blmode.7 (2010 09)

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blmode(7) blmode(7)
NAME
blmode - terminal block mode interface
DESCRIPTION
This terminal interface adds functionality to the current termio (7) functionality to allow for efficient emu-
lation of
MPE terminal driver functionality. Most importantly, it adds the necessary functionality to sup-
port block mode transfers with HP terminals. The block mode interface only affects input processing and
does not affect write requests. Write requests are always processed as described in termio (7). In charac-
ter mode the terminal sends each character to the system as it is typed. However, in block mode data is
buffered and possibly edited locally in the terminal memory as it is typed, then sent as a block of data
when the Enter key is pressed on the terminal. During block mode data transmissions, the incoming
data is not echoed and no special character processing is performed, other than recognizing a data block
terminator character. For subsequent character mode transmissions, the existing termio state continues
to determine echo and character processing.
There are two parts of the block mode protocol. The first part is the block mode handshake, which works
as follows:
• At the beginning of a read, a trigger character is sent to the terminal to notify it that the system
is requesting a block of data. (The trigger character, if defined, is sent at the beginning of all
reads, whether character or block. The trigger character must be defined for block mode reads.)
• After receiving the trigger character, the terminal waits until the user has typed data into the
terminal’s memory and pressed the terminal Enter key. The terminal then sends an alert char-
acter to the system to notify it that the terminal has a block of data to send.
• The system may then send user-definable cursor positioning or other data sequences to the ter-
minal. When that is done, the system sends another trigger character to the terminal, repeating
the cycle.
The second part of the block mode protocol is the block mode transmission. During this transmission of
data, the incoming data is not echoed and no special character processing is performed, other than recog-
nizing the data block termination character. It is possible to bypass the block mode handshake and have
the block mode transmission occur after the first trigger character is sent.
To prevent data loss,
XON/XOFF flow control should be used between the system and the terminal. The
IXOFF bit should be set and the terminal strapped appropriately. If flow control is not used, it is possible
for incoming data to overflow and be lost. (Note: some older terminals do not deal correctly with this flow
control.)
It is possible to intermix both character mode and block mode data transmissions. If block mode
transmissions are enabled, all transfers are handled as block mode transfers. When block mode
transmissions are not enabled, character mode transmissions are processed as described in termio (7). If
block mode transmissions are not enabled, but an alert character is received anywhere in the input data,
the transmission mode is switched to block mode automatically for a single transmission.
Read requests that receive data from block mode transmissions will not be returned until the transmis-
sion is complete; i.e., the terminal has transmitted all characters. If the read is satisfied by byte count or
if a data transmission error occurs, any subsequent data will be discarded. The read waits until comple-
tion of the data transmission before returning.
The data block terminator character is included in the data returned to the user, and is included in the
byte count. If the number of bytes transferred by the terminal in a block mode transfer exceeds the
number of bytes requested by the user, the read returns the requested number of bytes, and the remain-
ing bytes are discarded. The user can determine if data was discarded by checking the last character of
the returned data. If the last character is not the terminator character, more data was received than was
requested, and data was discarded.
If desired, the application program can provide its own handshake mechanism in response to the alert
character by selecting the
OWNTERM mode. With this mode selected, the driver completes a read request
when the alert character is received. The second trigger is sent by the driver when the application issues
the next read.
Several special characters (both input and output) are used with block mode. These characters and the
normal values used for block mode are described below. The initial value for these characters is 0377,
which causes them to be disabled.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1