public static int Decode(byte[] buffer, int offset, int max_length, out BacnetPtpFrameTypes frame_type, out int msg_length)
{
frame_type = (BacnetPtpFrameTypes)buffer[offset + 2];
msg_length = (buffer[offset + 3] << 8) | (buffer[offset + 4] << 0);
byte crc_header = buffer[offset + 5];
ushort crc_data = 0;
if (max_length < PTP_HEADER_LENGTH)
{
return(-1); //not enough data
}
if (msg_length > 0)
{
crc_data = (ushort)((buffer[offset + 6 + msg_length + 1] << 8) | (buffer[offset + 6 + msg_length + 0] << 0));
}
if (buffer[offset + 0] != PTP_PREAMBLE1)
{
return(-1);
}
if (buffer[offset + 1] != PTP_PREAMBLE2)
{
return(-1);
}
if (MSTP.CRC_Calc_Header(buffer, offset + 2, 3) != crc_header)
{
return(-1);
}
if (msg_length > 0 && max_length >= (PTP_HEADER_LENGTH + msg_length + 2) && MSTP.CRC_Calc_Data(buffer, offset + 6, msg_length) != crc_data)
{
return(-1);
}
return(8 + (msg_length) + (msg_length > 0 ? 2 : 0));
}
public static int Encode(byte[] buffer, int offset, BacnetPtpFrameTypes frame_type, int msg_length)
{
buffer[offset + 0] = PTP_PREAMBLE1;
buffer[offset + 1] = PTP_PREAMBLE2;
buffer[offset + 2] = (byte)frame_type;
buffer[offset + 3] = (byte)((msg_length & 0xFF00) >> 8);
buffer[offset + 4] = (byte)((msg_length & 0x00FF) >> 0);
buffer[offset + 5] = MSTP.CRC_Calc_Header(buffer, offset + 2, 3);
if (msg_length > 0)
{
//calculate data crc
ushort data_crc = MSTP.CRC_Calc_Data(buffer, offset + 6, msg_length);
buffer[offset + 6 + msg_length + 0] = (byte)(data_crc & 0xFF); //LSB first!
buffer[offset + 6 + msg_length + 1] = (byte)(data_crc >> 8);
}
return(PTP_HEADER_LENGTH + (msg_length) + (msg_length > 0 ? 2 : 0));
}
