HP 3000 Manuals HP 3000 Manuals
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.
* 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, etc.)
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.
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.