Modbus Addressing

Manually Addressing a Modbus Device

Modbus doesn't support tag browsing, this means you can not view the tags in the OPC Browser or Tag Browser of the Designer or from the OPC Connections > Quick Client in the Configure section of the Gateway.

There are two wa ys you can create tags so that you can browse them:

  1. By manually specifying each address
    This is done from the Designer by entering Modbus Specific Addresses into the OPC Item Path of an OPC Tag. See below for detailed information.

  2. By specifying the address mapping
    This is done from the Gateway, see the Modbus Address Mapping section.

To manually specify each address

You can enter Modbus Specific Adresses into the OPC Item Path of an OPC Tag by using the following designators along with the Modbus address:

  1. In the Tag Browser, right-click on Tags folder, and then go to New Tag > OPC Tag.

  2. In the Tag Editor window, as an example, you can set the following values:
    Name: Temp
    Data Type: Int4
    OPC Server: choose Ignition OPC-UA Server from the dropdown
    OPC Item Path: [Modbus]HR1, the Modbus device name goes in the square brackets then you give the address to PLC which in this case is the HR designator plus 1 as the Modbus address. The Modbus Specific Addressing section below, explains how your can construct these addresses.

  3. Click OK.
    Now you can see the Temp tag in the Tag Browser.

Modbus Specific Addressing

Per the Modbus protocol specification, the following four basic types of addresses can be read from a device:

  • Holding Registers (read/write 16 bit words)

  • Input Registers ( read only 16 bit words)

  • Coils (read/write bits)

  • Discrete Inputs (read only bits associated with device input points)

To manually create an address for a single tag

To manually enter Modbus Specific Addresses into the OPC Item Path of the Tag Editor window, use one of the following designators plus the Modbus address:

Note: Other OPC servers represent each type by starting the OPC address with a number, for example, 4 for holding registers.

Designator
Description

HR

for 16 bit signed Holding Register (HR1, equivalent to 40001 in other applications)

IR

for 16 bit signed Input Register (IR1, equivalent to 30001)

C

for Coil (C1, equivalent to 000001)

DI

for Discrete Input (DI1, equivalent to 100001)

An example of using these designators with the Modbus address is to enter HR1 in the OPC Item Path of an OPC Tag in the Tag Editor window, which is the HR designator plus the Modbus address 1.

Because some devices that support Modbus protocol store data in BCD format, there are two additional designators. These designators convert the data from BCD format to decimal when reading data from the device and convert data from decimal to BCD when writing to the device.

Designator
Description

HRBCD

for Holding Register with BCD conversion.

HRBCD_32

for 2 consecutive Holding Registers with BCD conversion.

IRBCD

for Input Register with BCD conversion.

IRBCD_32

for 2 consecutive Input Registers with BCD conversion.

To accommodate other data encoding commonly used by Modbus supported devices, the following designators are available for Modbus specific addressing:

Description

Holding Register Designator

Input Register Designator

Float/Double

2 consecutive Registers with Float conversion.

HRF

IRF

4 consecutive Registers with Double conversion.

HRD

IRD

Integer

Holding Registers with 16 bit unsigned integer conversion.

HRUS

IRUS

2 consecutive Registers with 32 bit integer conversion.

HRI

IRI

2 consecutive Registers with 32 bit unsigned integer conversion.

HRUI

IRUI

4 consecutive Registers with 64 bit integer conversion.

HR_64

IR_64

4 consecutive Registers with 64 bit unsigned integer conversion.

HRUI_64

IRUI_64

To read or write string values from/to a Modbus device, the following designation is available for Modbus specific addressing:

Designator
Description

HRS

read or write consecutive Holding Registers as a string value.

Note: There are 2 characters for each word and the order of which character comes first is controlled by the Reverse String Byte Order device setting as described in the Connecting to Modbus Device section. Because two characters are stored in a word, the string length must be an even number of characters.

HRS FORMAT: HRS<Modbus address>:<length>

Examples
Description

[DL240]HR1024

Read 16bit integer value from Holding Register 1024.

[DL240]HRBCD1024

Read BCD value from Holding Register 1024.

[DL240]IR512

Read 16bit integer value from Input Register 512.

[DL240]C3072

Read bit value from Coil 3072.

[DL240]IR0

Read 16bit integer value from Input Register 0.

[DL240]HRS1024:20

Read 20 character string value starting at Holding Register 1024.

Unit ID

You can also specify the Modbus unit ID by pre-pending it to the Modbus address. For example, to access Modbus unit ID 3 and read HR1024, the full OPC path is:

[DL240]3.HR1024

Bit-level addressing

You just append a period and the bit number you want to read and write to a bit, your Modbus device must support the Mask Write command, and your device documentation should specify if it does.

To read or write to a specific bit within a holding register, simply append the location of the bit as demonstrated in these examples:

[DL240]HR1024.0 will read and write to the first bit of the holding register.

[DL240]HR1024.10 will read and write to the 11th bit of the holding register.

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