Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90051)

Chapter 9 187
Describing Asynchronous Devices
How to Control the Flow of Data
How to Control the Flow of Data
Flow control is how the transfer of data between the computer and
the asynchronous device is regulated. Flow control protects the
computer and the device from data overruns. A data overrun occurs
when the sender of data transmits the data faster than the receiver can
accept it. Because the receiver cannot accept all of the data being sent,
it is said to be overrun with data. Therefore, some data may be lost.
When asynchronous devices communicate with a HP 3000 Series 900
computer, one main method of flow control is used: the XON/XOFF
protocol. This protocol controls the flow of data between devices and
the Datacommunications and Terminal Controller (DTC). Its purpose is
to protect devices from data overruns.
An additional mechanism is used for controlling the flow of data from a
device to the computer: the read trigger character. The read trigger
character tells a device when it is allowed to send data. Its purpose is to
protect the DTC from data overruns.
Flow control is also called pacing methods. For instance, terminal
settings for the XON/XOFF protocol are called transmit pacing
(XmitPace) and receive pacing (RecvPace).
Because flow control guards against data overruns, it is a means of data
protection. Other methods of data protection used with asynchronous
devices are parity and printer status requests. Refer to “How to Control
Printers” later in this chapter for more information on parity and
printer status requests.
The XON/XOFF Protocol
The XON/XOFF protocol is controlled by the recipient of the data being
transferred. The recipient sends an XOFF character to the sender of the
data if it is unable to continue to receive data. The sender then
suspends the transmission of data. When the receiver can accept data
again, it sends the XON character to the sender, and the transmission
of data is resumed.
Assuming that receive pacing is enabled at a terminal (this is not the
terminal’s default setting), the terminal will send an XOFF character if
any one of the following occur:
• The terminal is placed in local mode. (Remote mode is necessary to
communicate to the computer; refer to the manual that came with
the terminal for an explanation of local and remote modes.)
• The terminal’s buffer (a temporary data storage area) fills up
because data is being received faster than the terminal is able to
display it.