Embedded Protocol – XMODEM

Ward Christensen developed XMODEM, a straightforward file transfer protocol, in 1977. It frequently finds application between two computers employing the XMODEM protocol. The protocol initiates transmission with the ‘C’ character, followed by packets. The receiver provides proper acknowledgment (ACK) for each packet. Identification of the last packet is facilitated by EOT, and the completion of transmission is signaled by ETB.

Packet Format

Byte-1	Byte-2	Byte-3	Byte 4-129	Byte 130-131
Start of Header	Packet Number	Inverse Packet Number	Packet Data	16-bit CRC

Here Byte-1,2,3 are consider as XMODEM protocol Headers. XMODEM Transmission Reception : Image Link

Packet Size and Range

Total size of the packet is 132 Bytes and payload size is 128bytes.

• Start of the header: Size – 1Byte, Value – (0x01).
• Packet Number: Size – 1Byte, Value – 0 to 255, Packet numbering starts with 1, not Zero.
• Inverse of Packet Number: Size – 1Byte, Value – 255 to 0.
• Packet Data: Size – 128Byte.
• CRC XMODEM Size – 2Byte.

Control Characters Definitions

Control Character	Description	Value
SOH	Start of the Header	0x01
EOT	End of Transmission	0x04
ACK	Acknowledge	0x06
NAK	Not Acknowledge	0x15
ETB	End of Transmission Block	0x17
CAN	Cancel	0x18
C	ASCII “C”	0x43C

XMODEM CRC Calculation Code

CRC16 (XMODEM) information’s

• Check = 0x31C3.
• Poly = 0x1021.
• Init = 0x0000.
• RefIn = False.
• RefOut = False.
• XorOut = 0x0000.

int calcrc(char *ptr, int count)
{
    int  crc;
    char i;
    crc = 0;
    while (--count >= 0)
    {
        crc = crc ^ (int) *ptr++ << 8;
        i = 8;
        do
        {
            if (crc & 0x8000)
                crc = crc << 1 ^ 0x1021;
            else
                crc = crc << 1;
        } while(--i);
    }
    return (crc);
}

Function Name	Parameters	Description
calcrc	char *ptr, int count	Calculates the cyclic redundancy check (CRC) value for a given data block.

Explanation:

• *char ptr: A pointer to the data block for which the CRC is to be calculated.
• int count: Specifies the length of the data block.

The function initializes the CRC value to 0 and iterates over each byte in the data block. For each byte, it XORs the CRC value with the byte left-shifted by 8 bits (multiplied by 256). Then, it performs a bitwise left shift on the CRC value by 1 bit and checks the most significant bit.If the most significant bit is set, the function performs an XOR operation with the polynomial 0x1021. This process continues for 8 iterations (for each bit in the byte). After processing all the bytes in the data block, the function returns the calculated CRC value.