串行口能发送数据，但读取不到

能给下位机发送命令，但读取不到数据
用串口调试助手也如此

代码如下：


Private Sub Command1_Click()

Dim sts As String

If MSComm1.PortOpen = False Then MSComm1.PortOpen = True 'open the line

Call CMD_SEND("WRSTA") 'ＯＫ！下位机可以启动
Call STS_RD(sts) '读不进来数据，　ＳＴＳ＝＂＂

If sts = "OK" Then Text1.Text = "Start Measurement"

End Sub
Private Sub CMD_SEND(send_cmd As String)

Dim d$

d$ = send_cmd + Chr$(13)
MSComm1.Output = d$

End Sub
Private Sub STS_RD(ByRef status As String)

Dim a$, b$, d$
Dim L As Integer

Do
DoEvents
'Text1.Text = Text1.Text + "before input"
a$ = MSComm1.Input
'Text1.Text = Text1.Text + "after input"
b$ = Right(a$, 1)
d$ = d$ + a$
Loop Until (b$ = Chr$(13)) '陷入死循环

L = Len(d$)
status = Mid$(d$, 1, L - 1)

End Sub