版权声明：copyright@Leon_CSDN https://blog.csdn.net/LeonSUST/article/details/84980553
入手一份FreeModbus 已看呆 (๑Ő௰Ő๑)
文章按照FreeModbus协议栈的工作流程，对源代码进行总结解析；
FreeModbus协议栈作为从机，等待主机传送的数据；
当从机接收到一帧完整的报文后，对报文进行解析；
然后响应主机，发送报文给主机，实现主机和从机之间的通信；
1：demo.c中三个函数，完成协议栈的准备工作；
eMBInit()函数：(mb.c)
/*函数功能：
*1:实现RTU模式和ASCALL模式的协议栈初始化;
*2:完成协议栈核心函数指针的赋值，包括Modbus协议栈的使能和禁止、报文的接收和响应、3.5T定时器中断回调函数、串口发送和接收中断回调函数;
*3:eMBRTUInit完成RTU模式下串口和3.5T定时器的初始化，需用户自己移植;
*4:设置Modbus协议栈的模式eMBCurrentMode为MB_RTU，设置Modbus协议栈状态eMBState为STATE_DISABLED;
*/
eMBErrorCode
eMBInit( eMBMode eMode, UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{
//错误状态初始值
eMBErrorCode eStatus = MB_ENOERR;
//验证从机地址
if( ( ucSlaveAddress == MB_ADDRESS_BROADCAST ) ||
( ucSlaveAddress < MB_ADDRESS_MIN ) || ( ucSlaveAddress > MB_ADDRESS_MAX ))
{
eStatus = MB_EINVAL;
}
else
{
ucMBAddress = ucSlaveAddress; /*从机地址的赋值*/
switch ( eMode )
{
#if MB_RTU_ENABLED > 0
case MB_RTU:
pvMBFrameStartCur = eMBRTUStart; /*使能modbus协议栈*/
pvMBFrameStopCur = eMBRTUStop; /*禁用modbus协议栈*/
peMBFrameSendCur = eMBRTUSend; /*modbus从机响应函数*/
peMBFrameReceiveCur = eMBRTUReceive; /*modbus报文接收函数*/
pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
//接收状态机
pxMBFrameCBByteReceived = xMBRTUReceiveFSM; /*串口接收中断最终调用此函数接收数据*/
//发送状态机
pxMBFrameCBTransmitterEmpty = xMBRTUTransmitFSM; /*串口发送中断最终调用此函数发送数据*/
//报文到达间隔检查
pxMBPortCBTimerExpired = xMBRTUTimerT35Expired; /*定时器中断函数最终调用次函数完成定时器中断*/
//初始化RTU
eStatus = eMBRTUInit( ucMBAddress, ucPort, ulBaudRate, eParity );
break;
#endif
#if MB_ASCII_ENABLED > 0
case MB_ASCII:
pvMBFrameStartCur = eMBASCIIStart;
pvMBFrameStopCur = eMBASCIIStop;
peMBFrameSendCur = eMBASCIISend;
peMBFrameReceiveCur = eMBASCIIReceive;
pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
pxMBFrameCBByteReceived = xMBASCIIReceiveFSM;
pxMBFrameCBTransmitterEmpty = xMBASCIITransmitFSM;
pxMBPortCBTimerExpired = xMBASCIITimerT1SExpired;
eStatus = eMBASCIIInit( ucMBAddress, ucPort, ulBaudRate, eParity );
break;
#endif
default:
eStatus = MB_EINVAL;
}
//
if( eStatus == MB_ENOERR )
{
if( !xMBPortEventInit() )
{
/* port dependent event module initalization failed. */
eStatus = MB_EPORTERR;
}
else
{
//设定当前状态
eMBCurrentMode = eMode; //设定RTU模式
eMBState = STATE_DISABLED; //modbus协议栈初始化状态,在此初始化为禁止
}
}
}
return eStatus;
}
eMBEnable()函数：(mb.c)
/*函数功能
*1:设置Modbus协议栈工作状态eMBState为STATE_ENABLED;
*2:调用pvMBFrameStartCur()函数激活协议栈
*/
eMBErrorCode
eMBEnable( void )
{
eMBErrorCode eStatus = MB_ENOERR;
