPS is connected with PCMCIA card and shows up as COM2 in the device.
"Index" is the virtual serial port COM number. In this example it is COM6.
To deploy the application to the device we need to build a cab file. This is simple INF file that is used to build the cab file:
Collapse
; sample.inf
;
;==================================================
[Version]
Signature = "$Windows NT$"
Provider = "Gps"
CESignature = "$Windows CE$"
;
; Supports only WinCE 2.00
;
[CEDevice]
VersionMin=2.00
VersionMax=2.00
[CEStrings]
AppName = "Virtual Driver"
InstallDir = %CE2%
[Strings]
reg_path = Drivers\Builtin\Serial6
;==================================================
[DefaultInstall]
CopyFiles = Dllfiles
Addreg = Regkeys
;==================================================
[SourceDisksNames]
1 =, "Common Files",, .
[SourceDisksFiles]
GpsVirtualDriver.dll = 1
;==================================================
; Ouput directories for files & shortcuts
[DestinationDirs]
Dllfiles = 0, %CE2%
[Dllfiles]
GpsVirtualDriver.dll
[Regkeys]
HKLM,%reg_path%,Dll,0x00000000,GpsVirtualDriver.dll
HKLM,%reg_path%,Prefix,0x00000000,COM
HKLM,%reg_path%,FriendlyName,0x00000000,GPS Virtual Driver
HKLM,%reg_path%,Index,0x00010001,6
HKLM,%reg_path%,Order,0x00010001,2
HKLM,%reg_path%,ManagePort,0x00010001,2
HKLM,%reg_path%,DeviceType,0x00010001,1
HKLM,%reg_path%,Tsp,0x00000000,Unimodem.dll
HKLM,%reg_path%,DevConfig,0x00000001,10,00,00,00,05,00,00,00,10,01,00,00,00,4b,00,00,00,00,08,00,00,00,00,00,00
After the cab file is installed a soft reset is required for the driver to start working.
Points of Interest
Doing this small project was full of fun and I learned how to build simple device drivers for Windows CE 2.00. Now I have my Cassiopeia running the TeleType GPS application with the new GPS-500 receiver. The GPS-500 is very compact and it works out of my PCMCIA extension slot from where it gets its power and doesn't require batteries and/or bulky cables. It also gives me the opportunity to continue using my old and brave Cassiopeia A-10 for GPS navigation.
References
TeleType GPS FAQ: http://www.teletype.com/pages/support/faq.html
NMEA Reference Manual: http://www.sparkfun.com/datasheets/GPS/NMEA Reference Manual1.pdf
