Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90057)

Describing Asynchronous Devices
How to Control the Flow of Data
Chapter 9
173
• The paper in the printer jams.
• The printer’s buffer fills up because data is being received faster than the printer is
able to process it.
When printers (using printer type files 21, 22, or 26) send an XOFF character to the
computer, an internal timer (called the XOFF timer) starts. If the timer expires before
the printer sends an XON character, a message is sent to the system console. The
message informs the operator that intervention is needed to get the printer to function
again. After a printer using printer type file 18 sends an XOFF character to the
computer, the computer waits indefinitely for the XON character, but no message is sent
to the system console.
Use of the XON/XOFF protocol assumes the following:
• Full-duplex communication lines are used.
• The attached devices are capable of using the XON/XOFF protocol.
• The attached devices are capable of transmitting and receiving simultaneously.
These requirements are met if supported devices and cables are used. If non-HP devices
are used, ensure that these requirements are filled.
NOTE The two handshake characters, XON (the ASCII DC1 character) and XOFF (the ASCII
DC3 character), are defined as special characters and are reserved for the purpose of
protocol. These characters cannot be used as data except in binary mode.
The Read Trigger Character
The read trigger character tells an asynchronous device when it can send data to the
computer. It prevents a device from sending data before the computer is ready to receive
it. Data sent before the read trigger character is received is not accepted unless
typeahead is enabled. If typeahead is enabled, data can be entered at any time and the
read trigger character is not necessarily sent to the terminal. After the computer sends
the read trigger character, the input of data can proceed.
The ASC software generates the read trigger character at the beginning of each read
when the computer is ready to accept data. The read trigger character is the ASCII DC1
character (the same as XON). If the device is operating in character mode, the device can
begin transmitting without further exchange of protocol characters.
However, if the data is sent through block mode, the device informs the DTC by sending
an ASCII DC2 character in response to the read trigger character. The DTC, informed
that a lot of data will be coming, sends another read trigger character when it is ready to
accept the block of data. This second read trigger character is called a block read trigger
character.