Merged second commit of pull request #25 from Alexander Wenger.

Comment from the author:
The Cameo will get lost in communication if any new data is send while the machine is busy cutting. Also data chunk size in Silhouette Studio is limited to 3k chars at most and 1k is standard. In response a method "safe_write" was added as a wrapper for the write method to further split large data words (from plot commands) into more digestible chunks. Also inkscape-silhouette will now wait for the cutter to be ready before sending new data.

This results in the cutter returning to the main menu after the complete job is finished (e.g. the nifty option to advance the media into a position where the very handy cross-cutter can be used is offered). Without waiting the machine will only offer to eject the media after cutting.

silhouette/Graphtec.py

@@ -348,6 +348,24 @@ def write(s, string, timeout=10000):
     if o != len(string):
       raise ValueError('write all %d bytes failed: o=%d' % (len(string), o))
 
+  def safe_write(s, string):
+    """wrapper for write with special emphasis on not to over-load the cutter with long commands."""
+    if s.dev is None: return None
+    # Silhouette Studio uses packet size of maximal 3k, 1k is default
+    safemaxchunksz = 1024
+    so = 0
+    delimiter = "\x03"
+    while so < len(string):
+      safechunksz = min(safemaxchunksz, len(string)-so)
+      candidate = string[so:so+safechunksz]
+      # strip string candidate of unfinished command at its end
+      safechunk = candidate[0:(candidate.rfind(delimiter) + 1)]
+      s.write(string = safechunk)
+      # wait for cutter to finish current chunk, otherwise blocking might occur
+      while not s.status() == "ready":
+        time.sleep(0.05)
+      so += len(safechunk)
+
   def read(s, size=64, timeout=5000):
     """Low level read method"""
     if s.dev is None: return None
@@ -823,8 +841,9 @@ def plot(s, mediawidth=210.0, mediaheight=297.0, margintop=None,
     if not 'ury' in bbox: bbox['ury'] = 0
     new_home = "M%d,%d\x03SO0\x03" % (int(0.5+bbox['lly']+end_paper_offset*20.), 0)
 
+    # potentially long command string needs extra care
+    s.safe_write(p)
 
-    s.write(p)
     s.write(new_home)
 
     return {