nanomodbus.h File Reference

#include <stdbool.h>
#include <stdint.h>
#include <string.h>

Include dependency graph for nanomodbus.h:

Go to the source code of this file.

Data Structures
struct	nmbs_platform_conf
struct	nmbs_callbacks
struct	nmbs_t

Macros
#define	nmbs_error_is_exception(e)   ((e) > 0 && (e) < 5)
#define	NMBS_BITFIELD_MAX   2000
#define	NMBS_BITFIELD_BYTES_MAX   (NMBS_BITFIELD_MAX / 8)
#define	nmbs_bitfield_read(bf, b)   ((bool) ((bf)[(b) >> 3] & (0x1 << ((b) & (8 - 1)))))
#define	nmbs_bitfield_set(bf, b)   (((bf)[(b) >> 3]) = (((bf)[(b) >> 3]) | (0x1 << ((b) & (8 - 1)))))
#define	nmbs_bitfield_unset(bf, b)   (((bf)[(b) >> 3]) = (((bf)[(b) >> 3]) & ~(0x1 << ((b) & (8 - 1)))))
#define	nmbs_bitfield_write(bf, b, v)   ((bf)[(b) >> 3] = ((bf)[(b) >> 3] & ~(1 << ((b) & 7))) | ((v) << ((b) & 7)))
#define	nmbs_bitfield_reset(bf)   memset(bf, 0, sizeof(bf))
#define	NMBS_DEVICE_IDENTIFICATION_STRING_LENGTH   128

Typedefs
typedef enum nmbs_error	nmbs_error
typedef uint8_t	nmbs_bitfield[NMBS_BITFIELD_BYTES_MAX]
typedef uint8_t	nmbs_bitfield_256[32]
typedef enum nmbs_transport	nmbs_transport
typedef struct nmbs_platform_conf	nmbs_platform_conf
typedef struct nmbs_callbacks	nmbs_callbacks
typedef struct nmbs_t	nmbs_t

Enumerations
enum	nmbs_error {   NMBS_ERROR_INVALID_REQUEST = -8 , NMBS_ERROR_INVALID_UNIT_ID = -7 , NMBS_ERROR_INVALID_TCP_MBAP = -6 , NMBS_ERROR_CRC = -5 ,   NMBS_ERROR_TRANSPORT = -4 , NMBS_ERROR_TIMEOUT = -3 , NMBS_ERROR_INVALID_RESPONSE = -2 , NMBS_ERROR_INVALID_ARGUMENT = -1 ,   NMBS_ERROR_NONE = 0 , NMBS_EXCEPTION_ILLEGAL_FUNCTION = 1 , NMBS_EXCEPTION_ILLEGAL_DATA_ADDRESS = 2 , NMBS_EXCEPTION_ILLEGAL_DATA_VALUE = 3 ,   NMBS_EXCEPTION_SERVER_DEVICE_FAILURE = 4 }
enum	nmbs_transport { NMBS_TRANSPORT_RTU = 1 , NMBS_TRANSPORT_TCP = 2 }

