GPS module + PIC

[flotulopex]

Hello,

I'm trying to communicate with a GPS module http://www.futurlec.com/GPS.shtml.

Before I connect it to a PIC+LCD, I try to communicate with the GPS module via a PC terminal.

Unfortunately, I can't get any "understandable" message from the module; I might receive SIRF information instead of NMEA messages, I don't know.

This is what I see on my terminal:
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4175&stc=1&d=127083345 4">

If my guess is right, I would need to switch from SIRF to NMEA protocol. There are to examples hereunder in the doc (full doc attached).
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4174&stc=1&d=127083336 4">

I don't understand how to calculate the checksum.

I found a cool checksum calculator, HashCalc from SlavaSoft Inc. But I can't find a similar checksum corresponding to both examples.

Can someone tell me in "simple words" how the checksum in both samples above have been calculated and the exact string I would have to send the data to the module?

Is it "$PSRF100,1,4800,8,1,0*0D0A" or "$PSRF100,1,4800,8,1,0*0D,0A" or "$PSRF100,1,4800,8,1,0*<0D><0A>" or anything else?

I've spent quite a few hours on this one and no success so far so any help or info is very welcome

[mackrackit]

You need an inverter chip or circuit . (MAX232)

[flotulopex]

Thanks Dave,

The MAX232 in my drawer did the trick

Any idea about the checksum calculation?

This is what I did up to now in EXCEL but trying to convert HEX to BIN will rise an error.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4178&stc=1&d=127089284 5">

[mackrackit]

10001000000
is the number excel failed on but other than that I do not know. I will have to look into it.

[ScaleRobotics]

Here is one check sum solution for gps:

gpsdata         var byte[82]       ' Max length of a GPS sentence is 82 bytes.

DoCS:
    i = 0                             ' Initialize the position counter
                
    WHILE gpsdata[i] != "*"           ' Count everthing until the *
        cs = cs ^gpsdata[i]           ' XOR for checksum
        i = i + 1                     ' Increment position counter
    WEND

    ' A quick and easy atoh function
    ' Make it a seperate label if we add more atoh conversions in the future
    LOOKDOWN gpsdata[i+1],["0123456789ABCDEF"],tempbyte
    gpscs = tempbyte * 16        
    LOOKDOWN gpsdata[i+2],["0123456789ABCDEF"],tempbyte                
    gpscs = gpscs + tempbyte
        
    ' Flag bad data "V" if checksum doesn't add up. 
    ' Or if the sentence is less then 20 chars long
    ' Which means there is no active waypoint to go to.
    ' Should be "A" otherwise.
    IF (cs != gpscs) OR (i < 20 )THEN 
          gpsvalid = 0
    ELSE 
          gpsvalid = 1
    ENDIF   
RETURN

[flotulopex]

- Reading & parsing NMEA sentences is ok;
- Calculating checksum is ok;
- Sending data to GPS module is not ok.

I have connected the GPS module via the MAX232 chip to my serial PC port both ways so I should be able to "talk" to my module.

Unfortunately, only reading out (receiving) the GPS module is working.

I checked the MAX chip with my oscillo and the signals are okay (TX & RX) on both levels (RS232 and TTL).

I'm sending an example from the module's datasheet to enable a VTG sentence but it won't work.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4205&stc=1&d=127141338 1">

I just never see this VTG data appear.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=4207&stc=1&d=127141381 2">

Any idea?

[flotulopex]

Just for the record, I found the supplier's website for the GPS module I use; it is http://www.globalsat.com.tw/ and the specific module is here: http://www.globalsat.com.tw/products...t_cnt_folder=2.