/// <summary>
/// Creates a new telegram for a modbus request or response.
/// All data except the function code specific user data is written into the given buffer.
/// </summary>
/// <param name="addr">Device address. 0 = Breadcast, 1..247 are valid device addresses.</param>
/// <param name="fkt">Function code. <see cref="ModbusFunctionCode"/></param>
/// <param name="dataLength">Number of bytes for function code sspecific user data.</param>
/// <param name="buffer">Buffer to write data into. The buffer must be at least MaxTelegramLength - MaxDataLength + dataLength bytes long.</param>
/// <param name="telegramLength">Returns the total length of the telegram in bytes.</param>
/// <param name="dataPos">Returns the offset of the function code specific user data in buffer.</param>
/// <param name="isResponse">true if this is a response telegram; false if this is a request telegram.</param>
/// <param name="telegramContext">
/// If isResponse == false, this parameter returns the interface implementation specific data which must be passed to the ParseTelegram method of the received response.
/// If isResponse == true, this parameter must be called with the telegramContext parameter returned by ParseTelegram of the request telegram.</param>
public void CreateTelegram(byte addr, byte fkt, short dataLength, byte[] buffer, out short telegramLength, out short dataPos,
bool isResponse, ref object telegramContext)
{
telegramLength = (short)(8 + dataLength);
if (isResponse)
{
// in a response we insert the given telegramContext as transaction id
if (telegramContext is ushort)
{
ModbusUtils.InsertUShort(buffer, 0, (ushort)telegramContext);
}
else
{
ModbusUtils.InsertUShort(buffer, 0, 0);
}
}
else
{
// insert new transaction id into request
telegramContext = _NextTransactionId;
if (_NextTransactionId == 0xffff)
{
ModbusUtils.InsertUShort(buffer, 0, 0xffff);
_NextTransactionId = 0;
}
else
{
ModbusUtils.InsertUShort(buffer, 0, _NextTransactionId++);
}
}
ModbusUtils.InsertUShort(buffer, 2, 0);
ModbusUtils.InsertUShort(buffer, 4, (ushort)(telegramLength - 6));
buffer[6] = addr;
buffer[7] = fkt;
dataPos = 8;
}