Functions
void	nmbs_set_read_timeout (nmbs_t *nmbs, int32_t timeout_ms)
void	nmbs_set_byte_timeout (nmbs_t *nmbs, int32_t timeout_ms)
void	nmbs_platform_conf_create (nmbs_platform_conf *platform_conf)
void	nmbs_set_platform_arg (nmbs_t *nmbs, void *arg)
void	nmbs_callbacks_create (nmbs_callbacks *callbacks)
nmbs_error	nmbs_server_create (nmbs_t *nmbs, uint8_t address_rtu, const nmbs_platform_conf *platform_conf, const nmbs_callbacks *callbacks)
nmbs_error	nmbs_server_poll (nmbs_t *nmbs)
void	nmbs_set_callbacks_arg (nmbs_t *nmbs, void *arg)
nmbs_error	nmbs_client_create (nmbs_t *nmbs, const nmbs_platform_conf *platform_conf)
void	nmbs_set_destination_rtu_address (nmbs_t *nmbs, uint8_t address)
nmbs_error	nmbs_read_coils (nmbs_t *nmbs, uint16_t address, uint16_t quantity, nmbs_bitfield coils_out)
nmbs_error	nmbs_read_discrete_inputs (nmbs_t *nmbs, uint16_t address, uint16_t quantity, nmbs_bitfield inputs_out)
nmbs_error	nmbs_read_holding_registers (nmbs_t *nmbs, uint16_t address, uint16_t quantity, uint16_t *registers_out)
nmbs_error	nmbs_read_input_registers (nmbs_t *nmbs, uint16_t address, uint16_t quantity, uint16_t *registers_out)
nmbs_error	nmbs_write_single_coil (nmbs_t *nmbs, uint16_t address, bool value)
nmbs_error	nmbs_write_single_register (nmbs_t *nmbs, uint16_t address, uint16_t value)
nmbs_error	nmbs_write_multiple_coils (nmbs_t *nmbs, uint16_t address, uint16_t quantity, const nmbs_bitfield coils)
nmbs_error	nmbs_write_multiple_registers (nmbs_t *nmbs, uint16_t address, uint16_t quantity, const uint16_t *registers)
nmbs_error	nmbs_read_file_record (nmbs_t *nmbs, uint16_t file_number, uint16_t record_number, uint16_t *registers, uint16_t count)
nmbs_error	nmbs_write_file_record (nmbs_t *nmbs, uint16_t file_number, uint16_t record_number, const uint16_t *registers, uint16_t count)
nmbs_error	nmbs_read_write_registers (nmbs_t *nmbs, uint16_t read_address, uint16_t read_quantity, uint16_t *registers_out, uint16_t write_address, uint16_t write_quantity, const uint16_t *registers)
nmbs_error	nmbs_read_device_identification_basic (nmbs_t *nmbs, char *vendor_name, char *product_code, char *major_minor_revision, uint8_t buffers_length)
nmbs_error	nmbs_read_device_identification_regular (nmbs_t *nmbs, char *vendor_url, char *product_name, char *model_name, char *user_application_name, uint8_t buffers_length)
nmbs_error	nmbs_read_device_identification_extended (nmbs_t *nmbs, uint8_t object_id_start, uint8_t *ids, char **buffers, uint8_t ids_length, uint8_t buffer_length, uint8_t *objects_count_out)
nmbs_error	nmbs_read_device_identification (nmbs_t *nmbs, uint8_t object_id, char *buffer, uint8_t buffer_length)
nmbs_error	nmbs_send_raw_pdu (nmbs_t *nmbs, uint8_t fc, const uint8_t *data, uint16_t data_len)
nmbs_error	nmbs_receive_raw_pdu_response (nmbs_t *nmbs, uint8_t *data_out, uint8_t data_out_len)
uint16_t	nmbs_crc_calc (const uint8_t *data, uint32_t length, void *arg)
const char *	nmbs_strerror (nmbs_error error)

Macro Definition Documentation

nmbs_bitfield_read

#define nmbs_bitfield_read	(		bf,
			b
	)		((bool) ((bf)[(b) >> 3] & (0x1 << ((b) & (8 - 1)))))

Read a bit from the nmbs_bitfield bf at position b

nmbs_bitfield_reset

#define nmbs_bitfield_reset

(

bf

)

memset(bf, 0, sizeof(bf))

Reset (zero) the whole bitfield

nmbs_bitfield_set

#define nmbs_bitfield_set	(		bf,
			b
	)		(((bf)[(b) >> 3]) = (((bf)[(b) >> 3]) | (0x1 << ((b) & (8 - 1)))))

Set a bit of the nmbs_bitfield bf at position b

nmbs_bitfield_unset

#define nmbs_bitfield_unset	(		bf,
			b
	)		(((bf)[(b) >> 3]) = (((bf)[(b) >> 3]) & ~(0x1 << ((b) & (8 - 1)))))

