Hex checksum

[Jarhead]

Hey all,
I'm completely new to programming of any kind and I decided to try and write a driver for a XM tuner.
I'm trying to figure out how to send a hex array which has 4 bytes that will change with each command. The 4 bytes will be tuner number (appears twice), the actual event wanting to happen, and the checksum.
To start I want to just figure out how to calculate the checksum and take it from there.
This is the formula:
Step 1: Add byte values: 0xF0 + 0x00 + 0x67 + 0x7C + 0xF1 + 0x0F = 0x2D3
Step 2: Add byte count: 0x2D3 + 6 (Decimal value byte count) = 0x2D9
Step 3: Bitwise AND with 0x7F: 0x02D9 & 0x007F = 0x59 for the Checksum

Any ideas?
Thanks

[RedSon]

Hey check this out to see if it will help you:

http://en.wikipedia.org/wiki/Bitwise_operation

[Jarhead]

Anyone else have any ideas, or preferably, some examples?

[dennisoleksyuk]

You posted question in C# thread.
Here is C# implementation of your three steps.

            byte[] byteArray = { 0xF0, 0x00, 0x67, 0x7C, 0xF1, 0x0F };

            //Variable with result of your calculation.
            int result = 0;

            //Step1: Add byte values.            
            foreach (byte value in byteArray) {
                result += value;
            }

            //Step2: Add byte count.
            result += 6;

            //Step3: Bitwise AND with 0x7F.
            result &= 0x7F;

[Jarhead]

Almost...
It's just not returning the correct checksum.

If you try this:

byte[] byteArray = { 0xF0, 0x00, 0x7D, 0x00, 0x00, 0x00, 0x70, 0x05, 0x02, 0x01, 0x00, 0x02, 0x01, 0x00, 0x3B, 0x00, 0x70, 0x00, 0x00, 0x00, 0x01 };

And change the byte count to 21, it should return 29 but it returns 41?

[dennisoleksyuk]

Decimal 41 is hex 29. Console.WriteLi ne print all numbers as decimal