Protocol Documentation

This document describes the protocol used in this module.

Implementing a protocol for a command line interface is not easy. One has to know what a protocol and a state machine is and how they work. Please see the following modules for more information:

• Module 5.3.1 - Protocols
• Module 5.3.2 - Analysis and Selection
• Module 5.4.1 - Finate-State Machines

Refer to Module 5.3.3 for an implementation example with a Visual Studio and a Code::Blocks solution.

Protocol Definition

• Start Condition: O, F, B
• Stop Condition: space
• Message Format: 'COMMAND''ARGUMENT''TERMINAL' or 'COMMAND''ARGUMENT1''ARGUMENT2''TERMINAL'
  • O num - Turn LED num on.
  • F num - Turn LED num off.
  • B num1 num2 - Let LED num1 blink num2 times.

Protocol Implementation

The protocol is implemented by the following finite state machine:

The description of the states is:

State	Description
C	Parse command.
A	Parse argument of LED command.
A1	Parse first argument of blink command.
A2	Parse second argument of blink command.
T	Parse terminal character.
E	Execute command.

The transitions are:

State	Condition	Transition	Description
C	token = O	A	Command is LED on. → Parse LED argument.
	token = F	A	Command is LED off. → Parse LED argument.
	token = B	A1	Command is blink. → Parse first blink argument.
A	token = digit	T	LED argument parsed. → Parse terminal.
A1	token = digit	A2	First blink argument parsed. → Parse second blink argument.
A2	token = digit	T	Second blink argument parsed. → Parse terminal.
T	token = space	E	Terminal parsed. → Execute.
E	-	C	Command executed. → Parse next command.

Every parsing function gets the new token and returns the subsequent state. Some functions return extra information (command, argument) via a reference parameter. The basic workflow is as follows:

while (1)
{
  switch (state) {
    case CLI_STATE_GET_COMMAND:
      token = new_token();
      state = cli_parse_command(token, &command);
      break;
    case CLI_STATE_GET_ARGUMENT:
      token = new_token();
      state = cli_parse_argument(token, &argument);
      break;
    case CLI_STATE_GET_TERMINAL:
      token = new_token();
      state = cli_parse_terminal(token);
      break;
    case CLI_STATE_EXECUTE:
      state = cli_execute(command, argument);
      break;
  }
}