我用PC/PPi电缆与计算机相连，采用自由通信协议。用VB编程，PLC发回来在VB程序中接收正常，可是发送的始终不成功？请问是怎么回事？
程序如下：
主:
LD SM0.0
MOVB 9, SMB30 //自由通讯协议 9600bps
MOVB 16#A0, SMB87
MOVB 16#FF, SMB89
MOVB 100, SMB94
ATCH INT_0, 23 //若接收完成转中断1
ENI
RCV VB100, 0


INT_0:
NETWORK 1
LDB= SMB86, 16#20 //接收成功则接通线圈Q0.0
= Q0.0

VB程序定义了 发送按钮 文本框 使用 Microsoft Communications Control 控件通过串行口2进行发送。
程序如下：
Private Sub Exit_Click()
If MSComm1.PortOpen = True Then MSComm1.PortOpen = False
End Sub

Private Sub Form_Load()
Text1.Text = ""
With MSComm1
.CommPort = 2
.Settings = "9600,n,8,1" //通信设置为9600bps
.InputMode = comInputModeText
.RThreshold = 1
.InputLen = 0
.InBufferCount = 0
.OutBufferCount = 0 //串行口初始化
End With
If MSComm1.PortOpen = False Then MSComm1.PortOpen = True //打开串行口
End Sub

Private Sub Send_Click()
MSComm1.Output = Text1.Text //发送所输入的字符
End Sub