Modbus Units Tab

Use

On this tab, you enter the devices that you want to connect to the PCo system as Modbus servers. In this case, PCo is set with the Modbus type Modbus client and therefore acts as leading system.

Procedure

Make the settings for the first Modbus server unit:

Field	Description
Unit ID	ID of the programmable logic controller (PLC) or the device that is to be connected to PCo as a Modbus server.
Output Coils	Indicates the number of single bit outputs of the Modbus server that is to be connected. A maximum of 65536 coils can be defined. Each coil is assigned to a single bit. This data type can be changed by an application program.
Input Contacts	Indicates the number of single bit input contacts of the Modbus server that is to be connected. These input contacts can only be used in read-only mode. A maximum of 65536 contacts can be defined. Each contact is assigned to a single bit. This data type is usually provided by an input/output system.
Input Registers	Indicates the read-only area of 16-bit registers. A maximum of 65536 input registers can be defined. This data type can only be provided by an input/output system.
Holding Registers	The holding registers are the most frequently used 16-bit registers. They can be used in read and write mode. They enable, for example, the reading of measured values, counter readings, or the reading of device configuration. You can define a maximum of 65536 registers. This data type can be changed by an application program.
Files	Specifies the number of additional register areas. You can define up to 65535 files.
File Size	Specifies the number of 16-bit registers in each file. Each file can contain up to 10000 registers.

Enter the string format. You use the fields for the string format to define how the strings are displayed:

Field	Description
Little Endian Order	If you select this checkbox, the little endian order is used for the byte order. If you do not select the checkbox, the big endian order is used.
Encoding	You select the encoding format you want here: ASCII UTF-8 UTF-16 (See also: Encoding (BC-I18).)

Field

Description

Little Endian Order

If you select this checkbox, the little endian order is used for the byte order. If you do not select the checkbox, the big endian order is used.

Encoding

You select the encoding format you want here:

ASCII
UTF-8
UTF-16

(See also: Encoding (BC-I18).)

Define the swapping mode for the various data types.

The Modbus protocol usually uses the big endian order for address and data values (if you have not selected the Little Endian Order checkbox). This means that whenever a numerical quantity that is larger than one byte is to be transferred, the byte with the largest value needs to be sent first. Therefore, the byte order always needs to be clearly defined in such a case.

In the swapping mode table, each Word has two bytes and each DWord (Double Word) has 2 words. If you select a checkbox, for example, the Words checkbox, this means that the relevant data structure needs to be swapped.

Example

The following example shows how the swapping mode can influence the 32-bit values if you have chosen the swapping modes Bytes or Words or both:

In the Input row, the bytes are displayed in the original order (without swapping mode). In the Swap Bytes row, the bytes have been swapped. In the Swap Words row, the words (byte pairs) have been swapped. In the Swap Bytes and Words option, both are swapped.
To enter an additional device, choose the Create New Modbus Unit pushbutton.