TBox RTU Configuration with Modbus

Overview

• Modbus is a communication protocol created by Modicon in 1979, now Schneider Electric
• Modbus is used for industrial automation applications because it transfers data efficiently
• The application note provides a generic workflow for configuring TBox RTU with Modbus slave and master functionalities
• Methodologies apply to all TBox RTU models, using TBox MS as an example

Modbus Slave

• All Modbus protocols (RTU, TCP, ASCII) exist within TBox
• Modbus slave is a built-in functionality of the TBox RTU
• Communication to TBox RTU is possible via Modbus RTU (on serial ports) and Modbus TCP (on Ethernet ports) without configuration
• TWinSoft communicates with TBox in Modbus without any configuration

Station Address

• The 'Station Address' identifies each Modbus slave on the network (serial or TCP/IP)
• Each Modbus slave must have a unique station address in a multi-drop network
• The range of Modbus station addresses is 1 to 254
• Address 0 is the broadcast address which receives no answer from the slave
• Address 255 gives a normal reply, from any address, and is used in Modbus-TCP protocol
• The station address of TBox RTU is configured in the RTU Properties window

TCP Port

• TBox RTU acts as a TCP server listening for incoming TCP connections on port 502 from the TCP client (Modbus TCP master) when configured as a Modbus TCP slave over a TCP/IP network.
• The port number can be changed via RTU Properties->Advanced
• A global reset will restore the default Modbus TCP slave port to 502

Modbus Address

• 'Modbus Address' refers to the addressing of data points in the Modbus protocol
• Data points are presented as tags within TBox
• When a new tag is created, a specific Modbus Address can be assigned to the data point on the tag properties window
• The default assigned Modbus address can also be used for each created tag to prevent duplicated Modbus addresses
• Modbus addresses use 16 bits, enabling numbers between 0 and 65535
• Modbus addresses can be identical for digital and analog tags because the protocol uses different functions for access
• A 32-bit variable uses 2 x Modbus addresses

Configuration Procedure

• Since Modbus slave is built into TBox, there is nothing to configure
• Still, the way TBox is configured needs to be known to access it
• Check the 'Station Address' in RTU Properties
• If Modbus TCP is used, check the TCP port in RTU Properties->Advanced->Ports page
• With serial ports, check the configuration Type (RS232/485), Baudrate, Data bits, Parity, Stop bits, and Protocol (Modbus RTU/Modbus ASCII)
• Establish the connection between the master device and TBox via Ethernet or serial
• Use TBox Modbus simulators for testing or troubleshooting

Modbus Master

• Modbus master does not assign any dedicated station address
• Any device can be a master device on a serial Modbus bus with strictly one master device
• When Modbus protocol extends to TCP/IP network, the IP address identifies each device
• To access a slave, the station address used can be 255
• If Modbus routing to access a device behind TBox is required, the real station address must be addressed

Modbus Device

• External Modbus slave devices are treated as I/O when TBox RTU is a Modbus master
• I/O is managed in the Resources category
• A Modbus Device is a physical module connected to the TBox RTU via serial or Ethernet connection
• Add a Modbus slave device by selecting Resources->I/O, right clicking Add an I/O Card and selecting Modbus Device

Modbus Transactions

• In TBox Modbus configuration, one 'Modbus Transaction' means one specific communication that TBox RTU (as Modbus master) will execute as per one ‘Modbus Device’
• A Modbus Device can be configured with multiple Modbus Transactions
• TBox RTU executes communications sequentially (one after each other) to this Modbus Device
• Communications can read or write one or more data points from or to the Modbus device, either digital or analog
• Multiple, successive Modbus addresses can be read/write in one transaction
• To add a Modbus transaction, select Modbus Transactions, and double click Add a Modbus Transaction

Polling Management Principles

• Modbus communication is processed on different communication ports (RS232, RS485, Ethernet) simultaneously
• Only one Modbus transaction is under process at a time on a serial port
• On IP communication (Ethernet, GPRS, 3G, 4G), up to 64 Modbus TCP slaves can be handled in parallel, with transactions on each device carried out sequentially
• Modbus transactions of a Modbus device are completed sequentially, since Modbus is designed as a half-duplex protocol
• The configurable trigger controls the start/stop of the external Modbus device

Trigger

• The trigger of Modbus Device controls the Modbus polling
• The trigger is mapped to a tag so the polling is controlled by the value of the trigger tag
• The tag assignment is found in the dialog of Modbus Device configuration
• Methods to use the trigger include positive edge: all Modbus transactions of the Modbus device are executed once when Trigger changes from 0 to 1.
• Negative edge: all Modbus transactions of this remote device are executed once when Trigger changes from 1 to 0
• High state: all Modbus transactions are executed permanently if Trigger=1
• Low state: all Modbus transactions are executed permanently if Trigger=0
• When the trigger works in edge mode, the trigger value will be restored automatically after all the Modbus transactions of the Modbus device have been executed, with or without error
• With a 'negative edge' trigger and the trigger changes from 1 to 0, all Modbus transactions of the device execute once, then the trigger is restored to 1 automatically

Useful Hints – Diagnostics

• Assign tags to Cycle count, Status and Communication error diagnostics items to use them in the logic
• Cycle count increments by 1 when all transactions of a device end
• It monitors if Modbus transactions are executed normally
• If a permanent level is used, the change in cycle count can be detected
• Status and Communication errors are evaluated after each Modbus transaction
• This is used to diagnose if Modbus communication is healthy

Useful Hints - Timing

• TBox RTU uses 3 timing parameters to control the Modbus polling process:
• Packet reception delay is the maximum time the master waits for a complete packet from the slave
• IFG (Inter Frame Gap) is the time the master RTU must wait before its next Modbus query to the slave device
• Rx Timeout is the longest time the master RTU waits for the response from a slave
• These 3 parameters are found in the Advanced tab page of the properties window of a serial or Ethernet port

Master Configuration Procedure

• Define the Modbus master configuration of TBox RTU, including:
  • The quantity of external Modbus slave devices that need to poll
  • The data types and ranges needed to read or write in each Modbus slave device
  • Whether each Modbus slave device uses a serial or Ethernet connection
• If using a serial port, identify if RS232 or RS485 is required
  • The RS232 port of TBox RTU connects to one external device given it is a point-to-point connection
  • The RS485 port of TBox RTU connects to multiple slave devices as it’s a multiple-drop communication bus
• Configure the Baudrate, data bits, parity, and stop bits on the TBox RTU
• With Ethernet, make sure how many devices need polling and their IP addresses
  • TBox RUT cannot have the same subnets on different Ethernet ports
• Add a Modbus Device based on requirement and configure the communication port
• Check the TCP port, and adapt it in RTU properties -> Advanced -> TCP ports

Troubleshooting

• The diagnostic tags can be used, as well as other utilities like Wireshark and Modbus simulator

Special Notes – Keep Socket Open

• With the TBox RTU as a Modbus TCP master, 'Keep socket open' should be enabled to increase data throughput
• This saves bandwidth on the TCP/IP network
• The option is located in RTU Properties->Advanced->Modbus Transactions window
• When enabled, the socket stays open until there are no activities on this socket in 30s
• When disabled, the socket closes after every transaction so a new socket opens in the next transaction
• Some Modbus slave devices do not like frequent socket open/close actions over Ethernet ports, so enabling Keep socket open is a better choice

Illegal Data Address

• When TBox RTU is a Modbus slave and the polled data address (tag) does not exist, an exception code returns, following the Modbus protocol standard
• With 'Enable Advanced Modbus Mapping' checked, TBox RTU will fill the non-existent addresses with 0, instead of returning the exception code