diff --git a/backend/Makefile.in b/backend/Makefile.in
index b286f47e8..6748e59a0 100644
--- a/backend/Makefile.in
+++ b/backend/Makefile.in
@@ -260,6 +260,7 @@ libsane-sm3600.la: ../sanei/sanei_constrain_value.lo
 libsane-snapscan.la: ../sanei/sanei_config2.lo
 libsane-snapscan.la: ../sanei/sanei_constrain_value.lo
 libsane-snapscan.la: ../sanei/sanei_scsi.lo
+libsane-snapscan.la: ../sanei/sanei_usb.lo
 libsane-sp15c.la: ../sanei/sanei_config2.lo
 libsane-sp15c.la: ../sanei/sanei_constrain_value.lo
 libsane-sp15c.la: ../sanei/sanei_scsi.lo
diff --git a/backend/snapscan-usb.c b/backend/snapscan-usb.c
index eca1e3b36..df2440866 100644
--- a/backend/snapscan-usb.c
+++ b/backend/snapscan-usb.c
@@ -125,12 +125,26 @@ static SANE_Status atomic_usb_cmd(int fd, const void *src, size_t src_size,
 
 static SANE_Status snapscani_usb_open(const char *dev, int *fdp)
 {
-    return usb_open(dev,fdp);
+    static const char me[] = "snapscani_usb_open";
+
+    DBG (DL_CALL_TRACE, "%s(%s)\n", me, dev);
+
+    if((sem_id = semget( ftok(dev,0x1234), 1, IPC_CREAT | 0660 )) == -1) {
+        DBG (DL_MAJOR_ERROR, "%s: Can't get semaphore\n", me);
+        return SANE_STATUS_INVAL;
+    }
+    semop(sem_id, &sem_signal, 1);
+    sanei_usb_init();
+    return sanei_usb_open(dev, fdp);
 }
 
 
 static void snapscani_usb_close(int fd) {
-    usb_close(fd);
+    static const char me[] = "snapscani_usb_close";
+
+    DBG (DL_CALL_TRACE, "%s(%d)\n", me, fd);
+    semctl(sem_id, 0, IPC_RMID, 0);
+    sanei_usb_close(fd);
 }
 
 
@@ -170,33 +184,6 @@ static char *usb_debug_data(char *str,const char *data, int len) {
     return str;
 }
 
-
-static SANE_Status usb_open(const char *dev, int *fdp)
-{
-static const char me[] = "usb_open";
-
-DBG (DL_CALL_TRACE, "%s(%s)\n", me, dev);
-
-    if((sem_id = semget( ftok(dev,0x1234), 1, IPC_CREAT | 0660 )) == -1) {
-        DBG (DL_MAJOR_ERROR, "%s: Can't get semaphore\n", me);
-        return SANE_STATUS_INVAL;
-    }
-    semop(sem_id, &sem_signal, 1);
-
-    *fdp = open(dev, O_RDWR);
-    if( *fdp < 0)
-        return SANE_STATUS_INVAL;
-    return SANE_STATUS_GOOD;
-}
-
-static void usb_close(int fd) {
-    static const char me[] = "usb_close";
-
-    DBG (DL_CALL_TRACE, "%s(%d)\n", me, fd);
-    semctl(sem_id, 0, IPC_RMID, 0);
-    close(fd);
-}
-
 /*
 static int usb_status(char *status_buf) {
     int status;
@@ -419,6 +406,9 @@ static void dequeue_bq()
 }
 /*
  * $Log$
+ * Revision 1.4  2001/10/27 09:08:13  oliverschwartz
+ * Check USB vendor IDs to avoid hanging scanners, fix bug in dither matrix computation
+ *
  * Revision 1.3  2001/10/10 07:30:06  oliverschwartz
  * fix compiler warnings
  *
diff --git a/backend/snapscan-usb.h b/backend/snapscan-usb.h
index 3a77f95de..af56a237a 100644
--- a/backend/snapscan-usb.h
+++ b/backend/snapscan-usb.h
@@ -75,10 +75,6 @@ static int is_queueable(const char *src);
 
 static SANE_Status atomic_usb_cmd(int fd, const void *src, size_t src_size,
                     void *dst, size_t * dst_size);
-static SANE_Status usb_open(const char *dev, int *fdp);
-
-static void usb_close(int fd);
-
 static SANE_Status usb_cmd(int fd, const void *src, size_t src_size,
                     void *dst, size_t * dst_size);
 
@@ -86,6 +82,9 @@ static SANE_Status usb_cmd(int fd, const void *src, size_t src_size,
 
 /*
  * $Log$
+ * Revision 1.3  2001/10/27 09:08:14  oliverschwartz
+ * Check USB vendor IDs to avoid hanging scanners, fix bug in dither matrix computation
+ *
  * Revision 1.2  2001/10/09 09:45:15  oliverschwartz
  * update snapscan to snapshot 20011008
  *
diff --git a/backend/snapscan.c b/backend/snapscan.c
index 17c51c4c4..3c78f503b 100644
--- a/backend/snapscan.c
+++ b/backend/snapscan.c
@@ -70,6 +70,7 @@
 #include "../include/sane/sane.h"
 #include "../include/sane/sanei.h"
 #include "../include/sane/sanei_scsi.h"
+#include "../include/sane/sanei_usb.h"
 
 #ifndef PATH_MAX
 #define PATH_MAX        1024
@@ -941,8 +942,15 @@ static void mkDn (u_char *Dn, u_char *Dn2, unsigned n)
     unsigned f;
 
