串口通信问题，急寻解答！

[leepone]

程序框图如附件所示。现在我可以控制下位机，并接收下位机传上来的完整数据。

      问题是：（1）程序最初运行时，第一次点击合闸按钮或分闸按钮时，下位机动作，可读取缓冲区没有数据显示；点击第二次按钮时，下位机动作，读取缓冲区才显示正确数据。（2）当点击合闸按钮读取缓冲区有数据显示时，接着点击分闸按钮，读取缓冲区显示的数据还是合闸数据，再点击一次才显示分闸数据。

     不知道大家是否明白我的意思，总之，就是我的预期结果是点击合闸按钮，下位机动作，接收合闸数据；点击分闸按钮，下位机动作，接收分闸数据；可现在的状况如（1）（2）所述，上下不同步。

     请大家指点一下！谢谢

[ian_fung]

Hi,

That is because you perform a VISA Read right after VISA Write; the expected (full) set of data may not be already available for reading back.

I assume that you already know how many bytes of data are expected (to be read back), you may at a sub-loop with timeout after VISA Write to check if Bytes at Port is equal or greater than the expected byte count. Perform a VISA Read only when Bytes at Port is equal or greater than the set byte count, else wait till timeout.

You may also perform a VISA Read of Bytes at Port right before the VISA Write to clear whatever data in the buffer. 

[leepone]

可否用简单的程序框图明示一下啊？谢谢啊！

[ian_fung]

Attached is one example code for (Serial) VISA Read with timeout.

 

[leepone]

非常感谢你的耐心指点，能不能把程序visa read with timeout的例子通过附件的形式附上啊！

谢谢！

[ian_fung]

I hope that you can re-construct the exact code by yourself... this way, you might be able to really understand 😉

You may omit the hidden Case(s). It will still works!

[leepone]

握已经尝试重建了你的例子，错误的地方请您指正，谢谢！

第一个case，没想明白它的条件，还有左上角的局部变量。

[leepone]

今天我按你的建议修改了自己的程序，调试了一下，还是不能解决先前的问题！麻烦你看看！

[ian_fung]

All you need is the code inside the Fixed Bytes Case. Please remove the Byte Count control and the first Case.

As for the local variable (top-left corner), it is merely to reset the Timeout error at entry of this VI. 

Message Edited by ian fung on 01-11-2010 01:19 PM

[leepone]

现在还是不能做到发送命令与接收数据的同步