Universal Serial BUS USB Optional Interface, On/X-Off Flow Control

Section 5. Interface Specifications

host must be capable of supporting this flow control method for it to function properly.

X-On/X-Off FLOW CONTROL

X-On/X-Offflow control is used whenever hardware (Ready/Busy) flow control is not available or desirable. Instead of a voltage going high/low at pin 20, control characters representing “Printer Ready” (X-On=11 hexadecimal) or “Printer Busy” (X-Off= 13 hexadecimal) are transmitted by the printer on pin 2 (Transmit Data) to the host. In order for this method of flow control to function correctly, the host must be capable of supporting it. X-On/X-Offoperates in a manner similar to the function of pin 20 (DTR) as previously explained. When the printer is first powered on it sends an X-Offwhen the “Buffer Near Full” level is reached and a X-Onwhen the data level of the buffer drops below the “Buffer Available” mark. When the printer is taken off-line manually, it transmits an X-Offindicating it cannot accept data. When it is placed back on line manually, it sends an X-On, indicating it is again available for receipt of data. If an error occurs during printing (paper out, ribbon out), the printer sends an X-Offas soon as an error condition is detected. When the error is cleared and the printer is placed back on-line, it transmits an X-Onindicating it is again ready to accept data.

Upon power up if no error conditions are present, the printer will continually send X-Oncharacters at five millisecond intervals until it receives a transmission from the host.

Data Streams

The data streams for X-On/X-Offand Ready/Busy flow control are constructed in the same way as they are for Ready/Busy flow control.

<ESC>A . . Job#1 . . <ESC>Z<ESC>A . . Job#n . . <ESC>Z

Example: <ESC>A . . Job#1 . . <ESC>Z

NOTE: All charactersare in ASCII.