Connecting to Modbus Device
To connect Ignition to a Modbus device
Go to the Configure section of the Gateway webpage.
Scroll down and select OPC-UA Server > Devices.
On the Devices page, find the orange arrow and click on Create new Device.
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.
- Modbus RTU over TCP
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 10.20.8.117You can check the box for Show advanced properties? to see the additional settings, but you can keep all the defaults.
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 section 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 |
Driver Properties
Connectivity
| Property | Description |
|---|---|
| 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
| Property | Description |
|---|---|
| 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 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 high word comes before the low word. By checking this option, the low word comes before the high 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
| Property | Description |
|---|---|
| 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. |