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The Data-Ready Protocol The data-ready protocol works by controlling the signals on pins of
the interface. The pin functions with the data-ready protocol are shown in below. The DTR pin
controls the flow of information to the printer.
When the remaining buffer capacity is less than 256 bytes the printer changes the DTR signal to
LOW. When the data in the buffer falls to below 256 bytes the signal is again changed to HIGH.
Your computer must stop sending information within 256 characters after the printer makes the
DTR signal LOW or the information that overfills that buffer will be lost.
When the printer goes on line the printer changes the DTR signal from LOW to HIGH to signal
to the computer that it is ready to receive information.
The XON/XOFF Protocol This is the simplest of the software protocols. When the printer’s
buffer fills up it sends the ASCII code XOFF (13 HEX) to the computer to tell it to stop sending
information. When there is again some room in the buffer the printer sends the ASCII code XON
(11 HEX) to the computer to tell it to resume sending information. If the computer doesn’t stop
sending information within 256 characters after it receives an XOFF code the information that
overfills the buffer will be lost.
The printer will send an XON code when it goes on line to get things started. It will send an
XOFF code to stop the computer from sending information if there is an error, such as running
out of paper.
Serial Interface Data Ready Protocol Pin Functions
Pin Signal Direction Function
1 GND - Printer’s chassis ground
2 TXD - Not used
3 RXD IN Carries data to the printer
4 RTS OUT High when the printer is on line
7 GND - Signal ground
20 DTR OUT HIGH when the printer is ready to receive data
Pin functions With The XON/XOFF Software Protocols
Pin Signal Direction Function
1 GND - Printer’s chassis ground
2 TXD OUT Carries data from the printer
3 RXD IN Carries data to the printer
4 RTS OUT ON when the printer is ready to receive data
7 GND - Signal ground
20 DTR OUT Turned ON when the printer is ready to receive data.
Signal Levels The serial interface uses standard RS232C signal levels as shown below
Signal Level Input Output
LOW (0 or mark) -25V to -3V -12V
HIGH (1 or space) +3V to +25V +12V