GPS Virtual Driver

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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

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

The last field "Mode" does not exist 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 make 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 this article. (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:

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 a simple INF file that is used to build the cab file:

; 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

History

• 12-Jun-2007 - First release