Defining the peripheral addresses of the Modbus/TCP driver

The panel shown in the figure below allows the configuration 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.

Figure: Panel for defining a peripheral address for Modbus/TCP

Type
MODBUS or 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 register

1 read coils
2 read input discretes

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

UNICOS

not applicably

1 status

17 events32 (32 bit event frame)

18 events (16 bit event frame)

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.

Caution

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.

When a string is written, then only the number of bytes according to the string length, inclusive the Zero at the end, is written. If the transformation type "string" or "blob" is used for polling an input, the number of bytes can be set. For example: M.1.3.10:100 polls 100 bytes beginning with the address M.1.3.10. Note that the entry is only possible for "string" or "blob".

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
byte	59
float	59
boolean	1919	FC 1 (read coils) FC 2 (read input discretes) FC 15 (force multiple coils)
double	29	FC 5 (write coil) FC 6 (write single register) FC 15 (force multiple coils) FC 16 (write multiple registers)
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	FC 7 (read exception status)

Note

In the driver versions before WinCC OA version 3.9 it was important that several data point elements were summarized by means of the subindex in order to guarantee efficient polling queries. This is not longer required since version 3.9, but is still supported due to compatibility reasons. When polling, the driver automatically summarizes addresses to as optimal blocks as possible.

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) via the Direction radio buttons.
Receive mode
There are three different receive modes for the input: Spontaneous, Polling or 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 an own 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
The Address active check box is chosen. The address is used by the driver (see Reference 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.
OK: Applies the settings or changes and closes the panel.
Cancel: Discards the settings or changes and closes the panel.
Apply: Applies the settings without closing the panel.
Help: Opens this page of the Online Help.

V 3.11 SP1