Reset a bit of the nmbs_bitfield bf at position b

nmbs_bitfield_write

#define nmbs_bitfield_write	(		bf,
			b,
			v
	)		((bf)[(b) >> 3] = ((bf)[(b) >> 3] & ~(1 << ((b) & 7))) | ((v) << ((b) & 7)))

Write value v to the nmbs_bitfield bf at position b

nmbs_error_is_exception

#define nmbs_error_is_exception

(

e

)

((e) > 0 && (e) < 5)

Return whether the nmbs_error is a modbus exception nmbs_error to check

Typedef Documentation

nmbs_bitfield

typedef uint8_t nmbs_bitfield[NMBS_BITFIELD_BYTES_MAX]

Bitfield consisting of 2000 coils/discrete inputs

nmbs_bitfield_256

typedef uint8_t nmbs_bitfield_256[32]

Bitfield consisting of 256 values

nmbs_callbacks

typedef struct nmbs_callbacks nmbs_callbacks

Modbus server request callbacks. Passed to nmbs_server_create().

These methods accept a pointer to arbitrary user data, which is the arg member of the nmbs_platform_conf that was passed to nmbs_server_create together with this struct.

unit_id is the RTU unit ID of the request sender. It is always 0 on TCP.

nmbs_error

typedef enum nmbs_error nmbs_error

nanoMODBUS errors. Values <= 0 are library errors, > 0 are modbus exceptions.

nmbs_platform_conf

typedef struct nmbs_platform_conf nmbs_platform_conf

nanoMODBUS platform configuration struct. Passed to nmbs_server_create() and nmbs_client_create().

read() and write() are the platform-specific methods that read/write data to/from a serial port or a TCP connection.

Both methods should block until either:

• count bytes of data are read/written
• the byte timeout, with byte_timeout_ms >= 0, expires

A value < 0 for byte_timeout_ms means infinite timeout. With a value == 0 for byte_timeout_ms, the method should read/write once in a non-blocking fashion and return immediately.

Their return value should be the number of bytes actually read/written, or < 0 in case of error. A return value between 0 and count - 1 will be treated as if a timeout occurred on the transport side. All other values will be treated as transport errors.

Additionally, an optional crc_calc() function can be defined to override the default nanoMODBUS CRC calculation function.

These methods accept a pointer to arbitrary user-data, which is the arg member of this struct. After the creation of an instance it can be changed with nmbs_set_platform_arg().

nmbs_t

typedef struct nmbs_t nmbs_t

nanoMODBUS client/server instance type. All struct members are to be considered private, it is not advisable to read/write them directly.

nmbs_transport

typedef enum nmbs_transport nmbs_transport

Modbus transport type.

Enumeration Type Documentation

nmbs_error

enum nmbs_error

nanoMODBUS errors. Values <= 0 are library errors, > 0 are modbus exceptions.

Enumerator
NMBS_ERROR_INVALID_REQUEST	Received invalid request from client
NMBS_ERROR_INVALID_UNIT_ID	Received invalid unit ID in response from server
NMBS_ERROR_INVALID_TCP_MBAP	Received invalid TCP MBAP
NMBS_ERROR_CRC	Received invalid CRC
NMBS_ERROR_TRANSPORT	Transport error
NMBS_ERROR_TIMEOUT	Read/write timeout occurred
NMBS_ERROR_INVALID_RESPONSE	Received invalid response from server
NMBS_ERROR_INVALID_ARGUMENT	Invalid argument provided
NMBS_ERROR_NONE	No error
NMBS_EXCEPTION_ILLEGAL_FUNCTION	Modbus exception 1
NMBS_EXCEPTION_ILLEGAL_DATA_ADDRESS	Modbus exception 2
NMBS_EXCEPTION_ILLEGAL_DATA_VALUE	Modbus exception 3
NMBS_EXCEPTION_SERVER_DEVICE_FAILURE	Modbus exception 4

nmbs_transport

enum nmbs_transport

Modbus transport type.

Function Documentation

nmbs_callbacks_create()

void nmbs_callbacks_create

(

nmbs_callbacks *

callbacks

)

Create a new nmbs_callbacks struct.

