public void Send(SnapPacket packet)
{
byte c;
int n;
int i;
int padding = 0;
double x;
byte[] header = new byte[8];
// Header byte 2
c = SNAP_DAB | SNAP_SAB | SNAP_PFB | SNAP_ACK_REQUEST;
header[0] = c;
// Determine the size of the packet. The protocol allows 0 to 512 bytes of
// data but not all sizes within that are supported. In the header, if bit 3 = 0,
// then bits 2..0 indicate the exact size in bytes. If bit 3 = 1, then bits 2..0
// the size of the data is 2^(bits+3). If the data is greater than 7 bytes then
// we will pick the next largest size.
if (packet.Data.Count > 512)
{
throw new ApplicationException("Data is too long. 512 byte maximum.");
}
// Handle sizes >= 8 differently, see comments above
if (packet.Data.Count < 8)
{
n = packet.Data.Count;
}
else
{
x = Math.Log(packet.Data.Count, 2);
if (x > (int)x)
{
n = ((int)x) + 1;
}
else
{
n = (int)x;
}
padding = (int)(Math.Pow(2, n) - packet.Data.Count);
n = (byte)(n - 3) | 0x08; // Set bit 3 to indicate size is 2^(bits+3) bytes
}
// Header byte 1
c = (byte)(SNAP_CMD | SNAP_EDM | n);
header[1] = c;
// Destination address (3 bytes)
header[2] = (byte)(packet.Destination >> 16);
header[3] = (byte)(packet.Destination >> 8);
header[4] = (byte)(packet.Destination >> 0);
// Source address (3 bytes)
packet.Source = SnapNodeAddress;
header[5] = (byte)(packet.Source >> 16);
header[6] = (byte)(packet.Source >> 8);
header[7] = (byte)(packet.Source >> 0);
// Create the packet
ByteString bytes = new ByteString();
// Add the preamble and sync bytes
bytes.Append(SNAP_PREAMBLE1);
bytes.Append(SNAP_PREAMBLE2);
bytes.Append(SNAP_SYNC);
// Add the header and data
bytes.AddRange(header);
bytes.AddRange(packet.Data);
// Pad the data as necessary
for (i = padding; i > 0; i--)
{
bytes.Append(0);
}
// Calculate and add the CRC (ignore the preamble/sync bytes)
packet.Crc = Crc.Crc16(bytes.GetBytes(3));
bytes.Append((byte)(packet.Crc >> 8));
bytes.Append((byte)(packet.Crc >> 0));
// Add a newline for easier debugging
// The packet is done, these characters should be ignored by receivers
bytes.Append((byte)'\n');
// Send the packet
serialPort.Write(bytes.GetBytes());
}
protected void findSnapPacket()
{
// TODO: Find several packets per call
byte b;
int numDataBytes;
int x;
SnapPacket packet = new SnapPacket();
// Search for packet header
int c = receiveBuffer.IndexOf("!#T");
if (c >= 0)
{
// Drop any data before beginning of packet
// (drop the preamble/sync bytes so we don't try
// and parse this packet again).
receiveBuffer.RemoveRange(0, c);
if (receiveBuffer.Count >= 3 + 2 + 3 + 3)
{
b = (byte)receiveBuffer[3];
b = (byte)(b & SNAP_DAB_MASK);
if (((byte)receiveBuffer[3] & SNAP_DAB_MASK) == SNAP_DAB)
{
if (((byte)receiveBuffer[3] & SNAP_SAB_MASK) == SNAP_SAB)
{
if (((byte)receiveBuffer[3] & SNAP_PFB_MASK) == SNAP_PFB)
{
if (((byte)receiveBuffer[4] & SNAP_CMD_MASK) == SNAP_CMD)
{
if (((byte)receiveBuffer[4] & SNAP_EDM_MASK) == SNAP_EDM)
{
x = ((byte)receiveBuffer[4]) & SNAP_NDB_MASK;
if (x <= 8)
{
numDataBytes = x;
}
else
{
x = x - 8;
numDataBytes = 8;
for ( ; x > 0; x--)
{
numDataBytes = numDataBytes * 2;
}
}
packet.Destination = (uint)(byte)receiveBuffer[5] << 16;
packet.Destination += (uint)(byte)receiveBuffer[6] << 8;
packet.Destination += (uint)(byte)receiveBuffer[7] << 0;
packet.Source = (uint)(byte)receiveBuffer[8] << 16;
packet.Source += (uint)(byte)receiveBuffer[9] << 8;
packet.Source += (uint)(byte)receiveBuffer[10] << 0;
if (receiveBuffer.Count >= 3 + 2 + 3 + 3 + numDataBytes + 2)
{
packet.Data = receiveBuffer.GetRange(3 + 2 + 3 + 3, numDataBytes);
packet.Crc = (ushort)((uint)(byte)receiveBuffer[3 + 2 + 3 + 3 + numDataBytes + 0] << 8);
packet.Crc += (ushort)((uint)(byte)receiveBuffer[3 + 2 + 3 + 3 + numDataBytes + 1] << 0);
if (packet.Crc == Crc.Crc16(receiveBuffer.GetBytes(3, 2 + 3 + 3 + numDataBytes)))
{
if (packet.Destination == SnapNodeAddress)
{
PacketReceived(this, new SnapPacketReceivedEventArgs(packet));
}
}
receiveBuffer.RemoveRange(0, 3 + 2 + 3 + 3 + numDataBytes);
}
}
else
{
Debug.WriteLine("Only CRC16 is supported");
}
}
else
{
Debug.WriteLine("Command mode not supported");
}
}
else
{
Debug.WriteLine("Protocol specific flag bytes not supported");
}
}
else
{
Debug.WriteLine("Source address not 3 bytes");
}
}
else
{
Debug.WriteLine("Destination address not 3 bytes");
}
}
}
}