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、初始化协议栈---eMBInit函数（mb.c中），以RTU为例
eMBErrorCode eMBInit( eMBMode eMode, UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{
1
/*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 eStatus = MB_ENOERR;
/* check preconditions */
if( ( ucSlaveAddress == MB_ADDRESS_BROADCAST ) ||
( ucSlaveAddress < MB_ADDRESS_MIN ) || ( ucSlaveAddress > MB_ADDRESS_MAX ) ) //验证从机地址
{
eStatus = MB_EINVAL; //地址错误
}
else
{
ucMBAddress = ucSlaveAddress;
switch ( eMode )
{
#if MB_RTU_ENABLED > 0
case MB_RTU:
pvMBFrameStartCur = eMBRTUStart;
pvMBFrameStopCur = eMBRTUStop;
peMBFrameSendCur = eMBRTUSend;
peMBFrameReceiveCur = eMBRTUReceive;
pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
pxMBFrameCBByteReceived = xMBRTUReceiveFSM; //接收状态机，串口接受中断最终调用此函数接收数据
pxMBFrameCBTransmitterEmpty = xMBRTUTransmitFSM; //发送状态机，串口发送中断最终调用此函数发送数据
pxMBPortCBTimerExpired = xMBRTUTimerT35Expired; //报文到达间隔检查，定时器中断函数最终调用次函数完成定时器中断
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;
}
eMBInit函数中pvMBFrameStartCur、pvMBFrameStartCur等为协议栈函数的接口，对于不同的通信模式使用不同的函数进行初始化！！！此编程模式可以借鉴学习！！！
eMBInit函数对底层驱动（串口和定时器）进行初始化。初始化完成并且成功之后对事件也进行了初始化，完成后全局变量eMBState=STATE_DISABLED。
2、启动协议栈----eMBEnable函数（mb.c函数）
eMBErrorCode
eMBEnable( void )
{
/*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 eStatus = MB_ENOERR;
if( eMBState == STATE_DISABLED )
{
/* Activate the protocol stack. */
pvMBFrameStartCur( ); //激活协议栈
eMBState = STATE_ENABLED; //设置Modbus协议栈工作状态eMBState为STATE_ENABLED
}
else
{
eStatus = MB_EILLSTATE;
}
return eStatus;
}
---eMBRTUStart函数 （mbrtu.c）
void
eMBRTUStart( void )
{
/*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;
* */
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 ); //开启串口接收，发送未开启
vMBPortTimersEnable( ); //启动定时器
EXIT_CRITICAL_SECTION( );
}
eMBEnable函数启动协议栈pvMBFrameStartCur，对于RTU模式，pvMBFrameStartCur函数指针指向 eMBRTUStart，在eMBRTUStart函数中，全局变量eRcvState=STATE_RX_INIT,并使能串口和定时器。注意！！！此时定时器将开始工作！！！。eMBEnable函数中将把全局变量改为 eMBState=STATE_ENABLED。
3、状态机轮训---eMBPoll函数（mb.c）
eMBErrorCode eMBPoll( void )
{
/*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;
*/
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;
eMBEventType eEvent; //错误码
/* 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. */
if( xMBPortEventGet( &eEvent ) == TRUE ) //判断事件是否发生
{
switch ( eEvent ) //查询哪个事件发生
{
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 ); //接收数据，并检验报文长度和CRC校验是否正确
/*
* ucRcvAddress 主站要读取的从站的地址
* ucMBFrame 指向PDU的头部
* usLength PDU的长度
*/
if( eStatus == MB_ENOERR )
{
/* Check if the frame is for us. If not ignore the frame. */
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: //修改事件标志为EV_EXECUTE执行事件
ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF]; //提取功能码
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;
}
在eMBPoll函数中，首先由 xMBPortEventGet( &eEvent ) == TRUE 判断时间是否发生，若无事件发生则不进入状态机；若有时间发生则进入状态机开始轮询。状态机的时间转换在定时中断服务函数中实现。
在eMBEnable函数中启动定时器后，定时器开始工作（见 eMBRTUStart函数）， 在定时器第一次超时之后将会发送xNeedPoll = xMBPortEventPost( EV_READY ) 事件，然后关闭定时器，接收机状态（全局变量）eRcvState为STATE_RX_IDLE。此时，主循环eMBPoll中将执行一次EV_READY下的操作。至此，完成Modbus协议栈的初始化准备工作，协议栈开始运行，eMBPoll()函数轮询等待接收完成事件发生。
