概述

上篇文章分析了mb.c文件，我们知道，mb.c只是实现modbus协议的一个框架，具体的RTU、ASCII和TCP相关的代码再具体的文件夹中。这里我们以使用比较多的RTU为例，来分析一下RTU的实现。

一、RTU文件夹的文件

rtu文件夹下有四个文件，其说明如下：

文件名称	说明
mbcrc.c	这个文件只包含一个函数，就是标准的CRC16校验函数
mbcrc.h	包含CRC校验函数的函数声明
mbrtu.c	实现RTU协议的具体函数，rtu协议相关的实现函数都在这个文件中
mbrtu.h	头文件，包含rtu函数的声明

这里我们主要分析mbrtu.c文件，其他文件都比较简单。

二、mbrtu.c文件

先贴上代码，再一一说明。

/* * FreeModbus Libary: A portable Modbus implementation for Modbus ASCII/RTU.* Copyright (c) 2006-2018 Christian Walter <cwalter@embedded-solutions.at>* All rights reserved.** Redistribution and use in source and binary forms, with or without* modification, are permitted provided that the following conditions* are met:* 1. Redistributions of source code must retain the above copyright*    notice, this list of conditions and the following disclaimer.* 2. Redistributions in binary form must reproduce the above copyright*    notice, this list of conditions and the following disclaimer in the*    documentation and/or other materials provided with the distribution.* 3. The name of the author may not be used to endorse or promote products*    derived from this software without specific prior written permission.** THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.**//* ----------------------- System includes ----------------------------------*/
#include "stdlib.h"
#include "string.h"/* ----------------------- Platform includes --------------------------------*/
#include "port.h"/* ----------------------- Modbus includes ----------------------------------*/
#include "mb.h"
#include "mbrtu.h"
#include "mbframe.h"#include "mbcrc.h"
#include "mbport.h"/* ----------------------- Defines ------------------------------------------*/
#define MB_SER_PDU_SIZE_MIN     4       /*!< Minimum size of a Modbus RTU frame. */
#define MB_SER_PDU_SIZE_MAX     256     /*!< Maximum size of a Modbus RTU frame. */
#define MB_SER_PDU_SIZE_CRC     2       /*!< Size of CRC field in PDU. */
#define MB_SER_PDU_ADDR_OFF     0       /*!< Offset of slave address in Ser-PDU. */
#define MB_SER_PDU_PDU_OFF      1       /*!< Offset of Modbus-PDU in Ser-PDU. *//* ----------------------- Type definitions ---------------------------------*/
typedef enum
{STATE_RX_INIT,              /*!< Receiver is in initial state. */STATE_RX_IDLE,              /*!< Receiver is in idle state. */STATE_RX_RCV,               /*!< Frame is beeing received. */STATE_RX_ERROR              /*!< If the frame is invalid. */
} eMBRcvState;typedef enum
{STATE_TX_IDLE,              /*!< Transmitter is in idle state. */STATE_TX_XMIT               /*!< Transmitter is in transfer state. */
} eMBSndState;/* ----------------------- Static variables ---------------------------------*/
static volatile eMBSndState eSndState;
static volatile eMBRcvState eRcvState;volatile UCHAR  ucRTUBuf[MB_SER_PDU_SIZE_MAX];static volatile UCHAR *pucSndBufferCur;
static volatile USHORT usSndBufferCount;static volatile USHORT usRcvBufferPos;/* ----------------------- Start implementation -----------------------------*/
eMBErrorCode
eMBRTUInit( UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{eMBErrorCode    eStatus = MB_ENOERR;ULONG           usTimerT35_50us;( void )ucSlaveAddress;ENTER_CRITICAL_SECTION(  );/* Modbus RTU uses 8 Databits. */if( xMBPortSerialInit( ucPort, ulBaudRate, 8, eParity ) != TRUE ){eStatus = MB_EPORTERR;}else{/* If baudrate > 19200 then we should use the fixed timer values* t35 = 1750us. Otherwise t35 must be 3.5 times the character time.*/if( ulBaudRate > 19200 ){usTimerT35_50us = 35;       /* 1800us. */}else{/* The timer reload value for a character is given by:** ChTimeValue = Ticks_per_1s / ( Baudrate / 11 )*             = 11 * Ticks_per_1s / Baudrate*             = 220000 / Baudrate* The reload for t3.5 is 1.5 times this value and similary* for t3.5.*/usTimerT35_50us = ( 7UL * 220000UL ) / ( 2UL * ulBaudRate );}if( xMBPortTimersInit( ( USHORT ) usTimerT35_50us ) != TRUE ){eStatus = MB_EPORTERR;}}EXIT_CRITICAL_SECTION(  );return eStatus;
}void
eMBRTUStart( void )
{ENTER_CRITICAL_SECTION(  );/* Initially the receiver is in the state STATE_RX_INIT. we start* the timer and if no character is received within t3.5 we change* to STATE_RX_IDLE. This makes sure that we delay startup of the* modbus protocol stack until the bus is free.*/eRcvState = STATE_RX_INIT;vMBPortSerialEnable( TRUE, FALSE );vMBPortTimersEnable(  );EXIT_CRITICAL_SECTION(  );
}void
eMBRTUStop( void )
{ENTER_CRITICAL_SECTION(  );vMBPortSerialEnable( FALSE, FALSE );vMBPortTimersDisable(  );EXIT_CRITICAL_SECTION(  );
}eMBErrorCode
eMBRTUReceive( UCHAR * pucRcvAddress, UCHAR ** pucFrame, USHORT * pusLength )