Parameters

callbacks

pointer to the nmbs_callbacks instance

nmbs_client_create()

nmbs_error nmbs_client_create	(	nmbs_t *	nmbs,
		const nmbs_platform_conf *	platform_conf
	)

Create a new Modbus client.

Parameters

nmbs	pointer to the nmbs_t instance where the client will be created.
platform_conf	nmbs_platform_conf struct with platform configuration. It may be discarded after calling this method.

Returns

NMBS_ERROR_NONE if successful, NMBS_ERROR_INVALID_ARGUMENT otherwise.

nmbs_crc_calc()

uint16_t nmbs_crc_calc	(	const uint8_t *	data,
		uint32_t	length,
		void *	arg
	)

Calculate the Modbus CRC of some data.

Parameters

data	Data
length	Length of the data

nmbs_platform_conf_create()

void nmbs_platform_conf_create

(

nmbs_platform_conf *

platform_conf

)

Create a new nmbs_platform_conf struct.

Parameters

platform_conf

pointer to the nmbs_platform_conf instance

nmbs_read_coils()

nmbs_error nmbs_read_coils	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	quantity,
		nmbs_bitfield	coils_out
	)

Send a FC 01 (0x01) Read Coils request

Parameters

nmbs	pointer to the nmbs_t instance
address	starting address
quantity	quantity of coils
coils_out	nmbs_bitfield where the coils will be stored

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_device_identification()

nmbs_error nmbs_read_device_identification	(	nmbs_t *	nmbs,
		uint8_t	object_id,
		char *	buffer,
		uint8_t	buffer_length
	)

Send a FC 43 / 14 (0x2B / 0x0E) Read Device Identification to retrieve a single Object Id value (Read Device ID code 4)

Parameters

nmbs	pointer to the nmbs_t instance
object_id	requested Object Id
buffer	char array where the resulting value will be stored
buffer_length	length of the char array

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_device_identification_basic()

nmbs_error nmbs_read_device_identification_basic	(	nmbs_t *	nmbs,
		char *	vendor_name,
		char *	product_code,
		char *	major_minor_revision,
		uint8_t	buffers_length
	)

Send a FC 43 / 14 (0x2B / 0x0E) Read Device Identification to read all Basic Object Id values (Read Device ID code 1)

Parameters

nmbs	pointer to the nmbs_t instance
vendor_name	char array where the read VendorName value will be stored
product_code	char array where the read ProductCode value will be stored
major_minor_revision	char array where the read MajorMinorRevision value will be stored
buffers_length	length of every char array

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_device_identification_extended()

nmbs_error nmbs_read_device_identification_extended	(	nmbs_t *	nmbs,
		uint8_t	object_id_start,
		uint8_t *	ids,
		char **	buffers,
		uint8_t	ids_length,
		uint8_t	buffer_length,
		uint8_t *	objects_count_out
	)

Send a FC 43 / 14 (0x2B / 0x0E) Read Device Identification to read all Extended Object Id values (Read Device ID code 3)

Parameters

nmbs	pointer to the nmbs_t instance
object_id_start	Object Id to start reading from
ids	array where the read Object Ids will be stored
buffers	array of char arrays where the read values will be stored
ids_length	length of the ids array and buffers array
buffer_length	length of each char array
objects_count_out	retrieved Object Ids count

Returns

NMBS_ERROR_NONE if successful, NMBS_INVALID_ARGUMENT if buffers_count is less than retrieved Object Ids count, other errors otherwise.

nmbs_read_device_identification_regular()

nmbs_error nmbs_read_device_identification_regular	(	nmbs_t *	nmbs,
		char *	vendor_url,
		char *	product_name,
		char *	model_name,
		char *	user_application_name,
		uint8_t	buffers_length
	)

Send a FC 43 / 14 (0x2B / 0x0E) Read Device Identification to read all Regular Object Id values (Read Device ID code 2)

