Epson 4100 manual Software Handshaking, On/X-Off Etx/Ack

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.