diff --git a/master/fsm_sii.c b/master/fsm_sii.c
index 044d978e..270f5670 100644
--- a/master/fsm_sii.c
+++ b/master/fsm_sii.c
@@ -105,7 +105,7 @@ void ec_fsm_sii_read(ec_fsm_sii_t *fsm, /**< finite state machine */
 void ec_fsm_sii_write(ec_fsm_sii_t *fsm, /**< finite state machine */
                       ec_slave_t *slave, /**< slave to read from */
                       uint16_t offset, /**< offset to read from */
-                      uint16_t *value, /**< pointer to 2 bytes of data */
+                      const uint16_t *value, /**< pointer to 2 bytes of data */
                       ec_fsm_sii_addressing_t mode /**< addressing scheme */
                       )
 {
diff --git a/master/fsm_sii.h b/master/fsm_sii.h
index 977b032d..96f36ba7 100644
--- a/master/fsm_sii.h
+++ b/master/fsm_sii.h
@@ -84,8 +84,8 @@ void ec_fsm_sii_clear(ec_fsm_sii_t *);
 
 void ec_fsm_sii_read(ec_fsm_sii_t *, ec_slave_t *,
                      uint16_t, ec_fsm_sii_addressing_t);
-void ec_fsm_sii_write(ec_fsm_sii_t *, ec_slave_t *, uint16_t, uint16_t *,
-                      ec_fsm_sii_addressing_t);
+void ec_fsm_sii_write(ec_fsm_sii_t *, ec_slave_t *, uint16_t,
+        const uint16_t *, ec_fsm_sii_addressing_t);
 
 int ec_fsm_sii_exec(ec_fsm_sii_t *);
 int ec_fsm_sii_success(ec_fsm_sii_t *);