Here is the full version of the program with clock daisy-chaining. Before switching to the external clock, it checks if the clock really is there (by reading an internal scaler), and only then enables it. Note that the code also works without clock daisy-chaining. But without clock daisy-chaining your have some 400 ps time resolution between boards, and with clock daisy-chaining you get some 60 ps.

/********************************************************************\

  Name:         drs_exam_multi.cpp
  Created by:   Stefan Ritt

  Contents:     Simple example application to read out a several
                DRS4 evaluation board in daisy-chain mode

  $Id: drs_exam_multi.cpp 21509 2014-10-15 10:11:36Z ritt $

\********************************************************************/

#include <math.h>

#ifdef _MSC_VER

#include <windows.h>

#elif defined(OS_LINUX)

#define O_BINARY 0

#include <unistd.h>
#include <ctype.h>
#include <sys/ioctl.h>
#include <errno.h>

#define DIR_SEPARATOR '/'

#endif

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include "strlcpy.h"
#include "DRS.h"

/*------------------------------------------------------------------*/

int main()
{
   int i, j, k;
   DRS *drs;
   DRSBoard *b, *mb;
   float time_array[8][1024];
   float wave_array[8][1024];
   FILE  *f;

   /* do initial scan, sort boards accordning to their serial numbers */
   drs = new DRS();
   drs->SortBoards();

   /* show any found board(s) */
   for (i=0 ; i<drs->GetNumberOfBoards() ; i++) {
      b = drs->GetBoard(i);
      printf("Found DRS4 evaluation board, serial #%d, firmware revision %d\n", 
         b->GetBoardSerialNumber(), b->GetFirmwareVersion());
      if (b->GetBoardType() < 8) {
         printf("Found pre-V4 board, aborting\n");
         return 0;
      }
   }
   
   /* exit if no board found */
   if (drs->GetNumberOfBoards() == 0) {
      printf("No DRS4 evaluation board found\n");
      return 0;
   }

   /* exit if only one board found */
   if (drs->GetNumberOfBoards() == 1) {
      printf("Only one DRS4 evaluation board found, please use drs_exam program\n");
      return 0;
   }
   
   /* use first board with highest serial number as the master board */
   mb = drs->GetBoard(0);
   
   /* common configuration for all boards */
   for (i=0 ; i<drs->GetNumberOfBoards() ; i++) {
      b = drs->GetBoard(i);
      
      /* initialize board */
      b->Init();
      
      /* select external reference clock for slave modules */
      /* NOTE: this only works if the clock chain is connected */
      if (i > 0) {
         if (b->GetFirmwareVersion() >= 21260) { // this only works with recent firmware versions
            if (b->GetScaler(5) > 300000)        // check if external clock is connected
               b->SetRefclk(true);               // switch to external reference clock
         }
      }
      
      /* set sampling frequency */
      b->SetFrequency(5, true);
      
      /* set input range to -0.5V ... +0.5V */
      b->SetInputRange(0);

      /* enable hardware trigger */
      b->EnableTrigger(1, 0);
      
      if (i == 0) {
         /* master board: enable hardware trigger on CH1 at 50 mV positive edge */
         b->SetTranspMode(1);
         b->SetTriggerSource(1<<0);        // set CH1 as source
         b->SetTriggerLevel(0.05);         // 50 mV
         b->SetTriggerPolarity(false);     // positive edge
         b->SetTriggerDelayNs(0);          // zero ns trigger delay
      } else {
         /* slave boards: enable hardware trigger on Trigger IN */
         b->SetTriggerSource(1<<4);        // set Trigger IN as source
         b->SetTriggerPolarity(false);     // positive edge
      }
   }

   /* open file to save waveforms */
   f = fopen("data.txt", "w");
   if (f == NULL) {
      perror("ERROR: Cannot open file \"data.txt\"");
      return 1;
   }
   
   /* repeat ten times */
   for (i=0 ; i<10 ; i++) {

      /* start boards (activate domino wave), master is last */
      for (j=drs->GetNumberOfBoards()-1 ; j>=0 ; j--)
         drs->GetBoard(j)->StartDomino();

      /* wait for trigger on master board */
      printf("Waiting for trigger...");
      fflush(stdout);
      while (mb->IsBusy());

      fprintf(f, "Event #%d =====================================================\n", j);

      for (j=0 ; j<drs->GetNumberOfBoards() ; j++) {
         b = drs->GetBoard(j);
         if (b->IsBusy()) {
            i--; /* skip that event, must be some fake trigger */
            break;
         }
         
         /* read all waveforms from all boards */
         b->TransferWaves(0, 8);
         
         for (k=0 ; k<4 ; k++) {
            /* read time (X) array in ns */
            b->GetTime(0, k*2, b->GetTriggerCell(0), time_array[k]);

            /* decode waveform (Y) arrays in mV */
            b->GetWave(0, k*2, wave_array[k]);
         }

         /* Save waveform: X=time_array[i], Channel_n=wave_array[n][i] */
         fprintf(f, "Board #%d ---------------------------------------------------\n t1[ns]  u1[mV]  t2[ns]  u2[mV]  t3[ns]  u3[mV]  t4[ns]  u4[mV]\n", b->GetBoardSerialNumber());
         for (k=0 ; k<1024 ; k++)
            fprintf(f, "%7.3f %7.1f %7.3f %7.1f %7.3f %7.1f %7.3f %7.1f\n",
                    time_array[0][k], wave_array[0][k],
                    time_array[1][k], wave_array[1][k],
                    time_array[2][k], wave_array[2][k],
                    time_array[3][k], wave_array[3][k]);
      }

      /* print some progress indication */
      printf("\rEvent #%d read successfully\n", i);
   }

   fclose(f);
   
   printf("Program finished.\n");

   /* delete DRS object -> close USB connection */
   delete drs;
}