diff --git a/camlibs/sierra/PROTOCOL b/camlibs/sierra/PROTOCOL
index a6493f5de..63444e123 100644
--- a/camlibs/sierra/PROTOCOL
+++ b/camlibs/sierra/PROTOCOL
@@ -1,3 +1,4 @@
+For further information , see http://photopc.sourceforge.net/protocol.html.
 
  register	interpretation
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