diff --git a/modbus/mb_controller/common/include/esp_modbus_master.h b/modbus/mb_controller/common/include/esp_modbus_master.h
index 2558df3..75349f3 100644
--- a/modbus/mb_controller/common/include/esp_modbus_master.h
+++ b/modbus/mb_controller/common/include/esp_modbus_master.h
@@ -133,10 +133,11 @@ typedef enum {
     PARAM_SIZE_FLOAT = 0x04,                /*!< Float 32 size */
     PARAM_SIZE_ASCII = 0x08,                /*!< ASCII size default*/
     PARAM_SIZE_ASCII24 = 0x18,              /*!< ASCII24 size */
+    PARAM_SIZE_ASCII_MAX = 0xFF,            /*!< ASCII maximum size */
     PARAM_SIZE_I64 = 0x08,                  /*!< Signed integer 64 size */
     PARAM_SIZE_U64 = 0x08,                  /*!< Unsigned integer 64 size */
     PARAM_SIZE_DOUBLE = 0x08,               /*!< Double 64 size */
-    PARAM_MAX_SIZE
+    PARAM_SIZE_MAX = 0x7FFF
 } mb_descr_size_t;
 
 /**
@@ -169,7 +170,7 @@ typedef struct {
     uint16_t            mb_size;            /*!< Size of mb parameter in registers */
     uint32_t            param_offset;       /*!< Parameter name (OFFSET in the parameter structure or address of instance) */
     mb_descr_type_t     param_type;         /*!< Float, U8, U16, U32, ASCII, etc. */
-    mb_descr_size_t     param_size;         /*!< Number of bytes in the parameter. */
+    size_t              param_size;         /*!< Number of bytes in the parameter. */
     mb_parameter_opt_t  param_opts;         /*!< Parameter options used to check limits and etc. */
     mb_param_perms_t    access;             /*!< Access permissions based on mode */
 } mb_parameter_descriptor_t;
diff --git a/test_apps/cpp/modbus_serial_cpp_test/main/serial_test.cpp b/test_apps/cpp/modbus_serial_cpp_test/main/serial_test.cpp
index 39f0a4f..66b3261 100644
--- a/test_apps/cpp/modbus_serial_cpp_test/main/serial_test.cpp
+++ b/test_apps/cpp/modbus_serial_cpp_test/main/serial_test.cpp
@@ -16,7 +16,8 @@ enum {
 
 // Enumeration of all supported CIDs for device (used in parameter definition table)
 enum {
-    CID_DEV_REG0 = 0
+    CID_DEV_REG0 = 0,
+    CID_DEV_STRING
 };
 
 #define STR(fieldname) ((const char*)( fieldname ))
@@ -26,11 +27,13 @@ static void *pmaster_handle = nullptr;
 static void *pslave_handle = nullptr;
 
 // Example Data (Object) Dictionary for Modbus parameters
-const mb_parameter_descriptor_t dummy_dict[] = {
+constexpr mb_parameter_descriptor_t dummy_dict[] = {
     // CID, Name, Units, Modbus addr, register type, Modbus Reg Start Addr, Modbus Reg read length, 
     // Instance offset (NA), Instance type, Instance length (bytes), Options (NA), Permissions
     { CID_DEV_REG0, STR("MB_hold_reg-0"), STR("Data"), MB_DEVICE_ADDR1, MB_PARAM_HOLDING, 0, 1,
                     0, PARAM_TYPE_U16, PARAM_SIZE_U16, OPTS( 0,0,0 ), PAR_PERMS_READ_WRITE_TRIGGER },
+    { CID_DEV_STRING, STR("MB_hold_reg string"), STR("String"), MB_DEVICE_ADDR1, MB_PARAM_HOLDING, 2, 30,
+                    0, PARAM_TYPE_ASCII, 60, OPTS( 0,0,0 ), PAR_PERMS_READ_WRITE_TRIGGER }
 };
 
 // Calculate number of parameters in the table