Break Character not generated in RS485 2 Wire Auto Mode

[Coatesy]

I am trying to generate a break character before bytes of data as part of a protocol I am using. I am using LabWindows/CVI with an NI RS485 4 port Serial Card which is fully installed and working as I have seen data transmitted from it. I am able to generate break characters in 2 wire ECHO Mode and data is sent, but my target hardware is unable to reply as the line appears to remain asserted by the computer. I want to use 2 wire AUTO mode but it seems unable to generate break characters.

The protocol uses 230400 baud, 1 start bit, 8 data bits, even parity, 1 stop bit. A break is sent before the data bytes.

Any suggestions are greatly appreciated, please let me know if I have omitted any required information 

[JonBowers]

Hi,

I have been having a look into this and its seems that there is some difference in implementing the break function. I connected an the TX lines of an RS485 port up to a scope and in four wire mode there was no problem in sending the break. In two wire DTR Controlled with and without echo the break is sent as long as flow control is set to DTR/DSR. In two wire Auto I did not see the break no matter what flow control was used.

Did you specify any flow control?

Regards

Jon B
Applications Engineer
NIUK