vb与单片机数据传输

   目的：上位机（vb编写）发送一个数据给单片机，单片机接收到数据后，将数据加一后返回给上位机，
   现象：发送正常，接收时，不论你发送是什么数据，单片机返回给上位机的数据一直是1？？？
大家帮忙看看我那里错了。
下面是我的vb程序
Private Sub Command1_Click()
Dim outbyte(0) As Byte
Dim number As Integer
number = Val(Text1.Text)
outbyte(0) = CByte(number)
MSComm1.Output = outbyte
End Sub
Private Sub Command2_Click()
MSComm1.PortOpen = False
Unload Me
End Sub
Private Sub Form_Load()
Form1.AutoRedraw = True
MSComm1.CommPort = 1
MSComm1.Settings = "9600,n,8,1"
MSComm1.InputLen = 0              ' 设置或返回一次从接收缓冲区中读取字节数，0表示一次读取所有数据
MSComm1.InBufferCount = 0
MSComm1.OutBufferCount = 0
MSComm1.RThreshold = 1            ' 每个字符到接收缓冲区都触发接收事件
MSComm1.InputMode = comInputModehexadecimal      '采用二进制传输
MSComm1.PortOpen = True
Text1 = ""
Text2 = ""
End Sub
Private Sub MSComm1_OnComm()
    Dim BytReceived(0) As Byte
    Dim strBuff As Variant
    Select Case MSComm1.CommEvent
        Case comEvReceive
        strBuff = MSComm1.InBufferCount
        BytReceived(0) = strBuff
        Text2 = BytReceived(0)
    End Select
End Sub

下面是我单片机程序
#include
#define uint unsigned int
#define uchar unsigned char
uchar a,flag;
void main()
{
TMOD=0X20;
TL1=0XFD;
TH1=0XFD;
SCON=0X50;
TR1=1;
EA=1;
ES=1;
while(1)
{
  if(flag)
  {
   ES=0;
   flag=0;
   SBUF=a+1;
   while(!TI);
    TI=0;
   ES=1;
  }
}
}
void xx() interrupt 4
{
RI=0;
a=SBUF;
P0=0XF0|a;
flag=1;
}

3