if( eMBState == STATE_DISABLED )
{
/* Activate the protocol stack. */
pvMBFrameStartCur( ); /*pvMBFrameStartCur = eMBRTUStart;调用eMBRTUStart函数*/
eMBState = STATE_ENABLED;
}
else
{
eStatus = MB_EILLSTATE;
}
return eStatus;
}
eMBRTUStart()函数：(mbrtu.c)
/*函数功能
*1:设置接收状态机eRcvState为STATE_RX_INIT；
*2:使能串口接收,禁止串口发送,作为从机,等待主机传送的数据;
*3:开启定时器，3.5T时间后定时器发生第一次中断,此时eRcvState为STATE_RX_INIT,上报初始化完成事件,然后设置eRcvState为空闲STATE_RX_IDLE;
*4:每次进入3.5T定时器中断,定时器被禁止，等待串口有字节接收后，才使能定时器;
*/
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( );
}
eMBPoll()函数：(mb.c)
/*函数功能:
*1:检查协议栈状态是否使能，eMBState初值为STATE_NOT_INITIALIZED，在eMBInit()函数中被赋值为STATE_DISABLED,在eMBEnable函数中被赋值为STATE_ENABLE;
*2:轮询EV_FRAME_RECEIVED事件发生，若EV_FRAME_RECEIVED事件发生，接收一帧报文数据，上报EV_EXECUTE事件，解析一帧报文，响应(发送)一帧数据给主机;
*/
eMBErrorCode
eMBPoll( void )
{
static UCHAR *ucMBFrame; //接收和发送报文数据缓存区
static UCHAR ucRcvAddress; //modbus从机地址
static UCHAR ucFunctionCode; //功能码
static USHORT usLength; //报文长度
static eMBException eException; //错误码响应枚举
int i;
eMBErrorCode eStatus = MB_ENOERR; //modbus协议栈错误码
eMBEventType eEvent; //事件标志枚举
/* Check if the protocol stack is ready. */
if( eMBState != STATE_ENABLED ) //检查协议栈是否使能
{
return MB_EILLSTATE; //协议栈未使能，返回协议栈无效错误码
}
/* Check if there is a event available. If not return control to caller.
* Otherwise we will handle the event. */
//查询事件
if( xMBPortEventGet( &eEvent ) == TRUE ) //查询哪个事件发生
{
switch ( eEvent )
{
case EV_READY:
break;
case EV_FRAME_RECEIVED: /*接收到一帧数据，此事件发生*/
eStatus = peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength );
if( eStatus == MB_ENOERR ) /*报文长度和CRC校验正确*/
{
/* Check if the frame is for us. If not ignore the frame. */
/*判断接收到的报文数据是否可接受，如果是，处理报文数据*/
if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
{
( void )xMBPortEventPost( EV_EXECUTE ); //修改事件标志为EV_EXECUTE执行事件
}
}
break;
case EV_EXECUTE: //对接收到的报文进行处理事件
ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF]; //获取PDU中第一个字节，为功能码
eException = MB_EX_ILLEGAL_FUNCTION; //赋错误码初值为无效的功能码
for( i = 0; i < MB_FUNC_HANDLERS_MAX; i++ )
{
/* No more function handlers registered. Abort. */
if( xFuncHandlers[i].ucFunctionCode == 0 )
{
break;
}
else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode ) /*根据报文中的功能码，处理报文*/
{
eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength );/*对接收到的报文进行解析*/
break;
}
}
/* If the request was not sent to the broadcast address we
* return a reply. */
if( ucRcvAddress != MB_ADDRESS_BROADCAST )
{
