diff --git a/include/ecrt.h b/include/ecrt.h
index 24c82eb4..e2e4f0bc 100644
--- a/include/ecrt.h
+++ b/include/ecrt.h
@@ -2213,7 +2213,7 @@ void ecrt_reg_request_read(
  * \param data EtherCAT data pointer
  * \return EtherCAT data value
  */
-float ecrt_read_real(void *data);
+float ecrt_read_real(const void *data);
 
 /** Read a 32-bit floating-point value from EtherCAT data.
  *
@@ -2227,7 +2227,7 @@ float ecrt_read_real(void *data);
  * \param data EtherCAT data pointer
  * \return EtherCAT data value
  */
-double ecrt_read_lreal(void *data);
+double ecrt_read_lreal(const void *data);
 
 /** Read a 64-bit floating-point value from EtherCAT data.
  *
diff --git a/lib/common.c b/lib/common.c
index d04b5ebb..a70f3559 100644
--- a/lib/common.c
+++ b/lib/common.c
@@ -138,7 +138,7 @@ void ecrt_release_master(ec_master_t *master)
 
 /*****************************************************************************/
 
-float ecrt_read_real(void *data)
+float ecrt_read_real(const void *data)
 {
     uint32_t raw = EC_READ_U32(data);
     return *(float *) (const void *) &raw;
@@ -146,7 +146,7 @@ float ecrt_read_real(void *data)
 
 /*****************************************************************************/
 
-double ecrt_read_lreal(void *data)
+double ecrt_read_lreal(const void *data)
 {
     uint64_t raw = EC_READ_U64(data);
     return *(double *) (const void *) &raw;