mbfuncdisc.c主要是对离散量的一些描述。当主机接收到02等功能码时，会进入对应功能码处理函数。

eMBException eMBFuncReadDiscreteInputs( UCHAR * pucFrame, USHORT * usLen );

/* ----------------------- System includes ----------------------------------*/
#include "stdlib.h"
#include "string.h"

/* ----------------------- Platform includes --------------------------------*/
#include "port.h"

/* ----------------------- Modbus includes ----------------------------------*/
#include "mb.h"
#include "mbframe.h"
#include "mbproto.h"
#include "mbconfig.h"

/* ----------------------- Defines ------------------------------------------*/
#define MB_PDU_FUNC_READ_ADDR_OFF           ( MB_PDU_DATA_OFF )
#define MB_PDU_FUNC_READ_DISCCNT_OFF        ( MB_PDU_DATA_OFF + 2 )
#define MB_PDU_FUNC_READ_SIZE               ( 4 )
#define MB_PDU_FUNC_READ_DISCCNT_MAX        ( 0x07D0 )

/* ----------------------- Static functions ---------------------------------*/
eMBException    prveMBError2Exception( eMBErrorCode eErrorCode );

/* ----------------------- Start implementation -----------------------------*/
/*
Function Code：02，读离散量输入
发送：1丛机地址+1功能码+2起始地址+2线圈数量+2CRC
返回：1丛机地址+1功能码+1字节数+2线圈状态+2CRC
*/
#if MB_FUNC_READ_COILS_ENABLED > 0
eMBException
eMBFuncReadDiscreteInputs( UCHAR * pucFrame, USHORT * usLen )
{
    USHORT          usRegAddress;
    USHORT          usDiscreteCnt;
    UCHAR           ucNBytes;
    UCHAR          *pucFrameCur;

    eMBException    eStatus = MB_EX_NONE;
    eMBErrorCode    eRegStatus;

    if( *usLen == ( MB_PDU_FUNC_READ_SIZE + MB_PDU_SIZE_MIN ) )
    {
        usRegAddress = ( USHORT )( pucFrame[MB_PDU_FUNC_READ_ADDR_OFF] << 8 );
        usRegAddress |= ( USHORT )( pucFrame[MB_PDU_FUNC_READ_ADDR_OFF + 1] );
        usRegAddress++;

        usDiscreteCnt = ( USHORT )( pucFrame[MB_PDU_FUNC_READ_DISCCNT_OFF] << 8 );
        usDiscreteCnt |= ( USHORT )( pucFrame[MB_PDU_FUNC_READ_DISCCNT_OFF + 1] );

        /* Check if the number of registers to read is valid. If not
         * return Modbus illegal data value exception. 
         */
        if( ( usDiscreteCnt >= 1 ) &&
            ( usDiscreteCnt < MB_PDU_FUNC_READ_DISCCNT_MAX ) )
        {
            /* Set the current PDU data pointer to the beginning. */
            pucFrameCur = &pucFrame[MB_PDU_FUNC_OFF];
            *usLen = MB_PDU_FUNC_OFF;

            /* First byte contains the function code. */
            *pucFrameCur++ = MB_FUNC_READ_DISCRETE_INPUTS;
            *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. */
            if( ( usDiscreteCnt & 0x0007 ) != 0 )
            {
                ucNBytes = ( UCHAR ) ( usDiscreteCnt / 8 + 1 );
            }
            else
            {
                ucNBytes = ( UCHAR ) ( usDiscreteCnt / 8 );
            }
            *pucFrameCur++ = ucNBytes;
            *usLen += 1;

            eRegStatus =
                eMBRegDiscreteCB( pucFrameCur, usRegAddress, usDiscreteCnt );

            /* 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;
}
#endif