Modbus/TCP Driver - Periphery Address Panel

The configuration panel of a peripheral address for the Modbus/TCP driver.

You can enter the address directly in the Reference text field or create it with the spin buttons.

type

• MODBUS
• UNICOS

PLC

Number of PLC that has been set on the internal data point of the type _Mod_Plc during configuration.

FC/SFC

The possible function codes. All combinations of function codes, modes and addresses are not possible. The used function code depends on the PLC in the slave mode of the WinCC OA driver. The allowed values of the FC/SFC field are:

	Output	Input Polling	Input Spontaneous
MODBUS	5 write coil 6 write single register 15 force multiple coils 16 write multiple registers 23 atomic read/write	1 read coils 2 read input discrete 3 read multiple register 4 read input register 7 read exception status 24 read FIFO queue	5 write coil 6 write single 15 force multiple coils 16 write multiple coils 23 atomic read/write
UNICOS	not applicably	not applicably	1 status 17 events32 (32 bit event frame) 18 events (16 bit event frame)

Note:

The function code 23 is only used for atomic write/read operation, which means that a read operation is always combined with a write operation. Therefore, Polling or Single Query mode are not supported by these addresses.

The receive mode is always "Unsolicited", because the reading is an implicit consequence of the write.

Further information on the function code 23 (0x17) can be found in the chapter FC23

Reference Number

The reference number indicates the address within the PLC. It is important to note that the reference number is dependent on the function code. The reference number is an address with 16 bit word (a 2 byte integer without sign) for register based function codes and a bit number (a 2 byte integer without sign) for bit based function codes.

Note: Assure that the used function codes are compatible with the data types! The driver only checks the compatibility of the function code with the transformation type.

Note: By spontaneous input mode and function code FC 15 (force multiple coils) only use reference numbers divisible by 16 (an error message is shown when using numbers not divisible by 16) . The sub index can lie between 0 and 1919. For all other cases this modulo 16 limitation does not exist.

Type of transformation

Choose the transformation type of the Modbus driver from the combo box (see Reference tables). The subindices for the Modbus are dependent on the transformation type. If the current value is bigger than the upper boundary when changing the type of transformation the subindex is set to the new maximum value.

The Modbus driver now supports the access to bit groups in a Modbus register. This can be used for the realization of multi-state variables, which contain a configured number of bits. These bits must be consecutive in the register and must not exceed the register boundary. To configure such an address the transformation type "boolean" must be selected.

The "Subindex" defines the offset inside the register and the "Bits" field allows to configure the number of bits.

Specialities with the transformation type "blob" and "string"

When reading a Modbus register with the transformation type "string" or "blob", the number of bytes to be read can be set via an optional parameter [:<number of bytes>] in the address reference. E.g.: the address reference >M. 1. 3. 10:100 reads 100 bytes starting with the address M. 1. 3. 10. By default, the driver does always read the maximum data length of 240 bytes.

When writing a blob or string, by default only the number of bytes according to the blob or string length, inclusive the zero at the end, is written. For write operations the optional parameter [:<number of bytes>] can also be used in the address reference. E.g.: the address reference M.1.3.10:20 writes a maximum of 20 bytes starting at the address M.1.3.10. In this example, if a blob or string with a data length > 20 bytes is written, then the driver issues an error message in the log.

Subindex

The subindex of the Modbus is dependent on the transformation type. The restriction results from the maximum data length of 240 bytes in a Modbus frame as well as from the function codes:

Transformation type	Maximum subindex	Possible function code
boolean	1919	FC 3 (read multiple registers) FC 4 (read input registers) FC 16 (write multiple registers) FC 24 (read FIFO queue)
char	239
int16	119
uint16	119
int32	59
uint32	59
int64	29
uint64	29
byte	59
float	59
double	29
boolean	1919	FC 1 (read coils) FC 2 (read input discretes) FC 15 (force multiple coils)
boolean as byte	0	FC 5 (write coil)
boolean	15	FC 6 (write single register)
char	3
int16	0
uint16	0
boolean	7	FC 7 (read exception status)
char	0
MOD10 Size 2	0	FC 3 (read multiple registers) FC 4 (read input registers) FC 16 (write multiple registers )
MOD10 Size 3	0
MOD10 Size 4	0

Low level comparison

You can choose this option only when the transfer direction is set to Input. If the option is chosen the data is only sent in case of changes. The comparison is based on the raw data without conversion.

Direction

Define if you want to send values of a data point element to the command direction (output) or to the alert direction (input) or both directions (in/out) via the Direction radio buttons.

Receive mode

There are following receive modes for the input:

• Unsolicited
• Polling
• Single query

Poll group

Choose already existing poll groups. If there are no poll groups available you have to create them with the specific polling parameters in a separate panel. The panel is opened by clicking on the Poll groups... button (see Poll groups for more information on poll groups and polling parameters).

Address active

TheAddress active check box is chosen. The address is used by the driver (seeReference tables). An inactive address exists and the attributes can be set and queried but the driver does not use them. This means that no values can be sent to remote system or received from the remote system for this data point.

Note: The Modbus driver will perform a general query when establishing a connection. This behavior can be changed using the config entry [mod] autoGQ.