Connecting to Modbus Device

To connect Ignition to a Modbus device

  1. Go to the Configure section of the Gateway webpage.

  2. Scroll down and select OPC-UA > Devices.

  3. On the Devices page, find the orange arrow and click on Create new Device.

  4. On the Add Device Step 1: Choose Type page, select Modbus TCP, and click Next.

    There are two modules to choose from

    • Modbus RTU over TCP
      Which connects to devices that implement the Modbus RTU protocol over TCP.

    • Modbus TCP
      Which connects to devices that implement the Modbus TCP protocol.

    Typically, you connect using the Modbus TCP driver as we have in step 4 .

  5. On the New Device page, leave all the default values and type in the following fields:
    Name: Modbus
    Hostname: type the IP address, for example 10.20.8.117

  6. You can check the box for Show advanced properties? to see the additional settings, but you can keep all the defaults.

  7. Click Create New Device.
    The Devices page is displayed showing the Modbus device is successfully created and added to Ignition. The Status will show as Disconnected and then Connected.

Unlike other PLCs, Modbus devices do not support browsing, therefore you can not browse the tags by going to the OPC Connections > Quick Client in the Configure sectio n of the Gateway. To see and browse the tags, you need to create the tags as described in the Modbus Addressing section.

Modbus Drivers using Ethernet

The generic Modbus driver allows the Ignition OPC-UA server to communicate with any device that supports Modbus TCP protocol. The Modbus driver can connect directly to devices that support Ethernet communications. It can also connect to Modbus devices through a Gateway.

It is important to only add one Modbus device in the Ignition Device List per IP address. When communicating to multiple Modbus devices through a Gateway each with a unique unit ID, either include the unit ID in the Modbus specific address or set it in the address mapping for the device.

Supported Functions Codes

The Modbus driver supports the functions codes listed below. In some cases, a device may not allow certain function codes. To remedy this, advanced properties on the device connection can restrict or force specific functions codes. See the Driver Properties table on this page for more details.

Function Name

Code

Hex

Read Discrete Inputs

02

0x02

Read Coils

01

0x01

Write Single Coil

05

0x05

Write Multiple Coils

15

0x0F

Read Input Register

04

0x04

Read Holding Register

03

0x03

Write Single Register

06

0x06

Write Multiple registers

16

0x10

Read/Write Multiple Registers

23

0x17

Driver Properties

Connectivity

 

Hostname

Is the IP Address of the Modbus device.

Port

Is the port to use when connecting to a Modbus device. The Modbus TCP port specified in the Modbus specification is 502, but it can be changed to a different port.

Communication Timeout

Is the amount of time in milliseconds to wait for a response before treating it as a failure, after sending a request to the Modbus device.

When working with a Modbus RTU over TCP connection, each "device" would be an individual Modbus device on the Modbus network.

Advanced

 

Max Holding Registers per Request

Is the the maximum number of Holding Registers the device can handle. Because some Modbus devices cannot handle the default of requesting 125 Holding Registers in one request, to accommodate this limitation you can change this setting.

 

Max Input Registers per Request

Is the the maximum number of Input Registers the device can handle. Because some Modbus devices cannot handle the default of requesting 125 Input Registers in one request, to accommodate this limitation you can change this setting.

Max Coils per Request

Is the the maximum number of Coils the device can handle. Because some Modbus devices cannot handle the default of requesting 2000 Coils in one request, to accommodate this limitation you can change this setting.

Max Discrete Inputs per Request

Is the the maximum number of Discrete Inputs the device can handle. Because some Modbus devices cannot handle the default of requesting 2000 Discrete Inputs in one request, to accommodate this limitation you can change this setting.

Reverse Word Order

When reading and writing 32bit values from/to a Modbus device, the low word comes before the high word. By checking this option, the high word comes before the low word. The Modbus specification does not include a section for reading and writing 32bit values and as a result device manufacturers have implemented both methods.

One-based Addressing

When this option is checked, the address range for each area starts at 1. If unchecked, the range starts at 0.

The Modbus specification states that Modbus addresses are to be zero based. Meaning Modbus addresses start at 0 instead of 1. To read a value from Modbus address 1024, 1023 is sent to the device. When connecting to devices that do not adhere to zero based addressing, make sure this option is not selected. This will cause 1024 to be sent to the device to read Modbus address 1024.

Span Gaps

When this option is checked, it spans address gaps when optimizing requests, reducing the number of requests but increasing the amount of data requested at once. If unchecked, it does not span the address gaps.

Allow Write Multiple Registers Request

Enable or disable Modbus function code 0x10, Write Multiple Registers. Some devices may not support this function code. Caution: disabling this option will break the ability to write 32-bit and String values correctly to registers.

Force Multiple Register Writes

Force the use of Modbus function code 0x10, Write Multiple Registers, on write requests.

Allow Write Multiple Coils Request

Enable or disable Modbus function code 0x0F, Write Multiple Coils. Some devices may not support this function code.

Allow Read Multiple Registers Request

If disabled all registers will be read in individual read requests. Disable with caution.

Allow Read Multiple Coils

If disabled all coils will be read in individual read requests. Disable with caution.

Allow Read Multiple Discrete Inputs

If disabled all discrete inputs will be read in individual read requests. Disable with caution.

Note: Function code 0x02 is always used to read Discrete Inputs, regardless what this property is set to.

Reconnect After Consecutive Timeouts

When checked forces a reconnect after 3 consecutive timeouts.

StringHandling

 

Reverse String Byte Order

When reading and writing string values from/to a Modbus device, the low byte comes before the high byte. By checking this option the high byte comes before the low byte. If reading a string value from a device should read ABCD but BADC appears in Ignition, then check this option.

Right Justify Strings

Strings stored in a Modbus device may contain leading spaces or trailing spaces. This can produce unwanted results so that Modbus driver removes spaces or zeros when reading string values. By default, left justify string handling is used when reading and writing strings. When you check this option, right justify string handling is used.

Read Raw Strings

Whether or not to read the entire length of a string, ignoring any null bytes encountered.