if( eException != MB_EX_NONE ) /*接收到的报文有错误*/
{
/* An exception occured. Build an error frame. */
usLength = 0; /*响应发送数据的首字节为从机地址*/
ucMBFrame[usLength++] = ( UCHAR )( ucFunctionCode | MB_FUNC_ERROR ); /*响应发送数据帧的第二个字节，功能码最高位置1*/
ucMBFrame[usLength++] = eException; /*响应发送数据帧的第三个字节为错误码标识*/
}
if( ( eMBCurrentMode == MB_ASCII ) && MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS )
{
vMBPortTimersDelay( MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS );
}
eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength ); /*modbus从机响应函数,发送响应给主机*/
}
break;
case EV_FRAME_SENT:
break;
}
}
return MB_ENOERR;
}
至此：完成Modbus协议栈的初始化准备工作，eMBPoll()函数轮询等待接收完成事件发生,接收机状态eRcvState为STATE_RX_IDLE空闲；
2：FreeModbus协议栈接收一帧完整报文机制：
FreeModbus协议栈通过淳口中断接收一帧数据，用户需在串口接收中断中回调prvvUARTRxISR()函数；
prvvUARTRxISR()函数：(portserial.c)
static void prvvUARTRxISR( void )
{
pxMBFrameCBByteReceived( );
}
在第一阶段中eMBInit()函数中赋值pxMBFrameCBByteReceived = xMBRTUReceiveFSM,发生接收中断时,最终调用xMBRTUReceiveFSM函数对数据进行接收；
xMBRTUReceiveFSM()函数：(mbrtu.c)
/*函数功能
*1:将接收到的数据存入ucRTUBuf[]中;
*2:usRcvBufferPos为全局变量，表示接收数据的个数;
*3:每接收到一个字节的数据，3.5T定时器清0
*/
BOOL
xMBRTUReceiveFSM( void )
{
BOOL xTaskNeedSwitch = FALSE;
UCHAR ucByte;
assert( eSndState == STATE_TX_IDLE ); /*确保没有数据在发送*/
( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte ); /*从串口数据寄存器读取一个字节数据*/
//根据不同的状态转移
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(); /*开启3.5T定时器*/
break;
/* In the error state we wait until all characters in the
* damaged frame are transmitted.
*/
case STATE_RX_ERROR: /*数据帧被损坏，重启定时器，不保存串口接收的数据*/
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: /*接收器空闲，开始接收，进入STATE_RX_RCV状态*/
usRcvBufferPos = 0;
ucRTUBuf[usRcvBufferPos++] = ucByte; /*保存数据*/
eRcvState = STATE_RX_RCV;
/* Enable t3.5 timers. */
vMBPortTimersEnable(); /*每收到一个字节，都重启3.5T定时器*/
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; /*接收数据*/
}
else
{
eRcvState = STATE_RX_ERROR; /*一帧报文的字节数大于最大PDU长度，忽略超出的数据*/
}
vMBPortTimersEnable(); /*每收到一个字节，都重启3.5T定时器*/
break;
}
return xTaskNeedSwitch;
}
当主机发送一帧完整的报文后，3.5T定时器中断发生，定时器中断最终回调xMBRTUTimerT35Expired函数;
xMBRTUTimerT35Expired()函数：(mbrtu.c)
/*函数功能
*1:从机接受完成一帧数据后，接收状态机eRcvState为STATE_RX_RCV；
*2:上报“接收到报文”事件(EV_FRAME_RECEIVED)
*3:禁止3.5T定时器，设置接收状态机eRcvState状态为STATE_RX_IDLE空闲;
*/
BOOL
xMBRTUTimerT35Expired( void )
{
BOOL xNeedPoll = FALSE;
switch ( eRcvState )
{
/* Timer t35 expired. Startup phase is finished. */
/*上报modbus协议栈的事件状态给poll函数,EV_READY:初始化完成事件*/
