diff --git a/vvp/schedule.cc b/vvp/schedule.cc
index aa5416b07..5e176ac14 100644
--- a/vvp/schedule.cc
+++ b/vvp/schedule.cc
@@ -1038,6 +1038,10 @@ extern void vpiStartOfSim();
 extern void vpiPostsim();
 extern void vpiNextSimTime(void);
 
+static bool sim_at_rosync = false;
+bool schedule_at_rosync(void)
+{ return sim_at_rosync; }
+
 /*
  * The scheduler uses this function to drain the rosync events of the
  * current time. The ctim object is still in the event queue, because
@@ -1050,6 +1054,7 @@ extern void vpiNextSimTime(void);
  */
 static void run_rosync(struct event_time_s*ctim)
 {
+      sim_at_rosync = true;
       while (ctim->rosync) {
 	    struct event_s*cur = ctim->rosync->next;
 	    if (cur->next == cur) {
@@ -1061,6 +1066,7 @@ static void run_rosync(struct event_time_s*ctim)
 	    cur->run_run();
 	    delete cur;
       }
+      sim_at_rosync = false;
 
       while (ctim->del_thr) {
 	    struct event_s*cur = ctim->del_thr->next;
diff --git a/vvp/schedule.h b/vvp/schedule.h
index cd0deb79c..f47057060 100644
--- a/vvp/schedule.h
+++ b/vvp/schedule.h
@@ -164,6 +164,13 @@ extern void schedule_simulate(void);
  */
 extern vvp_time64_t schedule_simtime(void);
 
+/*
+ * Indicate that the simulator is running the rosync callbacks. This is
+ * used to prevent the callbacks from performing any write operations
+ * in the current simulation time slot.
+ */
+extern bool schedule_at_rosync(void);
+
 /*
  * This function is the equivalent of the $finish system task. It
  * tells the simulator that simulation is done, the current thread
diff --git a/vvp/vpi_priv.cc b/vvp/vpi_priv.cc
index b914a73eb..c77ebee37 100644
--- a/vvp/vpi_priv.cc
+++ b/vvp/vpi_priv.cc
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008-2015 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2008-2016 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -1067,13 +1067,13 @@ vpiHandle vpi_put_value(vpiHandle obj, s_vpi_value*vp,
 	    vvp_time64_t dly;
 	    int scale;
 
-            if (vpi_get(vpiAutomatic, obj)) {
-                  fprintf(stderr, "vpi error: cannot put a value with "
-                                  "a delay on automatically allocated "
-                                  "variable '%s'\n",
-                                  vpi_get_str(vpiName, obj));
-                  return 0;
-            }
+	    if (vpi_get(vpiAutomatic, obj)) {
+		  fprintf(stderr, "VPI error: cannot put a value with "
+				  "a delay on automatically allocated "
+				  "variable '%s'.\n",
+				  vpi_get_str(vpiName, obj));
+		  return 0;
+	    }
 
 	    assert(when != 0);
 
@@ -1095,6 +1095,13 @@ vpiHandle vpi_put_value(vpiHandle obj, s_vpi_value*vp,
 		  break;
 	    }
 
+	    if ((dly == 0) && schedule_at_rosync()) {
+		  fprintf(stderr, "VPI error: attempted to put a value to "
+				  "variable '%s' during a read-only synch "
+				  "callback.\n", vpi_get_str(vpiName, obj));
+		  return 0;
+	    }
+
 	    vpip_put_value_event*put = new vpip_put_value_event;
 	    put->handle = obj;
 	    put->value = *vp;
@@ -1132,6 +1139,13 @@ vpiHandle vpi_put_value(vpiHandle obj, s_vpi_value*vp,
 	    return 0;
       }
 
+      if (schedule_at_rosync()) {
+            fprintf(stderr, "VPI error: attempted to put a value to "
+			    "variable '%s' during a read-only synch "
+			    "callback.\n", vpi_get_str(vpiName, obj));
+            return 0;
+      }
+
       obj->vpi_put_value(vp, flags);
 
       return 0;