diff --git a/.gitignore b/.gitignore
index 79dac986c..d9fd6c074 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,4 @@ xs/buildtmp
 *.o
 MANIFEST.bak
 xs/MANIFEST.bak
+xs/assertlib*
diff --git a/utils/send-gcode.pl b/utils/send-gcode.pl
index 63affd506..989b17872 100644
--- a/utils/send-gcode.pl
+++ b/utils/send-gcode.pl
@@ -10,17 +10,27 @@ BEGIN {
 
 use Slic3r;
 
+die "Usage: send-gcode.pl SERIALPORT BAUDRATE GCODE_FILE\n"
+    if @ARGV != 3;
+
 my $serial = Slic3r::GCode::Sender->new($ARGV[0], $ARGV[1]);
+1 until $serial->is_connected;
+print "Connected to printer\n";
 
-$serial->send($ARGV[2]);
-
-exit;
-
-while (1) {
-    $serial->send("1");
-    sleep 1;
-    $serial->send("0");
-    sleep 1;
+{
+    local $/ = "\n";
+    Slic3r::open(\my $fh, '<', $ARGV[2])
+        or die "Unable to open $ARGV[2]: $!\n";
+    binmode $fh, ':utf8';
+    while (<$fh>) {
+        $serial->send($_);
+    }
+    close $fh;
 }
 
+while ((my $queue_size = $serial->queue_size) > 0) {
+    printf "Queue size: %d\n", $queue_size;
+}
+$serial->disconnect;
+
 __END__
diff --git a/xs/MANIFEST b/xs/MANIFEST
index 808240797..fb06bec85 100644
--- a/xs/MANIFEST
+++ b/xs/MANIFEST
@@ -1,21 +1,3 @@
-assertlib2WNuZugY.dSYM/Contents/Info.plist
-assertlib2WNuZugY.dSYM/Contents/Resources/DWARF/assertlib2WNuZugY
-assertlib_1AsN89M.dSYM/Contents/Info.plist
-assertlib_1AsN89M.dSYM/Contents/Resources/DWARF/assertlib_1AsN89M
-assertlibCHIyOgiI.dSYM/Contents/Info.plist
-assertlibCHIyOgiI.dSYM/Contents/Resources/DWARF/assertlibCHIyOgiI
-assertlibePik23eL.dSYM/Contents/Info.plist
-assertlibePik23eL.dSYM/Contents/Resources/DWARF/assertlibePik23eL
-assertlibk8tKRIAD.dSYM/Contents/Info.plist
-assertlibk8tKRIAD.dSYM/Contents/Resources/DWARF/assertlibk8tKRIAD
-assertlibNTySykmH.dSYM/Contents/Info.plist
-assertlibNTySykmH.dSYM/Contents/Resources/DWARF/assertlibNTySykmH
-assertlibpBddNlll.dSYM/Contents/Info.plist
-assertlibpBddNlll.dSYM/Contents/Resources/DWARF/assertlibpBddNlll
-assertlibtdgvvqTu.dSYM/Contents/Info.plist
-assertlibtdgvvqTu.dSYM/Contents/Resources/DWARF/assertlibtdgvvqTu
-assertlibxg2L4e5_.dSYM/Contents/Info.plist
-assertlibxg2L4e5_.dSYM/Contents/Resources/DWARF/assertlibxg2L4e5_
 Build.PL
 include/boost/aligned_storage.hpp
 include/boost/array.hpp
diff --git a/xs/MANIFEST.SKIP b/xs/MANIFEST.SKIP
index 0123bf375..14a73467e 100644
--- a/xs/MANIFEST.SKIP
+++ b/xs/MANIFEST.SKIP
@@ -71,3 +71,5 @@
 
 # Avoid archives of this distribution
 \bSlic3r-XS-[\d\.\_]+
+
+^assertlib
diff --git a/xs/src/libslic3r/GCodeSender.cpp b/xs/src/libslic3r/GCodeSender.cpp
index 7ba4a68bf..f5398a699 100644
--- a/xs/src/libslic3r/GCodeSender.cpp
+++ b/xs/src/libslic3r/GCodeSender.cpp
@@ -1,13 +1,18 @@
 #ifdef BOOST_LIBS
 #include "GCodeSender.hpp"
 #include <iostream>
+#include <istream>
+#include <string>
+#include <boost/algorithm/string/predicate.hpp>
+#include <boost/algorithm/string/trim.hpp>
+#include <boost/lexical_cast.hpp>
 
 namespace Slic3r {
 
 namespace asio = boost::asio;
 