{BOOL            xFrameReceived = FALSE;eMBErrorCode    eStatus = MB_ENOERR;ENTER_CRITICAL_SECTION(  );assert( usRcvBufferPos < MB_SER_PDU_SIZE_MAX );/* Length and CRC check */if( ( usRcvBufferPos >= MB_SER_PDU_SIZE_MIN )&& ( usMBCRC16( ( UCHAR * ) ucRTUBuf, usRcvBufferPos ) == 0 ) ){/* Save the address field. All frames are passed to the upper layed* and the decision if a frame is used is done there.*/*pucRcvAddress = ucRTUBuf[MB_SER_PDU_ADDR_OFF];/* Total length of Modbus-PDU is Modbus-Serial-Line-PDU minus* size of address field and CRC checksum.*/*pusLength = ( USHORT )( usRcvBufferPos - MB_SER_PDU_PDU_OFF - MB_SER_PDU_SIZE_CRC );/* Return the start of the Modbus PDU to the caller. */*pucFrame = ( UCHAR * ) & ucRTUBuf[MB_SER_PDU_PDU_OFF];xFrameReceived = TRUE;}else{eStatus = MB_EIO;}EXIT_CRITICAL_SECTION(  );return eStatus;
}eMBErrorCode
eMBRTUSend( UCHAR ucSlaveAddress, const UCHAR * pucFrame, USHORT usLength )
{eMBErrorCode    eStatus = MB_ENOERR;USHORT          usCRC16;ENTER_CRITICAL_SECTION(  );/* Check if the receiver is still in idle state. If not we where to* slow with processing the received frame and the master sent another* frame on the network. We have to abort sending the frame.*/if( eRcvState == STATE_RX_IDLE ){/* First byte before the Modbus-PDU is the slave address. */pucSndBufferCur = ( UCHAR * ) pucFrame - 1;usSndBufferCount = 1;/* Now copy the Modbus-PDU into the Modbus-Serial-Line-PDU. */pucSndBufferCur[MB_SER_PDU_ADDR_OFF] = ucSlaveAddress;usSndBufferCount += usLength;/* Calculate CRC16 checksum for Modbus-Serial-Line-PDU. */usCRC16 = usMBCRC16( ( UCHAR * ) pucSndBufferCur, usSndBufferCount );ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 & 0xFF );ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 >> 8 );/* Activate the transmitter. */eSndState = STATE_TX_XMIT;vMBPortSerialEnable( FALSE, TRUE );}else{eStatus = MB_EIO;}EXIT_CRITICAL_SECTION(  );return eStatus;
}BOOL
xMBRTUReceiveFSM( void )
{BOOL            xTaskNeedSwitch = FALSE;UCHAR           ucByte;assert( eSndState == STATE_TX_IDLE );/* Always read the character. */( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte );switch ( eRcvState ){/* If we have received a character in the init state we have to* wait until the frame is finished.*/case STATE_RX_INIT:vMBPortTimersEnable(  );break;/* In the error state we wait until all characters in the* damaged frame are transmitted.*/case STATE_RX_ERROR:vMBPortTimersEnable(  );break;/* In the idle state we wait for a new character. If a character* is received the t1.5 and t3.5 timers are started and the* receiver is in the state STATE_RX_RECEIVCE.*/case STATE_RX_IDLE:usRcvBufferPos = 0;ucRTUBuf[usRcvBufferPos++] = ucByte;eRcvState = STATE_RX_RCV;/* Enable t3.5 timers. */vMBPortTimersEnable(  );break;/* We are currently receiving a frame. Reset the timer after* every character received. If more than the maximum possible* number of bytes in a modbus frame is received the frame is* ignored.*/case STATE_RX_RCV:if( usRcvBufferPos < MB_SER_PDU_SIZE_MAX ){ucRTUBuf[usRcvBufferPos++] = ucByte;}else{eRcvState = STATE_RX_ERROR;}vMBPortTimersEnable(  );break;}return xTaskNeedSwitch;
}BOOL
xMBRTUTransmitFSM( void )
{BOOL            xNeedPoll = FALSE;assert( eRcvState == STATE_RX_IDLE );switch ( eSndState ){/* We should not get a transmitter event if the transmitter is in* idle state.  */case STATE_TX_IDLE:/* enable receiver/disable transmitter. */vMBPortSerialEnable( TRUE, FALSE );break;case STATE_TX_XMIT:/* check if we are finished. */if( usSndBufferCount != 0 ){xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );pucSndBufferCur++;  /* next byte in sendbuffer. */usSndBufferCount--;}else{xNeedPoll = xMBPortEventPost( EV_FRAME_SENT );/* Disable transmitter. This prevents another transmit buffer* empty interrupt. */vMBPortSerialEnable( TRUE, FALSE );eSndState = STATE_TX_IDLE;}break;}return xNeedPoll;
}BOOL
xMBRTUTimerT35Expired( void )
{BOOL            xNeedPoll = FALSE;switch ( eRcvState ){/* Timer t35 expired. Startup phase is finished. */case STATE_RX_INIT:xNeedPoll = xMBPortEventPost( EV_READY );break;/* A frame was received and t35 expired. Notify the listener that* a new frame was received. */case STATE_RX_RCV:xNeedPoll = xMBPortEventPost( EV_FRAME_RECEIVED );break;/* An error occured while receiving the frame. */case STATE_RX_ERROR:break;/* Function called in an illegal state. */default:assert( ( eRcvState == STATE_RX_INIT ) ||( eRcvState == STATE_RX_RCV ) || ( eRcvState == STATE_RX_ERROR ) );}vMBPortTimersDisable(  );eRcvState = STATE_RX_IDLE;return xNeedPoll;
}