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: /*一帧数据接收完成*/
xNeedPoll = xMBPortEventPost( EV_FRAME_RECEIVED ); /*上报协议栈事件,接收到一帧完整的数据*/
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( ); /*当接收到一帧数据后，禁止3.5T定时器，只到接受下一帧数据开始，开始计时*/
eRcvState = STATE_RX_IDLE; /*处理完一帧数据，接收器状态为空闲*/
return xNeedPoll;
}
至此：从机接收到一帧完整的报文，存储在ucRTUBuf[MB_SER_PDU_SIZE_MAX]全局变量中，定时器禁止，接收机状态为空闲；
3：解析报文机制
在第二阶段，从机接收到一帧完整的报文后，上报“接收到报文”事件，eMBPoll函数轮询，发现“接收到报文”事件发生，调用peMBFrameReceiveCur函数，此函数指针在eMBInit被赋值eMBRTUReceive函数，最终调用eMBRTUReceive函数，从ucRTUBuf中取得从机地址、PDU单元和PDU单元的长度，然后判断从机地址地是否一致，若一致，上报“报文解析事件”EV_EXECUTE,(xMBPortEventPost( EV_EXECUTE ));“报文解析事件”发生后，根据功能码，调用xFuncHandlers[i].pxHandler( ucMBFrame, &usLength )对报文进行解析，此过程全部在eMBPoll函数中执行；
eMBPoll()函数：(mb.c)
/*函数功能:
*1:检查协议栈状态是否使能，eMBState初值为STATE_NOT_INITIALIZED，在eMBInit()函数中被赋值为STATE_DISABLED,在eMBEnable函数中被赋值为STATE_ENABLE;
*2:轮询EV_FRAME_RECEIVED事件发生，若EV_FRAME_RECEIVED事件发生，接收一帧报文数据，上报EV_EXECUTE事件，解析一帧报文，响应(发送)一帧数据给主机;
*/
eMBErrorCode
eMBPoll( void )
{
static UCHAR *ucMBFrame; //接收和发送报文数据缓存区
static UCHAR ucRcvAddress; //modbus从机地址
static UCHAR ucFunctionCode; //功能码
static USHORT usLength; //报文长度
static eMBException eException; //错误码响应枚举
int i;
eMBErrorCode eStatus = MB_ENOERR; //modbus协议栈错误码
eMBEventType eEvent; //事件标志枚举
/* Check if the protocol stack is ready. */
if( eMBState != STATE_ENABLED ) //检查协议栈是否使能
{
return MB_EILLSTATE; //协议栈未使能，返回协议栈无效错误码
}
/* Check if there is a event available. If not return control to caller.
* Otherwise we will handle the event. */
//查询事件
if( xMBPortEventGet( &eEvent ) == TRUE ) //查询哪个事件发生
{
switch ( eEvent )
{
case EV_READY:
break;
case EV_FRAME_RECEIVED: /*接收到一帧数据，此事件发生*/
eStatus = peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength );
if( eStatus == MB_ENOERR ) /*报文长度和CRC校验正确*/
{
/* Check if the frame is for us. If not ignore the frame. */
/*判断接收到的报文数据是否可接受，如果是，处理报文数据*/
if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
{
( void )xMBPortEventPost( EV_EXECUTE ); //修改事件标志为EV_EXECUTE执行事件
}
}
break;
case EV_EXECUTE: //对接收到的报文进行处理事件
ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF]; //获取PDU中第一个字节，为功能码
eException = MB_EX_ILLEGAL_FUNCTION; //赋错误码初值为无效的功能码
for( i = 0; i < MB_FUNC_HANDLERS_MAX; i++ )
{
/* No more function handlers registered. Abort. */
if( xFuncHandlers[i].ucFunctionCode == 0 )
{
break;
}
else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode ) /*根据报文中的功能码，处理报文*/
{
eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength );/*对接收到的报文进行解析*/
break;
}
}
/* If the request was not sent to the broadcast address we
* return a reply. */
if( ucRcvAddress != MB_ADDRESS_BROADCAST )
{
if( eException != MB_EX_NONE ) /*接收到的报文有错误*/
{
/* An exception occured. Build an error frame. */
usLength = 0; /*响应发送数据的首字节为从机地址*/
ucMBFrame[usLength++] = ( UCHAR )( ucFunctionCode | MB_FUNC_ERROR ); /*响应发送数据帧的第二个字节，功能码最高位置1*/
ucMBFrame[usLength++] = eException; /*响应发送数据帧的第三个字节为错误码标识*/
}
if( ( eMBCurrentMode == MB_ASCII ) && MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS )
{
vMBPortTimersDelay( MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS );
}
eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength ); /*modbus从机响应函数,发送响应给主机*/
}
break;
case EV_FRAME_SENT:
break;
}
}
return MB_ENOERR;
}
eMBRTUReceive()函数：(mbrtu.c)
/*eMBPoll函数轮询到EV_FRAME_RECEIVED事件时,调用peMBFrameReceiveCur()，此函数是用户为函数指针peMBFrameReceiveCur()的赋值
*此函数完成的功能：从一帧数据报文中，取得modbus从机地址给pucRcvAddress，PDU报文的长度给pusLength，PDU报文的首地址给pucFrame，函数
*形参全部为地址传递*/
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 ); /*断言宏，判断接收到的字节数<256，如果>256，终止程序*/
/* 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]; //取接收到的第一个字节，modbus从机地址
/* 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 ); //减3
/* 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;
}
xMBPortEventPost()函数：(portevent.c)
BOOL
xMBPortEventPost( eMBEventType eEvent )
{
xEventInQueue = TRUE;
eQueuedEvent = eEvent;
return TRUE;
}
xFuncHandlers[i]是结构体数组，存放的是功能码以及对应的报文解析函数，原型如下：
typedef struct
{
UCHAR ucFunctionCode;
pxMBFunctionHandler pxHandler;
} xMBFunctionHandler;
以下列举读线圈函数举例：
eMBFuncReadCoils()读线圈寄存器函数： (mbfunccoils.c)
#if MB_FUNC_READ_COILS_ENABLED > 0
eMBException
eMBFuncReadCoils( UCHAR * pucFrame, USHORT * usLen )
{
USHORT usRegAddress;
USHORT usCoilCount;
UCHAR ucNBytes;
UCHAR *pucFrameCur;
eMBException eStatus = MB_EX_NONE;
eMBErrorCode eRegStatus;
if( *usLen == ( MB_PDU_FUNC_READ_SIZE + MB_PDU_SIZE_MIN ) )
{
/*线圈寄存器的起始地址*/
usRegAddress = ( USHORT )( pucFrame[MB_PDU_FUNC_READ_ADDR_OFF] << 8 );
usRegAddress |= ( USHORT )( pucFrame[MB_PDU_FUNC_READ_ADDR_OFF + 1] );
//usRegAddress++;
/*线圈寄存器个数*/
usCoilCount = ( USHORT )( pucFrame[MB_PDU_FUNC_READ_COILCNT_OFF] << 8 );
usCoilCount |= ( USHORT )( pucFrame[MB_PDU_FUNC_READ_COILCNT_OFF + 1] );
/* Check if the number of registers to read is valid. If not
* return Modbus illegal data value exception.
