HEX addition, subtraction LRC ASCII

Trying to do ASCII LRC calculation, so I have hex values stored in a QByteArray, something in the lines of 01050500FF00, I need to add these values together, then subtract the result from FF, and add 1 to get the LRC..

in short :
1- Step1 = Sum Hex bytes of given QByteArray
2- Step2 = "FF" - Step1 (take the right part)
3- Step3 = Step2 + 1

Example :

1 - "01050500FF00", LRC should be F6
0+1+0+5+0+5+0+0+F+F+0+0 = 010A
FF - 0A = F5
F5 +1 = F6

2 - "010505000000", LRC should be F5
ie 0+1+0+5+0+5+0+0+0+0+0+0 = 0B
FF - 0B = F4
F4 + 1 = F5

My question is there an easy way to do Hex addition and subtraction in QT? if so how? or do I convert to decimal... case 'A' : a[i] = 11;break;... and so on..

tnx for any help..

p.s. C++

Did You have in QByteArray characters ('0'..'9', 'A'..'F') or bytes (0..0xf) ? This is not the same.

Supposed to have bytes..

The QByteArray crcWord is formed as such.. where ;

Quote:

---

ui->lineEdit->text() is '01' (slave id)
functioncode is '05' (modbus function code)
ui->lineEdit_2->text() is '0500' (Register address hex)
ui->lineEdit_3->text() is '0000' (or 'FF00' from the examples I gave)

---

(data)

Code:

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QByteArray crcWord = QString(ui->lineEdit->text() + functionCode + ui->lineEdit_2->text() + ui->lineEdit_3->text()).toLatin1();

---

I later write to the port, it all works when I manually add the LRC manually (i.e. replace the crc.toUpper() with "F5" (or whatever the LRC value of the word is) )

Code:

---

..
 
QString sendWord = ":" + crcWord + crc.toUpper() + "\r\n";
 
QByteArray sndwrd = sendWord.toLatin1();
const char *cstr = sndwrd.data();
 
serial->write(cstr);
serial->close();

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Is this what you are trying to implement?

http://en.wikipedia.org/wiki/Longitu...dundancy_check

If so, you'll need to take your byte array which contains a string, and pull 2 characters out of it at a time to convert them to bytes. Then do your calculations on the bytes.

So in QByteArray You have ASCII characters (string) not binary data. You can convert them to binary data with QByteArray::fromHex method and then simply add byte by byte.

Your description of the process is wrong but you have given the correct result in both cases.

Code:

---

1 - "01050500FF00", LRC should be F6
0+1+0+5+0+5+0+0+F+F+0+0 = 010A  // wrong working, right answer
0x01 + 0x05 + 0x05 + 0x00 + 0xFF + 0x00 = 0x010A   // this is the correct working
FF - 0A = F5
F5 +1 = F6

---

Steps 2 and 3 are a description of negating a number in Two's-complement. Your computer will do this for you. Here is your example with the LRC appended to the input:

Code:

---

QByteArray input("01050500FF00");
 
    // Convert to binary
    QByteArray work = QByteArray::fromHex(input);
    // Step 1, sum bytes (relies on char losing any overflow)
    char sum = 0;
    foreach (char c, work)
        sum += c;
    // steps 2 and 3: negate result
    sum = -sum;
 
    qDebug() << "Input    =" << work.toHex().toUpper();
    work.append(sum); 
    qDebug() << "With LRC =" << work.toHex().toUpper();

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Quote:

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Originally Posted by ChrisW67

Your description of the process is wrong but you have given the correct result in both cases.

Code:

---

..
0+1+0+5+0+5+0+0+F+F+0+0 = 010A  // wrong working, right answer
0x01 + 0x05 + 0x05 + 0x00 + 0xFF + 0x00 = 0x010A   // this is the correct working
...

---

Steps 2 and 3 are a description of negating a number in Two's-complement. Your computer will do this for you. Here is your example with the LRC appended to the input:
....

---

Thanks for the reply, all is working now. I wrote the calculation wrong partially on purpose as oddly enough it still calculates the LRC correctly for all the samples that I tried it on..