Parameters

nmbs	pointer to the nmbs_t instance
vendor_url	char array where the read VendorUrl value will be stored
product_name	char array where the read ProductName value will be stored
model_name	char array where the read ModelName value will be stored
user_application_name	char array where the read UserApplicationName value will be stored
buffers_length	length of every char array

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_discrete_inputs()

nmbs_error nmbs_read_discrete_inputs	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	quantity,
		nmbs_bitfield	inputs_out
	)

Send a FC 02 (0x02) Read Discrete Inputs request

Parameters

nmbs	pointer to the nmbs_t instance
address	starting address
quantity	quantity of inputs
inputs_out	nmbs_bitfield where the discrete inputs will be stored

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_file_record()

nmbs_error nmbs_read_file_record	(	nmbs_t *	nmbs,
		uint16_t	file_number,
		uint16_t	record_number,
		uint16_t *	registers,
		uint16_t	count
	)

Send a FC 20 (0x14) Read File Record

Parameters

nmbs	pointer to the nmbs_t instance
file_number	file number (1 to 65535)
record_number	record number from file (0000 to 9999)
registers	array of registers to read
count	count of registers

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_holding_registers()

nmbs_error nmbs_read_holding_registers	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	quantity,
		uint16_t *	registers_out
	)

Send a FC 03 (0x03) Read Holding Registers request

Parameters

nmbs	pointer to the nmbs_t instance
address	starting address
quantity	quantity of registers
registers_out	array where the registers will be stored

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_input_registers()

nmbs_error nmbs_read_input_registers	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	quantity,
		uint16_t *	registers_out
	)

Send a FC 04 (0x04) Read Input Registers request

Parameters

nmbs	pointer to the nmbs_t instance
address	starting address
quantity	quantity of registers
registers_out	array where the registers will be stored

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_read_write_registers()

nmbs_error nmbs_read_write_registers	(	nmbs_t *	nmbs,
		uint16_t	read_address,
		uint16_t	read_quantity,
		uint16_t *	registers_out,
		uint16_t	write_address,
		uint16_t	write_quantity,
		const uint16_t *	registers
	)

Send a FC 23 (0x17) Read Write Multiple registers

Parameters

nmbs	pointer to the nmbs_t instance
read_address	starting read address
read_quantity	quantity of registers to read
registers_out	array where the read registers will be stored
write_address	starting write address
write_quantity	quantity of registers to write
registers	array of registers values to write

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_receive_raw_pdu_response()

nmbs_error nmbs_receive_raw_pdu_response	(	nmbs_t *	nmbs,
		uint8_t *	data_out,
		uint8_t	data_out_len
	)

Receive a raw response Modbus PDU.

Parameters

nmbs	pointer to the nmbs_t instance
data_out	response data. It's up to the caller to convert this data to host byte order. Can be NULL.
data_out_len	number of bytes to receive

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_send_raw_pdu()

nmbs_error nmbs_send_raw_pdu	(	nmbs_t *	nmbs,
		uint8_t	fc,
		const uint8_t *	data,
		uint16_t	data_len
	)

Send a raw Modbus PDU. CRC on RTU will be calculated and sent by this function.

Parameters

nmbs	pointer to the nmbs_t instance
fc	request function code
data	request data. It's up to the caller to convert this data to network byte order
data_len	length of the data parameter

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_server_create()

nmbs_error nmbs_server_create	(	nmbs_t *	nmbs,
		uint8_t	address_rtu,
		const nmbs_platform_conf *	platform_conf,
		const nmbs_callbacks *	callbacks
	)

Create a new Modbus server.

Parameters

nmbs	pointer to the nmbs_t instance where the client will be created.
address_rtu	RTU address of this server. Can be 0 if transport is not RTU.
platform_conf	nmbs_platform_conf struct with platform configuration. It may be discarded after calling this method.
callbacks	nmbs_callbacks struct with server request callbacks. It may be discarded after calling this method.

