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RS232 Flow control

By jbrown8328 ·
I am communicating with DCE via RS232 using win32 api's. One of my tasks is to send new firmware to the DCE essentially a string of ascii characters about 120 kb long. The DCE will send the Xoff when its buffer is full but transmission is not stopping from the DTE. I have verified that I can set Xon/Xoff flow control and Xon/Xoff characters are not placed in the input que. Is software flow control auotomatic or I have to respond to some predefined event? Where do the Xon/Xoff characters go if not the input que and what flags (if any) get set when they are recieved?

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by ozi Eagle In reply to RS232 Flow control


I don't know the details of XON XOFF, but why don't you set the flow control to hardware, RTS/CTS? That way the software is not involved.

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by jbrown8328 In reply to

The device I'm communicating with only has 2 wires Tx and Rx, so hardware control is not an option. Any idea where I can find the details of Xon/Xoff flow control? There has to be a flag set or some sort of event happening when Xoff is recieved because they are not placed in the input que. My other option is to try and invoke this 16 bit dos app that was designed for the device. Can this be done in win32 with vb or some api function.

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by ozi Eagle In reply to RS232 Flow control


I tried Googling for xon xoff specs and got some potentially useful sites. This one seems to have the definitions you need.

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Xon/Xoff Flow Control

by roubaldo In reply to RS232 Flow control

Software controls the data flow sometimes described as Xon/Xoff; it is connected with the fact that two characters Xon and Xoff are used. Xon is usually a 17 character, and Xoff is a 19 character . The modem will only have a small buffer as when the computer fills it, the modem sends a Xoff character to inform the computer about data transfer termination. As soon as the modem empties the most of the memory for data, it will send the Xon character to the computer and the latter starts the data transfer. This type of data flow control has the following advantages: it doesn't need any other wires as the characters are sent via TD/RD lines. But if the connection is slow, every character needs 10 bits, which can reduce the connection speed.

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