diff --git a/tcl/drivers/trun.tcl b/tcl/drivers/trun.tcl
index 585b0a1..53bc8ad 100644
--- a/tcl/drivers/trun.tcl
+++ b/tcl/drivers/trun.tcl
@@ -496,10 +496,12 @@ proc trun::read_run {} {
   }
   set now [DoubleTime]
   set tr [silent none sct target]
+  set prev_tr [silent $tr sct prev_tr]
+  sct prev_tr $tr
   if {$tr eq "none"} {
     set tr $tmain
   } else {
-    set dif [expr abs($tr) - [hvali [sct]/target]]
+    set dif [expr $tr - $prev_tr]
     updateval_u [sct]/target $tr 0
     set dblmode [silent -1 hval [sct]/dblctrl/mode]
     if {$dblmode >= 0} { # not disabled