---定时器中断服务函数
void BOARD_GPTA_HANDLER()
{
// BOOL bTaskWoken = FALSE;
PRINTF("\r\nTimer Expired:\n\n\r");
vMBPortSetWithinException( TRUE );
GPT_ClearStatusFlag(BOARD_GPTA_BASEADDR, gptStatusFlagOutputCompare1); //关闭定时器
( void )pxMBPortCBTimerExpired( ); //事件转换 vMBPortSetWithinException( FALSE );
}
---xMBRTUTimerT35Expired T3.5超时函数
BOOL xMBRTUTimerT35Expired( void )
{
/* 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 xNeedPoll = FALSE;
switch ( eRcvState ) //上报modbus协议栈的事件状态给poll函数
{
/* Timer t35 expired. Startup phase is finished. */
case STATE_RX_INIT:
xNeedPoll = xMBPortEventPost( EV_READY ); //初始化完成事件
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，直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;
}
4、报文接收
在定时器第一次中断之后，状态机为eRcvState=STATE_RX_IDLE，即读空闲状态，eMBPoll也阻塞在等待接收完成事件发生。而在eMBPoll之前的eMBRTUStart函数中已经开启了串口中断，因此在接收到数据之后，串口中断将会响应，在串口中断服务函数中将调用接收状态机函数xMBRTUReceiveFSM来接收数据。
---xMBRTUReceiveFSM函数
BOOL xMBRTUReceiveFSM( void )
{
/*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 xTaskNeedSwitch = FALSE;
UCHAR ucByte;
assert( eSndState == STATE_TX_IDLE ); //确保没有数据在发送
/* Always read the character. */
( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte ); //从串口数据寄存器读取一个字节数据
switch ( eRcvState ) //根据不同的状态转移
{
/* 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;
}
在串口中断前，状态机为eRcvState=STATE_RX_IDLE，接收状态机开始后，读取uart串口缓存中的数据，并进入STATE_RX_IDLE分支中存储一次数据后开启定时器，然后进入STATE_RX_RCV分支继续接收后续的数据，直至定时器超时！如果没有超时的话，状态不会转换，将还可以继续接收数据。超时之后，在T3.5超时函数xMBRTUTimerT35Expired 中将发送EV_FRAME_RECEIVED事件。然后eMBPoll函数将会调用eMBRTUReceive函数。
/* 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;
---eMBRTUReceive函数
eMBErrorCode
eMBRTUReceive( UCHAR * pucRcvAddress, UCHAR ** pucFrame, USHORT * pusLength )
{
/*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，
*/
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]; //保存从站地址
/* 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 ); //PDU长度
/* Return the start of the Modbus PDU to the caller. */
*pucFrame = ( UCHAR * ) & ucRTUBuf[MB_SER_PDU_PDU_OFF]; //pucFrame指向PDU起始位置
xFrameReceived = TRUE;
}
else
{
eStatus = MB_EIO;
}
EXIT_CRITICAL_SECTION( );
return eStatus;
}
eMBRTUReceive函数完成了CRC校验、帧数据地址和长度的赋值，便于给上层进行处理！之后eMBPoll函数发送 ( void )xMBPortEventPost( EV_EXECUTE )事件。在EV_EXECUTE 事件中，从站对接收到的数据进行处理，包括根据功能码寻找功能函数处理报文和调用eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength ) 发送应答报文。
--- eMBRTUSend函数
eMBErrorCode eMBRTUSend( UCHAR ucSlaveAddress, const UCHAR * pucFrame, USHORT usLength )
{
/*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 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; //在协议数据单元前加从机地址
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
//以下为新添加
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;
}
在 eMBRTUSend函数中会调用串口发送数据，在进入串口发送中断后会调用xMBRTUTransmitFSM发送状态机函数发送应答报文。
---xMBRTUTransmitFSM函数
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 ); //发送器处于空闲状态，使能接收，禁止发送
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。
作者：wj_hubei
- 该日志由 求索阁 于5年前发表在工业·编程分类下，最后更新于 2020年05月30日.
- 转载请注明: freemodbus-v1.5.0 源码分析 | 求索阁 +复制链接
- 关键字: freemodbus, modbus, 网络协议