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

Describing Asynchronous Devices
How to Control the Flow of Data
Chapter 9
172
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 e3000 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.
• The terminal user presses
[CTRL]-S
. (Pressing
[Stop]
on HP239x terminals also sends
the XOFF character to the computer. Note that this key is a toggle switch; the second
time it is pressed, it sends an XON character to the computer, the third time, an
XOFF character, and so forth.)
After a terminal sends an XOFF character, the DTC waits indefinitely for an XON
character.
A printer will send an XOFF character if any one of the following occur:
• The printer is placed off-line.
• The printer runs out of paper.