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B-14 Appendix B—Serial Interface
HANDSHAKING
Handshaking controls the data flow across the serial interface.
Control of the data flow is necessary because it is possible for computers
to send data faster than the printer can print. The printer contains a data
storage buffer to ease this potential problem, but data flow control is
necessary to totally eliminate the possibility of data loss.
The printer provides two kinds of handshaking:
•
Software handshaking
•
Hardware handshaking
SOFTWARE HANDSHAKING
Software handshaking occurs when a device controls data flow using the
content of the data itself.
The printer supports two software handshaking protocols:
•
X-ON/X-OFF
•
ETX/ACK
X-ON/X-OFF
X-ON/X-OFF is commonly used when printers control the handshake.
The printer accepts characters until its input buffer is nearly full, at which
time it initiates the handshake by sending a "turn-off" character, X-OFF,
back to the computer. When the computer receives this character, it
stops transmitting data. When the input buffer is nearly empty, the printer
sends a "turn-on" character, X-ON, to the computer. Upon receipt of this
character, the computer resumes sending characters.