diff --git a/posctl.cc b/posctl.cc
index 6f9bfcc..5c909da 100644
--- a/posctl.cc
+++ b/posctl.cc
@@ -639,7 +639,7 @@ void PosCtl::OutputWriteValue (int axis, double value) {
 	std::string s = setlocale(LC_ALL, NULL);
 	setlocale (LC_ALL, "C");
 
-	snprintf(outbuf, 254, "R%c%c%08.4f#\n", (axis == 0 ? 'A' : 'R'),
+	snprintf(outbuf, 254, ":R%c%c%08.4f#\n", (axis == 0 ? 'A' : 'R'),
 											(value < 0 ? '-' : '+'),
 											value);
 	outbuf[254] = 0;