Returns

NMBS_ERROR_NONE if successful, NMBS_ERROR_INVALID_ARGUMENT otherwise.

nmbs_server_poll()

nmbs_error nmbs_server_poll

(

nmbs_t *

nmbs

)

Handle incoming requests to the server. This function should be called in a loop in order to serve any incoming request. Its maximum duration, in case of no received request, is the value set with nmbs_set_read_timeout() (unless set to < 0).

Parameters

nmbs	pointer to the nmbs_t instance

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_set_byte_timeout()

void nmbs_set_byte_timeout	(	nmbs_t *	nmbs,
		int32_t	timeout_ms
	)

Set the timeout between the reception/transmission of two consecutive bytes.

Parameters

nmbs	pointer to the nmbs_t instance
timeout_ms	timeout in milliseconds. If < 0, the timeout is disabled.

nmbs_set_callbacks_arg()

void nmbs_set_callbacks_arg	(	nmbs_t *	nmbs,
		void *	arg
	)

Set the pointer to user data argument passed to server request callbacks.

Parameters

nmbs	pointer to the nmbs_t instance
arg	user data argument

nmbs_set_destination_rtu_address()

void nmbs_set_destination_rtu_address	(	nmbs_t *	nmbs,
		uint8_t	address
	)

Set the recipient server address of the next request on RTU transport.

Parameters

nmbs	pointer to the nmbs_t instance
address	server address

nmbs_set_platform_arg()

void nmbs_set_platform_arg	(	nmbs_t *	nmbs,
		void *	arg
	)

Set the pointer to user data argument passed to platform functions.

Parameters

nmbs	pointer to the nmbs_t instance
arg	user data argument

nmbs_set_read_timeout()

void nmbs_set_read_timeout	(	nmbs_t *	nmbs,
		int32_t	timeout_ms
	)

Set the request/response timeout. If the target instance is a server, sets the timeout of the nmbs_server_poll() function. If the target instance is a client, sets the response timeout after sending a request. In case of timeout, the called method will return NMBS_ERROR_TIMEOUT.

Parameters

nmbs	pointer to the nmbs_t instance
timeout_ms	timeout in milliseconds. If < 0, the timeout is disabled.

nmbs_strerror()

const char * nmbs_strerror

(

nmbs_error

error

)

Convert a nmbs_error to string

Parameters

error

error to be converted

Returns

string representation of the error

nmbs_write_file_record()

nmbs_error nmbs_write_file_record	(	nmbs_t *	nmbs,
		uint16_t	file_number,
		uint16_t	record_number,
		const uint16_t *	registers,
		uint16_t	count
	)

Send a FC 21 (0x15) Write File Record

Parameters

nmbs	pointer to the nmbs_t instance
file_number	file number (1 to 65535)
record_number	record number from file (0000 to 9999)
registers	array of registers to write
count	count of registers

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_write_multiple_coils()

nmbs_error nmbs_write_multiple_coils	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	quantity,
		const nmbs_bitfield	coils
	)

Send a FC 15 (0x0F) Write Multiple Coils

Parameters

nmbs	pointer to the nmbs_t instance
address	starting address
quantity	quantity of coils
coils	bitfield of coils values

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_write_multiple_registers()

nmbs_error nmbs_write_multiple_registers	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	quantity,
		const uint16_t *	registers
	)

Send a FC 16 (0x10) Write Multiple Registers

Parameters

nmbs	pointer to the nmbs_t instance
address	starting address
quantity	quantity of registers
registers	array of registers values

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_write_single_coil()

nmbs_error nmbs_write_single_coil	(	nmbs_t *	nmbs,
		uint16_t	address,
		bool	value
	)

Send a FC 05 (0x05) Write Single Coil request

Parameters

nmbs	pointer to the nmbs_t instance
address	coil address
value	coil value

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.

nmbs_write_single_register()

nmbs_error nmbs_write_single_register	(	nmbs_t *	nmbs,
		uint16_t	address,
		uint16_t	value
	)

Send a FC 06 (0x06) Write Single Register request

Parameters

nmbs	pointer to the nmbs_t instance
address	register address
value	register value

Returns

NMBS_ERROR_NONE if successful, other errors otherwise.