diff --git a/src/include/ngspice/sharedspice.h b/src/include/ngspice/sharedspice.h
index ca8f4b71e..efdc6ff60 100644
--- a/src/include/ngspice/sharedspice.h
+++ b/src/include/ngspice/sharedspice.h
@@ -209,6 +209,18 @@ typedef int (BGThreadRunning)(bool, void*);
    void*       return pointer received from caller
 */
 
+/* callback functions
+addresses received from caller with ngSpice_Init_Sync() function
+*/
+
+/* ask for VSRC EXTERNAL value */
+typedef int (GetVSRCData)(double*, double, char*, void*);
+/*
+   double*     return voltage value
+   double      actual time
+   char*       node name
+   void*       return pointer received from caller
+*/
 
 /* ngspice initialization, 
 printfcn: pointer to callback function for reading printf, fprintf
@@ -223,6 +235,12 @@ IMPEXP
 int  ngSpice_Init(SendChar* printfcn, SendStat* statfcn, ControlledExit* ngexit, 
                   SendData* sdata, SendInitData* sinitdata, BGThreadRunning* bgtrun, void* userData);
 
+/* initialization of synchronizing functions 
+vsrcdat: pointer to callback function for retrieving a voltage source value
+ident: pointer to integer unique to this shared library (defaults to 0)
+*/
+IMPEXP
+int  ngSpice_Init_Sync(GetVSRCData* vsrcdat, int* ident, void* userData);
 
 /* Caller may send ngspice commands to ngspice.dll.
 Commands are executed immediately */
@@ -263,6 +281,10 @@ char** ngSpice_AllVecs(char* plotname);
 IMPEXP
 bool ngSpice_running(void);
 
+/* set a breakpoint in ngspice */
+IMPEXP
+bool ngSpice_SetBkpt(double time);
+
 
 #ifdef __cplusplus
 }
diff --git a/src/sharedspice.c b/src/sharedspice.c
index a6ba69309..adc6579ea 100644
--- a/src/sharedspice.c
+++ b/src/sharedspice.c
@@ -198,6 +198,7 @@ int  sh_vfprintf(FILE *f, const char *fmt, va_list args);
 int sh_fputsll(const char *input, FILE* outf);
 
 int sh_ExecutePerLoop(void);
+double getvsrcval(double, char*);
 int sh_vecinit(runDesc *run);
 
 void shared_exit(int status);
@@ -206,6 +207,8 @@ void sighandler_sharedspice(int num);
 
 void wl_delete_first(wordlist **wlstart, wordlist **wlend);
 
+int add_bkpt(void);
+
 #if !defined(low_latency)
 static char* outstorage(char*, bool);
 static void printsend(void);
@@ -220,6 +223,7 @@ static ControlledExit* ngexit;
 static SendData* datfcn;
 static SendInitData* datinitfcn;
 static BGThreadRunning* bgtr;
+static GetVSRCData* getvdat;
 static pvector_info myvec = NULL;
 char **allvecs = NULL;
 char **allplots = NULL;
@@ -228,6 +232,7 @@ static bool nostatuswanted = FALSE;
 static bool nodatawanted = FALSE;
 static bool nodatainitwanted = FALSE;
 static bool nobgtrwanted = FALSE;
+static bool wantvdat = FALSE;
 static bool immediate = FALSE;
 static bool coquit = FALSE;
 static jmp_buf errbufm, errbufc;
@@ -247,6 +252,9 @@ mutexType fputsMutex;
 static bool is_initialized = FALSE;
 static char* no_init = "Error: ngspice is not initialized!\n   Run ngSpice_Init first";
 
+/* identifier for this ngspice invocation */
+static int ng_ident = 0;
+
 
 /*helper function*//*
 static struct plot *
@@ -499,6 +507,26 @@ ngSpice_running (void)
 }
 #endif
 
+/*  Initialise external voltage source          */
+IMPEXP
+int
+ngSpice_Init_Sync(GetVSRCData* vsrcdat, int* ident, void* userData)
+{
+    getvdat = vsrcdat;
+    /* set userdata, but don't overwrite with NULL */
+    if (userData)
+        userptr = userData;
+    /* set ngspice shared lib identification number */
+    ng_ident = *ident;
+    /* if caller sends NULL, don't try to retrieve voltage */
+    if (getvdat) {
+        wantvdat = TRUE;
+        return 0;
+    }
+    return 1;
+}
+
+
 /*  Initialise ngspice and setup native methods          */
 IMPEXP
 int
@@ -826,6 +854,65 @@ char** ngSpice_AllVecs(char* plotname)
 }
 
 
+static double *bkpttmp = NULL;
+static int bkpttmpsize = 0;
+
+/* set a breakpoint in ngspice */
+IMPEXP
+bool ngSpice_SetBkpt(double time)
+{
+    int error;
+    CKTcircuit *ckt = NULL;
+
+    if (!ft_curckt || !ft_curckt->ci_ckt) {
+        fprintf(cp_err, "Error: no circuit loaded.\n");
+        return(FALSE);
+    }
+
+    ckt = ft_curckt->ci_ckt;
+    if (ckt->CKTbreakSize == 0) {
+    /* breakpoints have not yet been set up, so store here preliminary 
+    and add with fcn add_bkpt() called from DCTran()*/
+        if (bkpttmp == NULL) {
+            bkpttmp = TMALLOC(double, bkpttmpsize + 1);
+            if(bkpttmp == NULL) 
+                return(FALSE);
+            bkpttmpsize++;
+        }
+        else {
+            bkpttmp = TREALLOC(double, bkpttmp, bkpttmpsize + 1);
+            bkpttmpsize++;
+        }
+        bkpttmp[bkpttmpsize-1] = time;
+        error = 0;
+    }
+    else
+        error = CKTsetBreak(ckt, time);
+    if(error)
+        return(FALSE);
+    return(TRUE);
+}
+
+/* add the preliminary breakpoints to the list.
+   called from dctran.c */
+int
+add_bkpt(void)
+{ 
+    int i;
+    int error = 0;
+    CKTcircuit *ckt =  ft_curckt->ci_ckt;
+
+    if((bkpttmp) && (bkpttmpsize > 0)) {
+        for (i = 0; i < bkpttmpsize; i++)
+            error = CKTsetBreak(ckt, bkpttmp[i]);
+        FREE(bkpttmp);
+        bkpttmpsize = 0;
+    }
+    if(error)
+        return(error);
+    return(OK);
+}
+
 /*------------------------------------------------------*/
 /* Redefine the vfprintf() functions for callback       */
 /*------------------------------------------------------*/
@@ -1612,3 +1699,18 @@ int sh_vecinit(runDesc *run)
     return 0;
 }
 
+/* issue callback to request external voltage data for source vname*/
+double getvsrcval(double time, char* vname)
+{
+    double vval;
+    if (!wantvdat) {
+        fprintf(stderr, "Error: No callback supplied for source %s\n", vname);
+        shared_exit(EXIT_BAD);
+        return(EXIT_BAD);
+    }
+    else {
+        /* callback fcn */
+        getvdat(&vval, time, vname, userptr);
+        return vval;
+    }
+}