 GCodeSender::GCodeSender(std::string devname, unsigned int baud_rate)
-    : io(), serial(io)
+    : io(), serial(io), can_send(false), sent(0), error(false), connected(false)
 {
     this->serial.open(devname);
     this->serial.set_option(asio::serial_port_base::baud_rate(baud_rate));
@@ -18,56 +23,188 @@ GCodeSender::GCodeSender(std::string devname, unsigned int baud_rate)
     this->serial.close();
     this->serial.open(devname);
     this->serial.set_option(asio::serial_port_base::parity(asio::serial_port_base::parity::none));
+    this->open = true;
     
-    std::string greeting;
-    this->read_line(&greeting);
+    // this gives some work to the io_service before it is started
+    // (post() runs the supplied function in its thread)
+    this->io.post(boost::bind(&GCodeSender::do_read, this));
+    
+    // start reading in the background thread
+    boost::thread t(boost::bind(&asio::io_service::run, &this->io));
+    this->background_thread.swap(t);
+}
+
+void
+GCodeSender::disconnect()
+{
+    if (!this->open) return;
+
+    this->open = false;
+    this->connected = false;
+    this->io.post(boost::bind(&GCodeSender::do_close, this));
+    this->background_thread.join();
+    this->io.reset();
+    if (this->error_status()) {
+        throw(boost::system::system_error(boost::system::error_code(),
+            "Error while closing the device"));
+    }
+}
+
+bool
+GCodeSender::is_connected() const
+{
+    return this->connected;
+}
+
+size_t
+GCodeSender::queue_size() const
+{
+    boost::lock_guard<boost::mutex> l(this->queue_mutex);
+    return this->queue.size();
+}
+
+void
+GCodeSender::do_close()
+{
+    boost::system::error_code ec;
+    this->serial.cancel(ec);
+    if (ec) this->set_error_status(true);
+    this->serial.close(ec);
+    if (ec) this->set_error_status(true);
+}
+
+void
+GCodeSender::set_error_status(bool e)
+{
+    boost::lock_guard<boost::mutex> l(this->error_mutex);
+    this->error = e;
+}
+
+bool
+GCodeSender::error_status() const
+{
+    boost::lock_guard<boost::mutex> l(this->error_mutex);
+    return this->error;
+}
+
+void
+GCodeSender::do_read()
+{
+    // read one line
+    asio::async_read_until(
+        this->serial,
+        this->read_buffer,
+        '\n',
+        boost::bind(
+            &GCodeSender::on_read,
+            this,
+            asio::placeholders::error,
+            asio::placeholders::bytes_transferred
+        )
+    );
+}
+
+void
+GCodeSender::on_read(const boost::system::error_code& error,
+    size_t bytes_transferred)
+{
+    if (error) {
+        // error can be true even because the serial port was closed.
+        // In this case it is not a real error, so ignore.
+        if (this->open) {
+            this->do_close();
+            this->set_error_status(true);
+        }
+        return;
+    }
+    
+    // copy the read buffer into string
+    std::string line((std::istreambuf_iterator<char>(&this->read_buffer)), 
+        std::istreambuf_iterator<char>());
+    
+    // parse incoming line
+    if (!this->connected
+        && (boost::starts_with(line, "start")
+         || boost::starts_with(line, "Grbl "))) {
+        this->connected = true;
+        this->can_send = true;
+        this->send();
+    } else if (boost::starts_with(line, "ok")) {
+        {
+            boost::lock_guard<boost::mutex> l(this->queue_mutex);
+            this->queue.pop();
+        }
+        this->can_send = true;
+        this->send();
+    } else if (boost::istarts_with(line, "resend")
+            || boost::istarts_with(line, "rs")) {
+        // extract the first number from line
+        using boost::lexical_cast;
+        using boost::bad_lexical_cast;
+        boost::algorithm::trim_left_if(line, !boost::algorithm::is_digit());
+        size_t toresend = lexical_cast<size_t>(line.substr(0, line.find_first_not_of("0123456789")));
+        if (toresend == this->sent) {
+            this->sent--;
+            this->can_send = true;
+            this->send();
+        } else {
+            printf("Cannot resend %lu (last was %lu)\n", toresend, this->sent);
+        }
+    }
+    
+    this->do_read();
 }
 
