diff --git a/master/domain.c b/master/domain.c
index 843e6787..e832c79f 100644
--- a/master/domain.c
+++ b/master/domain.c
@@ -266,22 +266,12 @@ int ec_domain_reg_pdo_range(ec_domain_t *domain, /**< EtherCAT domain */
 {
     ec_data_reg_t *data_reg;
     ec_sii_sync_t *sync;
-    unsigned int sync_index;
     uint16_t sync_length;
 
-    switch (dir) { // FIXME
-        case EC_DIR_OUTPUT: sync_index = 2; break;
-        case EC_DIR_INPUT:  sync_index = 3; break;
-        default:
-            EC_ERR("Invalid direction!\n");
-            return -1;
-    }
-
-    if (sync_index >= slave->sii_sync_count) {
+    if (!(sync = ec_slave_get_pdo_sync(slave, dir))) {
         EC_ERR("No sync manager found for PDO range.\n");
         return -1;
     }
-    sync = &slave->sii_syncs[sync_index];
 
     // Allocate memory for data registration object
     if (!(data_reg =
diff --git a/master/slave.c b/master/slave.c
index 0213e11d..5e2e7932 100644
--- a/master/slave.c
+++ b/master/slave.c
@@ -1113,6 +1113,36 @@ uint16_t ec_slave_calc_sync_size(const ec_slave_t *slave,
 
 /*****************************************************************************/
 
+/**
+ */
+
+ec_sii_sync_t *ec_slave_get_pdo_sync(
+        ec_slave_t *slave, /**< EtherCAT slave */
+        ec_direction_t dir /**< input or output */
+        )
+{
+    unsigned int sync_index;
+
+    switch (dir) {
+        case EC_DIR_OUTPUT: sync_index = 0; break;
+        case EC_DIR_INPUT:  sync_index = 1; break;
+        default:
+            EC_ERR("Invalid direction!\n");
+            return NULL;
+    }
+
+    if (slave->sii_mailbox_protocols) sync_index += 2;
+
+    if (sync_index >= slave->sii_sync_count) {
+        EC_ERR("No appropriate sync manager found.\n");
+        return NULL;
+    }
+
+    return &slave->sii_syncs[sync_index];
+}
+
+/*****************************************************************************/
+
 /**
    Initializes a sync manager configuration page with EEPROM data.
    The referenced memory (\a data) must be at least EC_SYNC_SIZE bytes.
diff --git a/master/slave.h b/master/slave.h
index c53ec8d8..578757f5 100644
--- a/master/slave.h
+++ b/master/slave.h
@@ -284,6 +284,7 @@ int ec_slave_fetch_pdo(ec_slave_t *, const uint8_t *, size_t,
 int ec_slave_locate_string(ec_slave_t *, unsigned int, char **);
 
 // misc.
+ec_sii_sync_t *ec_slave_get_pdo_sync(ec_slave_t *, ec_direction_t); 
 void ec_slave_sync_config(const ec_slave_t *, const ec_sii_sync_t *,
         uint8_t *);
 void ec_slave_fmmu_config(const ec_slave_t *, const ec_fmmu_t *, uint8_t *);