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modbus serial ASCII mode not work

Excuse me , I have two RS232 ports,COM3 and COM4, on my computer, I connect COM3 TXD,RXD,GND  to COM4 RXD,TXD,GND, one by one.  And I have installed Labview 2009, VISA 4.4.1,and modbus1.2.1.There are two example in modbus,one is called"MB Serial Example Slave",another called"MB Serial Example Master". I run these two vi together to check the example, the slave connect COM3, and the master connect COM4. The example work well in the mode "RTU", while when I change it into the mode"ASCII", one of vi which occur en error:6101. If the master is run after the slave, the master occur this error. Else if the slave run after the master,the slaver occur this error. Is there any way to solve the problem?

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Message 1 of 4
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Please have a look on this link, which gives you possible issues for error 1601.

 

http://digital.ni.com/public.nsf/allkb/628A4CF844B2382A862575D700349D99

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Message 2 of 4
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Thank you for your answer. But the vi works well in RTU mode. So I thought it might not be the hardware error.

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Message 3 of 4
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I have paired two virtual com ports, tried same examples with ASCII option, executed MB_Serial_Slave vi first and then MB_Serial_Master vi. worked fine.

 

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