Creating Checksum in PBP

[NavMicroSystems]

Tom,

I have dealt with NMEA a few times in the past.

The checksum is optional, and you do not really need it
(as long as your wires aren't too long and the environment isn't too "noisy")

What the example does is pretty simple:

-----
Num_To_ASCII:
Dec_lat1 = 119
C[7] = Dec_lat1 DIG 2+"0" ' element c[7] = ASCII 1
-----

DIG2 of Dec_lat1 in this example is decimal 1 (or $01)
the ASCII value for the character "1" is 31, so you need to add 30
(which equals the the decimal value of the ASCII character "0")
to get the ASCII value for this character

You could also use this:
-----
C[7] = Dec_lat1 DIG 2+30 ' element c[7] = ASCII 1
-----

(hope it is not too confusing)

[Bruce]

Just look at any ASCII chart to understand what's happening.

With PBP most people assume sending serial data goes something like this;

X VAR BYTE
X = 123

Main
    SEROUT2 0,16468,[DEC X]
    PAUSEUS 200
    GOTO Main

You would assume you're sending a single byte, but PBP is first converting your three decimal digits in X into three separate ASCII characters, and sending a total of three bytes. "1" then "2" then "3". That's what the DEC modifier does.

So your PC isn't receiving a single byte value of 123. It's receiving three separate bytes. ASCII characters 1 then 2 then 3. This is what it's using to calculate the checksum.

Since your PC checksum program runs a cumulative checksum calculated from each separate ASCII digit, you just need to do the same with your PIC-based cheksum program before sending the final checksum result to the PC.

Try running this with a PIC connected to your PC serial port, and see what displays in a terminal window.

X = 123

Main
    HSEROUT [DEC X,13,10]
    HSEROUT [X,13,10]    
    PAUSEUS 2000
    GOTO Main

[picnaut]

Hi,

The attached file may be of some help.

Cheers.