Re: [asio-users] How to Enable/Disable Serial Port DTR?

[Christopher K. <ch...@ko...>]

Robert Dailey wrote:
> However, the problem I have is that I'm using async_write(). In other 
> words, I have to enable the DTR before I call async_write(), and turn it 
> off in the write completion callback. This is error prone because if 
> writes are sent fast enough, DTR could be enabled several, consecutive 
> times, and also disabled several consecutive times. 

Do you mean you are starting new async_write operations before the 
previous ones have finished? If so, don't, as async_write is a composed 
operation (meaning it is made up of one or more calls to another async 
operation, namely async_write_some). You may garble your data.

So in that case the approach you use should work fine. You could also 
consider wrapping it in another async operation, e.g.:

   template <typename Handler>
   struct dtr_enabled_write_handler
   {
     serial_port& sp;
     Handler h;
     void operator()(error_code ec, size_t length)
     {
       serial_port::native_type handle = sp.native();
       EscapeCommFunction(handle, CLRDTR);
       h(ec, length);
     }
   };

   template <typename MutableBufferSequence, typename Handler>
   void dtr_enabled_async_write(serial_port& sp,
       const MutableBufferSequence& b, Handler h)
   {
     serial_port::native_type handle = sp.native();
     EscapeCommFunction(handle, SETDTR);
     dtr_enabled_write_handler<Handler> dewh = { sp, h };
     async_write(sp, b, dewh);
   }

Cheers,
Chris