2.1数据类型说明

接收器状态	说明
STATE_RX_INIT	接收器已初始化
STATE_RX_IDLE	接收器空闲
STATE_RX_RCV	接收器正在接收
STATE_RX_ERROR	接收器错误

发送器状态说明：

发送器状态	说明
STATE_TX_IDLE	发送器空闲
STATE_TX_XMIT	正在发送

接着定义了一个数据缓存区、发送缓存指针、发送缓存长度、数据接收长度。

2.2函数说明

三、总结

mbrtu.c函数实现了modbus-rtu协议栈。它在整个协议栈中占据承上启下的作用个，向上，给modbus协议提供数据，向下接收硬件接收到的数据和往硬件发送数据。超时处理等等。其中一些功能还需要平台相关的接口的支持。这些在后面具体的移植中再进行分析。

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FreeModbus开源协议栈的移植和详解（三）- RTU协议代码分析

概述

一、RTU文件夹的文件

二、mbrtu.c文件

2.1数据类型说明

2.2函数说明

2.1`eMBRTUInit()`函数

2.2`eMBRTUStart()`函数

2.3`eMBRTUStop()`函数

2.4`eMBRTUReceive()`函数

2.5`eMBRTUSend()`函数

2.6`xMBRTUReceiveFSM()`函数

2.7`xMBRTUTransmitFSM()`函数

2.8`xMBRTUTimerT35Expired()`函数

三、总结

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