 void
 GCodeSender::send(const std::vector<std::string> &lines)
 {
-    this->lines = lines;
-}
-
-void
-GCodeSender::send(const std::string &s)
-{
+    // append lines to queue
     {
-        std::stringstream ss(s);
-        std::string line;
-        while (std::getline(ss, line, '\n'))
-            this->lines.push_back(line);
-    }
-    
-    for (std::vector<std::string>::const_iterator line = this->lines.begin(); line != this->lines.end(); ++line) {
-        this->send_line(*line);
+        boost::lock_guard<boost::mutex> l(this->queue_mutex);
+        for (std::vector<std::string>::const_iterator line = lines.begin(); line != lines.end(); ++line)
+            this->queue.push(*line);
     }
+    this->send();
 }
 
 void
-GCodeSender::send_line(const std::string &line)
+GCodeSender::send(const std::string &line)
 {
+    // append line to queue
+    {
+        boost::lock_guard<boost::mutex> l(this->queue_mutex);
+        this->queue.push(line);
+    }
+    this->send();
+}
+
+void
+GCodeSender::send()
+{
+    // printer is not connected or we're still waiting for the previous ack
+    if (!this->can_send) return;
+    
+    boost::lock_guard<boost::mutex> l(this->queue_mutex);
+    if (this->queue.empty()) return;
+    
+    // get line and strip any comment
+    std::string line = this->queue.front();
+    if (size_t comment_pos = line.find_first_of(';') != std::string::npos)
+        line.erase(comment_pos, std::string::npos);
+    boost::algorithm::trim(line);
+    
+    // calculate checksum
+    int cs = 0;
+    for (std::string::const_iterator it = line.begin(); it != line.end(); ++it)
+       cs = cs ^ *it;
+    
+    sent++;
     asio::streambuf b;
     std::ostream os(&b);
-    os << line << "\n";
+    os << "N" << sent << " " << line
+       << "*" << cs << "\n";
     asio::write(this->serial, b);
-}
-
-void
-GCodeSender::read_line(std::string* line)
-{
-    for (;;) {
-        char c;
-        asio::read(this->serial, asio::buffer(&c, 1));
-        switch (c) {
-            case '\r':
-                break;
-            case '\n':
-                return;
-            default:
-                *line += c;
-        }
-    }
+    this->can_send = false;
 }
 
 #ifdef SLIC3RXS
diff --git a/xs/src/libslic3r/GCodeSender.hpp b/xs/src/libslic3r/GCodeSender.hpp
index c5a67fd51..ef68849fb 100644
--- a/xs/src/libslic3r/GCodeSender.hpp
+++ b/xs/src/libslic3r/GCodeSender.hpp
@@ -3,28 +3,47 @@
 #ifdef BOOST_LIBS
 
 #include <myinit.h>
+#include <queue>
 #include <string>
 #include <vector>
 #include <boost/asio.hpp>
+#include <boost/bind.hpp>
+#include <boost/thread.hpp>
 
 namespace Slic3r {
 
 namespace asio = boost::asio;
 
-class GCodeSender {
+class GCodeSender : private boost::noncopyable {
     public:
     GCodeSender(std::string devname, unsigned int baud_rate);
     void send(const std::vector<std::string> &lines);
     void send(const std::string &s);
-    
+    void disconnect();
+    bool error_status() const;
+    bool is_connected() const;
+    size_t queue_size() const;
     
     private:
     asio::io_service io;
     asio::serial_port serial;
-    std::vector<std::string> lines;
+    boost::thread background_thread;
+    boost::asio::streambuf read_buffer;
+    bool open;      // whether the serial socket is connected
+    bool connected; // whether the printer is online
+    bool error;
+    mutable boost::mutex error_mutex;
     
-    void send_line(const std::string &line);
-    void read_line(std::string* line);
+    mutable boost::mutex queue_mutex;
+    std::queue<std::string> queue;
+    bool can_send;
+    size_t sent;
+    
+    void set_error_status(bool e);
+    void do_close();
+    void do_read();
+    void on_read(const boost::system::error_code& error, size_t bytes_transferred);
+    void send();
 };
 
 }
diff --git a/xs/xsp/GCodeSender.xsp b/xs/xsp/GCodeSender.xsp
index 37cdd6fd3..150c8c64b 100644
--- a/xs/xsp/GCodeSender.xsp
+++ b/xs/xsp/GCodeSender.xsp
@@ -11,6 +11,9 @@
     GCodeSender(std::string port, unsigned int baud_rate);
     ~GCodeSender();
     
+    bool is_connected() const;
+    int queue_size() const;
+    void disconnect();
     void send(std::string s);
 };