     /* compute 4*D(n/2) */
+    /* Oliver Schwartz, 27 Oct. 2001: Changed code from
+          tmp[i] = (u_char) (4 * Dn2[i]);
+       to
+          tmp[i] = (u_char) (4 * Dn2[i/4]);
+       to avoid illegal indices in Dn2. Don't know if this
+       is the desired algorithm.
+    */
     for (i = 0;  i < nsq;  i++)
-        tmp[i] = (u_char) (4 * Dn2[i]);
+        tmp[i] = (u_char) (4 * Dn2[i/4]);
 
     /* now the dither matrix */
     for (r = 0, imin = 0, f = 0;  r < 2;  r++, imin += n2)
@@ -993,8 +1001,10 @@ static SANE_Status add_device (SANE_String_Const name)
     SnapScan_Device *pd;
     SnapScan_Model model_num = UNKNOWN;
     SnapScan_Bus bus_type = UNKNOWN_BUS;
+    SANE_Word vendor_id, product_id;
     int i;
     int supported_vendor = 0;
+    int supported_usb_vendor = 0;
     char vendor[8];
     char model[17];
 
@@ -1007,7 +1017,7 @@ static SANE_Status add_device (SANE_String_Const name)
     vendor[0] = model[0] = '\0';
 
     if((strstr (name, "usb")) || (strstr (name, "USB")))
-      {
+    {
         DBG (DL_VERBOSE, "%s: Detected (kind of) an USB device\n", me);
 
         bus_type = USB;
@@ -1018,31 +1028,52 @@ static SANE_Status add_device (SANE_String_Const name)
         }
         status = snapscani_usb_open (name, &fd);
         if (status != SANE_STATUS_GOOD)
-          {
+        {
             DBG (DL_MAJOR_ERROR,
                  "%s: error opening device %s: %s\n",
                  me,
                  name,
                  sane_strstatus (status));
             return status;
-          }
-      }
+        }
+        if (sanei_usb_get_vendor_product(fd, &vendor_id, &product_id) ==
+            SANE_STATUS_GOOD)
+        {
+            /* check for known USB vendors to avoid hanging scanners by
+               inquiry-command.
+            */
+            DBG(DL_INFO, "%s: Checking if 0x%04x is a supported USB vendor ID\n",
+                me, vendor_id);
+            for (i = 0; i < known_usb_vendor_ids; i++) {
+                if (vendor_id == usb_vendor_ids[i]) {
+                    supported_usb_vendor = 1;
+                }
+            }
+            if (!supported_usb_vendor) {
+                DBG(DL_MINOR_ERROR,
+                    "%s: USB vendor ID 0x%04x is currently NOT supported by the snapscan backend.\n",
+                    me, vendor_id);
+                snapscani_usb_close (fd);
+                return SANE_STATUS_INVAL;
+            }
+        }
+    }
     else
-      {
+    {
         DBG (DL_VERBOSE, "%s: Detected (kind of) a SCSI device\n", me);
         bus_type = SCSI;
 
         status = sanei_scsi_open (name, &fd, sense_handler, NULL);
         if (status != SANE_STATUS_GOOD)
-          {
+        {
             DBG (DL_MAJOR_ERROR,
                  "%s: error opening device %s: %s\n",
                  me,
                  name,
                  sane_strstatus (status));
             return status;
-          }
-      }
+        }
+    }
 
     /* check that the device is legitimate */
     if ((status = mini_inquiry (bus_type, fd, vendor, model)) != SANE_STATUS_GOOD)
@@ -1053,13 +1084,13 @@ static SANE_Status add_device (SANE_String_Const name)
              sane_strstatus (status));
 
         if(bus_type == SCSI)
-          {
+        {
             sanei_scsi_close (fd);
-          }
+        }
         else if(bus_type == USB)
-          {
+        {
             snapscani_usb_close (fd);
-          }
+        }
 
         return status;
     }
@@ -3092,6 +3123,9 @@ SANE_Status sane_get_select_fd (SANE_Handle h, SANE_Int * fd)
 
 /*
  * $Log$
+ * Revision 1.10  2001/10/27 09:08:14  oliverschwartz
+ * Check USB vendor IDs to avoid hanging scanners, fix bug in dither matrix computation
+ *
  * Revision 1.9  2001/10/25 10:56:39  oliverschwartz
  * Change snapscan backend version number to 1.4.0
  *
diff --git a/backend/snapscan.h b/backend/snapscan.h
index d9120bef7..55c381c15 100644
--- a/backend/snapscan.h
+++ b/backend/snapscan.h
@@ -142,6 +142,15 @@ static char *vendors[] =
 };
 #define known_vendors ((int) (sizeof(vendors)/sizeof(vendors[0])))
 
+static SANE_Word usb_vendor_ids[] =
+{
+    /* USB Vendor IDs */
+    0x06bd,     /* Agfa */
+    0x04a5      /* Acer */
+};
+#define known_usb_vendor_ids ((int) (sizeof(usb_vendor_ids)/sizeof(usb_vendor_ids[0])))
+
+
 typedef enum
 {
     OPT_COUNT = 0,        /* option count */
@@ -312,6 +321,9 @@ struct snapscan_scanner
 
 /*
  * $Log$
+ * Revision 1.7  2001/10/27 09:08:17  oliverschwartz
+ * Check USB vendor IDs to avoid hanging scanners, fix bug in dither matrix computation
+ *
  * Revision 1.6  2001/10/12 21:19:16  oliverschwartz
  * update to snapscan-20011012
  *