HEIDENHAIN TNC 306 Technical Manual - Manual (Page 434)

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TNC 406/TNC 306 2 TNC data interfaces 8-21
The software handshake can easily be achieved when transmitting with a BCC. At first the receiver
sends neither a positive (<ACK>) nor a negative acknowledgment (<NAK>), and the transmitter
waits until it receives one of these characters. When the buffer in the receiver is again capable of
accepting data, it again sends an <ACK> and the transmitter continues its data transmission.
It is also possible, however, to conduct the software handshake with control characters <DC1> and
<DC3>.