[ST] Custom serial protocol

When implementing the system, it is sometimes necessary to use a protocol that is not available in the Mervis environment. Such a situation can occur, for example, when you install a Unipi PLC as a replacement for another PLC and you need to connect some of the original peripherals, or when you need to connect the Unipi PLC to your own device with its own protocol.

The following article describes the basic information needed to successfully implement a custom serial protocol and a sample project.

Since the implementation of a serial protocol with a function block diagram (FBD) is more demanding and complex than the implementation in structured text (ST), we recommend the implementation in the ST language.

In order to create programs, functions or function blocks in the ST language, it is first necessary to switch the solution to full mode.

A total of 5 functions for working with (serial) ports are available in Mervis:

1. Open Port - opens a port communication according to the entered data
2. Get Port Status - gets the communication status of a handle
3. Write Port - writes data to a given communication port
4. Read Port - reads data from a given communication port
5. Close Port - closes communication at the given port

A description of the inputs and outputs of the above functions is available in the Mervis IDE help
(path: Software → Libraries → Communications → Input/Output Functions)

As part of the sample project, a state machine is implemented for communication via the selected serial port on the query/response principle, when a response is expected after sending a query. Therefore, this sample project does not include a state machine that expects a spontaneous flow of data from a remote device.

A table of serial ports is also implemented (according to the tables below) using derived data types (structures) and an HMI web interface simulating a terminal window for simple communication test.

The project is created in Mervis IDE 2.6.5.

FUNCTION_BLOCK Custom_Serial

  VAR
    serialPort : unipi_serial.v1_0.Patron_M567;
    str,def : string;
  END_VAR

  INIT
    def := 'serial:'; // the first part of the definition
    def := CONCAT(IN1:=def,IN2:=serialPort.RS485_1_1); // merge the first part of the definition with the serial port
    def := CONCAT(IN1:=def,IN2:=':9600,8,N,1'); //merge with the last part of the definition
  END_INIT

  str := serialPort.RS485_1_1;

END_FUNCTION_BLOCK