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

188 Chapter9

Describing Asynchronous Devices

How to Control the Flow of Data

• 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.

• 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.