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Interfaces and Terminals as of I7.8
System Interfaces
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2. 3. 2. 2
Flow control
Depending on the set mode, the data flow is controlled differently by the equip-
ment involved. The most common modes are Xon / Xoff or RTS /CTS.
Xon / Xoff mode
This mode is also known as a software handshake.
The data flow is controlled by the data receiving equipment. The data receiving
equipment sends an SW signal over the data line as soon as its input memory is full,
and an SW signal as soon as it is ready to receive again.
Xon / Xoff requires only a three-wire connection for signals RxD, TxD and SGND.
Drawback:
The data transmitting equipment cannot tell whether or not a receiving equip-
ment is connected.
RTS / CTS mode
This mode is also known as a hardware handshake.
The two equipment involved use the RTS and CTS signals to indicate that they are
ready to send and to receive, each signal being transmitted over a separate signal-
ling line.
Advantage:
Data exchange cannot take place if there is no equipment ready to receive. For
example no more data will be sent to the printer if the printer is switched off or out
of paper.
Note:
To exclude any transmission errors, always operate the printer on the PBX
in RTS / CTS mode.