MODBUS networking

The T5 runtime includes fully integrated MODBUS master and slave functions for enabling MODBUS communication on serial link or ETHERNET. Please refer to the following topics in that page for detailed information on how to use and configuration MODBUS networking:

Protocol specification
Architecture
Communication ports
Data exchange
Slave configuration
Master configuration
Status and command variables
Data types

Protocol specification:

The following protocols are supported:

- MODBUS-RTU on a serial link
- Open MODBUS on ETHERNET

The following MODBUS function codes are supported:

1  read coils (*)
2  read bit inputs
3  read holding registers (*)
4  read input registers
5  write 1 coil
6  write 1 register
15 write N coils
16 write N registers

(*) The reading of coils and holding registers has no meaning when T5 runtime is used as a server (slave), as only inputs are updated (copy from the VM database to the MODBUS map). Reading of coils and holding registers is fully supported when T5 runtime is used as a client (master).

As a default, the first valid address for each kind of data is 1. If you use MODBUS devices with other addressing conventions, you can change the base offset for each kind of data using the "Tools / Addresses" menu command.

Architecture:

The T5 runtime can be used either as a server (MODBUS slave) or as a client (MODBUS master). Both server and client may be active at the same time. This allows multiple applications such as:

- slave connection to a MODBUS master such as a SCADA system
- master connection to pilot MODBUS I/Os
- T5 to T5 communication for real time exchange of variables (binding)

The configuration of slave and master functions is done in the Workbench. Any data used by MODBUS is fully integrated in the application code and the VM database.

Communication ports:

The client side opens a new port (RS or ETHERNET) for each configured MODBUS port. The number of client ports is not limited.

On the server side, the management of low level communication (sending and receiving frames) is fully included in the T5 communication server, sharing the same ports for MODBUS requests and native requests. Thus the number of possible connected MODBUS maters fits the limitation of the communication server:

- one serial port
- several ports on ETHERNET (platform dependent - generally 7 ports max)

Other slave ports can be open by calling MBSlaveRTU or MBSlaveUDP function blocks.

Note that the same port can be freely used either for communication with a MODBUS master or for dialog with the workbench. Both workbench and MODBUS frames are recognized by the communication server.

Data exchange - configuration:

The T5 runtime manages a intermediate variable map representing MODBUS data, for both master and slave connections. A dedicated configuration tool is integrated in the Workbench. Run it using the "File / Open / Fieldbus Configuration" menu command from the main window.

The MODBUS configuration is represented as a tree:

- MODBUS Configuration
  - Slave (variables that can be accessed from external MODBUS masters)
    - Input bits data block (read by masters)
      - Variable (*)
    - Input words data block (read by masters)
      - Variable (*)
    - Coil bits data block (forced by masters)
      - Variable (*)
    - Holding bits data block (forced by masters)
      - Variable (*)
  - Master communication port (connected to external MODBUS slaves) (*)
    - Data block (read or write / bits or words) (*)
      - exchanged Variable (*)
      - Command variable (*)
      - Status variable (*)

(*) The items with this mark can appear several times in the configuration.

MODBUS Slave configuration:

You need to configurate the MODBUS slave in order to make variables visible from external MODBUS masters such as SCADA systems. Below is a simple example of slave configuration:

Double click on the "Local Server" item to setup the MODBUS slave number that will identify the runtime application. When the local server is selected, use the "Edit / New data block" menu command to insert MODBUS data blocks. The following kinds of block are available:

- input bits: bits read by external masters (function 2)
- coil bits: bits forced by by external masters (function 5 or 15)
- input registers: words read by external masters (function 4)
- holding registers: words forced by external masters (function 6 or 16)

Each data block is identified by a MODBUS base address and a number of items (bits or words).

When a server data block is selected, use the "Edit / New variable" command to map a variable to an item of the data block. Each variable is identified by a valid symbol of a variable in the open project and an offset in the data block according to MODBUS addressing.

For exchanging boolean variables through MODBUS words, a hexadecimal mask is available in order to define to which bit of a word a variable is attached. For example, enter the mask "0001" to map a boolean variable to the less significant bit of a word. For exchanging 32 bit variables (DINT, REAL...), you can select to map the variable on two consecutive words.

At any time you can sort the variables of each data block according to their offset using the "Edit / Sort symbols" menu command.

MODBUS Master configuration:

You need to configurate MODBUS master ports in order to drive external MODBUS I/Os with variables. Below is a simple example of master configuration with just one port defined:

Run the "Edit / New port" to create a MODBUS port. Each port corresponds to one logical or physical MODBUS network. A port is based on either serial link or ETHERNET network. For ETHERNET ports, you must enter the IP address of the slave and its ETHERNET port number (usually 502). For serial ports, you must enter the settings of the port, according to your target platform conventions (e.g. "COM1:9600,N,8,1" on a PC based platform). As an option, you can select that the master retrys to open the port each time the communication fails.

Another option at the "port" level enables the management by the MODBUS stack of diagnostic counters for each slave. See below the list of possible status information you can map to variables.

When a port is selected, you can add new MODBUS data blocks by running the "Edit / New data block" menu command. The following types of blocks are available:

- read input bits (function 2)
- read coil bits (function 1)
- write coil bits (function 5 or 15)
- read input registers (function 4)
- read holding registers (function 3)
- write holding registers (function 6 or 16)

Each data block is identified by a MODBUS slave number, a base address and a number of items (bits or words). The number of items is limited by MODBUS (2000 bits read, 1968 bits forced, 125 words read or 120 words forced).

For each request, you must define a timeout value in milliseconds, and an activation mode. Possible activation mode are:

- periodic: you must then enter the minimum period
- on change: in case of a "write" request, means that the request is changed each time a variable changes
- on call: the request is activated using a variable

For periodic requests, you must specify an alternate period to be used if the last exchange of the request has failed. This is typically used to slow down exchanges with faulty slaves.

When a server data block is selected, use the "Edit / New variable" command to map a variable to an item of the data block. Each variable is identified by a valid symbol of a variable in the open project and an offset in the data block according to MODBUS addressing.

For exchanging boolean variables through MODBUS words, a hexadecimal mask is available in order to define to which bit of a word a variable is attached. For example, enter the mask "0001" to map a boolean variable to the less significant bit of a word. For exchanging 32 bit variables (DINT, REAL...), you can select to map the variable on two consecutive words.

At any time you can sort the variables of each data block according to their offset using the "Edit / Sort symbols" menu command.

Status and command variables:

When you map a variable to a data block in a master configuration, you can select it to be:

- an exchange of data (exchange between the MODBUS map and the T5 variables)
- a status (diagnostic information provided to the application)
- a command (using a variable for driving the stack)

Below are possible diagnostic and status information available:

Below are the possible commands you can map:

Below are possible error codes for "status" variables:

Other specific error codes may be answered by some slaves. Please refer to the documentation of the device.

When you map a variable to a data block in a master configuration, you can select it to be a "command". In that case, the data block is exchanged only when the variable is TRUE (or non zero for numerical data types).

Data types:

You can freely map a variable of any data type to a MODBUS item. The runtime automatically converts the value to the type of the variable.

For exchanging boolean variables through MODBUS words, a hexadecimal mask is available in order to define to which bit of a word a variable is attached. For example, enter the mask "0001" to map a boolean variable to the less significant bit of a word. For exchanging 32 bit variables (DINT, REAL...), you can select to map the variable on two consecutive words.

STRING variables are not supported and cannot be exchanged with MODBUS. 64 bit variable (LINT and LREAL) cannot be extracted directly without lost of accuracy or data.