Set DTR by default when opening a serial port

CHANGELOG.md

@@ -12,6 +12,9 @@ project adheres to [Semantic Versioning](https://semver.org/).
 
 * Enumerate ports from more subsystems on Linux without libudev.
   [#238](https://github.com/serialport/serialport-rs/pull/238)
+* Set data terminal ready (DTR) signal when opening a port by default and allow
+  to customize this behavior through the builder.
+  [#239](https://github.com/serialport/serialport-rs/pull/239)
 
 ### Fixed
 ### Removed

src/lib.rs

@@ -331,6 +331,8 @@ pub struct SerialPortBuilder {
     stop_bits: StopBits,
     /// Amount of time to wait to receive data before timing out
     timeout: Duration,
+    /// The state to set DTR to when opening the device
+    dtr_on_open: Option<bool>,
 }
 
 impl SerialPortBuilder {
@@ -393,6 +395,22 @@ impl SerialPortBuilder {
         self
     }
 
+    /// Set data terminal ready (DTR) to the given state when opening the device
+    #[must_use]
+    pub fn dtr_on_open(mut self, state: bool) -> Self {
+        self.dtr_on_open = Some(state);
+        self
+    }
+
+    /// Preserve the state of data terminal ready (DTR) when opening the device. Your outcome may
+    /// vary depending on the operation system. For example, Linux sets DTR by default and Windows
+    /// doesn't.
+    #[must_use]
+    pub fn preserve_dtr_on_open(mut self) -> Self {
+        self.dtr_on_open = None;
+        self
+    }
+
     /// Open a cross-platform interface to the port with the specified settings
     pub fn open(self) -> Result<Box<dyn SerialPort>> {
         #[cfg(unix)]
@@ -837,6 +855,11 @@ pub fn new<'a>(path: impl Into<std::borrow::Cow<'a, str>>, baud_rate: u32) -> Se
         parity: Parity::None,
         stop_bits: StopBits::One,
         timeout: Duration::from_millis(0),
+        // By default, set DTR when opening the device. There are USB devices performing "wait for
+        // DTR" before sending any data and users stumbled over this multiple times (see issues #29
+        // and #204). We are expecting little to no negative consequences from setting DTR by
+        // default but less hassle for users.
+        dtr_on_open: Some(true),
     }
 }
 

src/posix/tty.rs

@@ -183,14 +183,23 @@ impl TTYPort {
         termios::set_termios(fd.0, &termios)?;
 
         // Return the final port object
-        Ok(TTYPort {
+        let mut port = TTYPort {
             fd: fd.into_raw(),
             timeout: builder.timeout,
             exclusive: true,
             port_name: Some(builder.path.clone()),
             #[cfg(any(target_os = "ios", target_os = "macos"))]
             baud_rate: builder.baud_rate,
-        })
+        };
+
+        // Ignore setting DTR for pseudo terminals (indicated by baud_rate == 0).
+        if builder.baud_rate > 0 {
+            if let Some(dtr) = builder.dtr_on_open {
+                port.write_data_terminal_ready(dtr)?;
+            }
+        }
+
+        Ok(port)
     }
 
     /// Returns the exclusivity of the port

src/windows/com.rs

@@ -85,6 +85,10 @@ impl COMPort {
         dcb::set_flow_control(&mut dcb, builder.flow_control);
         dcb::set_dcb(handle, dcb)?;
 
+        if let Some(dtr) = builder.dtr_on_open {
+            com.write_data_terminal_ready(dtr)?;
+        }
+
         com.set_timeout(builder.timeout)?;
         com.port_name = Some(builder.path.clone());
         Ok(com)