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quickly. Normally, the receiver will also have a large buffer for the incoming characters.
When this buffer is full, it transmits an X-OFF. After it has read from the buffer the X-ON is
transmitted, see Figure 13.10.
Data
transmission
X-OFF
X-ON
Data
transmission
Figure 13.10
Software handshaking using X-ON and X-OFF
13.3.5 Hardware handshaking
Hardware handshaking stops characters in the receiver buffer from being overwritten. The
control lines used are all active HIGH. Figure 13.11 shows how the nodes communicate.
When a node wishes to transmit data it asserts the RTS line active (that is, HIGH). It then
monitors the CTS line until it goes active (that is, HIGH). If the CTS line at the transmitter
stays inactive then the receiver is busy and cannot receive data, at the present. When the re-
ceiver reads from its buffer the RTS line will automatically go active indicating to the trans-
mitter that it is now ready to receive a character.
RTS
Are you ready
to receive ?
Transmitting
node
CTS
RTS
No
CTS
RTS
Yes
CTS
TD
Receiving
node
Send
character
RD
Figure 13.11
Handshaking lines used in transmitting data
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