*/
/*判断线圈寄存器个数是否合理*/
if( ( usCoilCount >= 1 ) &&
( usCoilCount < MB_PDU_FUNC_READ_COILCNT_MAX ) )
{
/* Set the current PDU data pointer to the beginning. */
/*为发送缓冲pucFrameCur赋值*/
pucFrameCur = &pucFrame[MB_PDU_FUNC_OFF];
*usLen = MB_PDU_FUNC_OFF;
/* First byte contains the function code. */
/*响应报文第一个字节赋值为功能码0x01*/
*pucFrameCur++ = MB_FUNC_READ_COILS;
*usLen += 1;
/* Test if the quantity of coils is a multiple of 8. If not last
* byte is only partially field with unused coils set to zero. */
/*usCoilCount%8有余数，ucNBytes加1,不够的位填充0*/
if( ( usCoilCount & 0x0007 ) != 0 )
{
ucNBytes = ( UCHAR )( usCoilCount / 8 + 1 );
}
else
{
ucNBytes = ( UCHAR )( usCoilCount / 8 );
}
*pucFrameCur++ = ucNBytes;
*usLen += 1;
eRegStatus =
eMBRegCoilsCB( pucFrameCur, usRegAddress, usCoilCount,
MB_REG_READ );
/* If an error occured convert it into a Modbus exception. */
if( eRegStatus != MB_ENOERR )
{
eStatus = prveMBError2Exception( eRegStatus );
}
else
{
/* The response contains the function code, the starting address
* and the quantity of registers. We reuse the old values in the
* buffer because they are still valid. */
*usLen += ucNBytes;;
}
}
else
{
eStatus = MB_EX_ILLEGAL_DATA_VALUE;
}
}
else
{
/* Can't be a valid read coil register request because the length
* is incorrect. */
eStatus = MB_EX_ILLEGAL_DATA_VALUE;
}
return eStatus;
}
至此：报文解析结束，得到ucMBFrame响应缓冲和usLength响应报文长度，等待发送报文；
4：发送响应报文
解析完一帧完整的报文后，eMBPoll()函数中调用peMBFrameSendCur()函数进行响应，eMBFrameSendCur()是函数指针，最终会调用eMBRTUSend()函数发送响应；
eMBRTUSend()函数：
/*函数功能
*1:对响应报文PDU前面加上从机地址;
*2:对响应报文PDU后加上CRC校;
*3:使能发送，启动传输;
*/
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. */
/*在协议数据单元前加从机地址*/
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; //发送状态
xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur ); /*发送一个字节的数据，进入发送中断函数，启动传输*/
pucSndBufferCur++; /* next byte in sendbuffer. */
usSndBufferCount--;
vMBPortSerialEnable( FALSE, TRUE ); /*使能发送，禁止接收*/
}
else
{
eStatus = MB_EIO;
}
EXIT_CRITICAL_SECTION( );
return eStatus;
}
进入发送中断，串口发送中断中调用prvvUARTTxReadyISR()函数，继续调用pxMBFrameCBTransmitterEmpty()函数，pxMBFrameCBTransmitterEmpty为函数指针，最终调用xMBRTUTransmitFSM()函数；
xMBRTUTransmitFSM()函数：(mbrtu.c)
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: /*发送器处于空闲状态，使能接收，禁止发送*/
/* enable receiver/disable transmitter. */
vMBPortSerialEnable( TRUE, FALSE );
break;
case STATE_TX_XMIT: /*发送器处于发送状态,在从机发送函数eMBRTUSend中赋值STATE_TX_XMIT*/
/* check if we are finished. */
if( usSndBufferCount != 0 )
{
//发送数据
xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );
pucSndBufferCur++; /* next byte in sendbuffer. */
usSndBufferCount--;
}
else
{
//传递任务，发送完成
xNeedPoll = xMBPortEventPost( EV_FRAME_SENT ); /*协议栈事件状态赋值为EV_FRAME_SENT,发送完成事件,eMBPoll函数会对此事件进行处理*/
/* Disable transmitter. This prevents another transmit buffer
* empty interrupt. */
vMBPortSerialEnable( TRUE, FALSE ); /*使能接收，禁止发送*/
eSndState = STATE_TX_IDLE; /*发送器状态为空闲状态*/
}
break;
}
return xNeedPoll;
}
至此：协议栈准备工作，从机接受报文，解析报文，从机发送响应报文四部分结束；