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Data flow control (Handshaking)

To configure port parameters, you should switch to the "Port - Data flow control" tab in the options window (fig. 3).



Fig. 3 Handshaking setup.


Handshaking is used when the NC cannot quickly process the received data. To prevent the input buffer in the NC from being overrun, it tells the program (DNC server) to pause sending data.


The method of exchanging signals between the NC and the DNC server is called handshaking. There are two handshaking methods: software (XOn/XOff) and hardware. For hardware handshaking, you will need a special cable with hardware flow control wires.


Software flow control (XOn/XOff) - Two bytes that are not usually used to transmit text are assigned to two special characters of pausing (XOff) and resuming (XOn) data transmission. When the NC wants the DNC Precision to pause sending data, it sends the character XOff (Transmission Off). When the buffer of the NC is empty, it sends the signal XOn (Transmission On), meaning that it is ready to receive data again. The default for XOff is ASCII 19, sometimes also called DC3 (Device Control 3), and ASCII 17 (DC1) for XOn. XOn/XOff are used only for text transmission. They cannot be used to transmit binary data, for binary files may contain the characters XOn or XOff.


Hardware flow control (RTS/CTS) - RTS/CTS is a normal form of hardware handshaking. The computer signals via the RTS (Request To Send) line that it wants to send data to the NC, while the NC sends the CTS (Clear To Send) signal when it is ready to receive data.


Hardware flow control (DTR/DSR) - The DTR (Data Terminal Ready) and DSR (Data Set Ready) signals can also be used for handshaking. The DNC server sends the DTR signal when it is ready to send and receive data, while the NC uses the DSR signal for that.


It is possible to use both handshaking methods simultaneously. The handshaking settings should match those on the NC.


XOn/XOff characters - Some machines use nonstandard characters for XOn/XOff. In this case, you can specify your code value for these characters. The character code is specified in the hexadecimal form #XX, where XX is the hexadecimal ASCII character code and can be calculated in the standard Windows calculator. If you are not confident, specify the default values of XOn = #11, and XOff = #13.


Set RTS high - If the RTS signal is not used for hardware handshaking, it can have either low or high level. The default "on" setting is high level.


Set DTR high - If the DTR signal is not used for hardware handshaking, it can have either low or high level. The default "on" setting is high level.