blmode - terminal block mode interface
This terminal interface adds functionality to the current termio(7)
functionality to allow for efficient emulation of MPE terminal driver
functionality. Most importantly, it adds the necessary functionality
to support 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 character 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 character 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 terminal. 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 recognizing 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
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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
Read requests that receive data from block mode transmissions will not
be returned until the transmission 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 completion of the data transmission
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 remaining 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.
CBTRIG1C (DC1) is the initial trigger character sent to the
terminal at the beginning of a read request.
CBTRIG2C (DC1) is the secondary trigger character sent to
the terminal after the alert character has been
CBALERTC (DC2) is the alert character sent by the terminal
in response to the first trigger character. It
signifies that the terminal is ready to send the
data block. The alert character can be escaped by
preceding it with a backslash (\).
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CBTERMC (RS) is sent by the terminal after the block mode
transfer is complete. It signifies the end of the
data block to the computer.
The two ioctl(2) requests that apply to block mode use the blmodeio
structure, which defined in <blmodeio.h>, and includes the following
unsigned long cb_flags; /* Modes */
unsigned char cb_trig1c; /* First trigger */
unsigned char cb_trig2c; /* Second trigger */
unsigned char cb_alertc; /* Alert character */
unsigned char cb_termc; /* Terminating char */
unsigned char cb_replen; /* cb_reply length */
char cb_reply; /* optional reply */
The cb_flags member controls the basic block mode protocol:
CB_BMTRANS 0000001 Enable mandatory block mode transmission.
CB_OWNTERM 0000002 Enable user control of handshake.
The CB_BMTRANS bit is only effective when the ICANON flag in
termio(7) is set. If ICANON is clear, all transfers are done in
raw mode, regardless of the CB_BMTRANS bit. If CB_BMTRANS is not
set, input processing is performed as described in termio(7).
During this time, if the alert character is defined and is
detected anywhere in the input stream, the input buffer is
flushed and block-mode handshake is invoked. The system then
sends the cb_trig2c character to the terminal, and a block mode
transfer follows. The alert character can be escaped by
preceding it with a backslash (\).
If CB_BMTRANS is set, then all transmissions are processed as
block mode transmissions. Block mode handshake is not required
and data read is processed as block mode transfer data. Block
mode handshake can still be invoked by receipt of an alert
character as the first character received. Reads issued while
the CB_BMTRANS bit is set cause any existing input buffer data to
If CB_OWNTERM is set, reads are terminated upon receipt of a
non-escaped alert character. No input buffer flushing is
performed and the alert character is returned in the data read.
This allows application code to perform its own block-mode
handshaking. If the bit is clear, an alert character causes
normal block mode handshaking to be used.
The initial cb_flags value is all-bits-cleared.
The cb_trig1c character is the initial trigger character sent to the
terminal at the beginning of a read request. The initial value is
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undefined (0377); i.e., no trigger character is sent.
The cb_trig2c character is the secondary trigger character sent to the
terminal after the alert character has been received. The initial
value is undefined (0377).
The cb_alertc character is the alert character sent by the terminal in
response to the first trigger character sent by the computer. It
signifies that the terminal is ready to transmit data. The initial
value is undefined (0377).
The cb_termc character is sent by the terminal after the block mode
transfer has completed. It signifies the end of the data block to the
computer. The initial value is undefined (0377).
The cb_replen member specifies the length in bytes of the cb_reply
array. The maximum length of the cb_reply array is NBREPLY bytes. If
set to zero, the cb_reply string is not used. It is initially set to
The cb_reply array contains a string to be sent out after receipt of
the alert character but before the second trigger character is sent by
the computer. Any character can be included in the reply string. The
number of characters sent is specified by cb_replen. The maximum
length of the cb_reply array is NBREPLY bytes. The initial value of
all characters in the cb_reply array is null.
On systems that support process group control, ioctl requests are
restricted from use by background processes, unless otherwise noted
for a specific request. An attempt to issue an ioctl request from a
background process causes the process to block and may cause a SIGTTOU
signal to be sent to the process group.
The primary ioctl(2) calls have the form:
int ioctl(int fildes, int request, struct blmodeio *arg);
Requests using this form include:
CBGETA Get the parameters associated with the block mode
interface and store them in the blmodeio structure
referenced by arg. This request is allowed from a
background process. However, the information may
be subsequently changed by a foreground process.
CBSETA Set the parameters associated with the block mode
interface from the blmodeio structure referenced
by arg. The change is immediate.
Refer to read(2), write(2), and ioctl(2).
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If an error value is returned during a read, it is possible for the
user's buffer to be altered. In this case, the data in the user's
buffer should be ignored because it is incomplete.
The global variable errno will be set to indicate the following error,
in addition to those errors described on read(2), write(2), and
[EIO] A read error occurred during the transmission of the
block mode data block.
The EIO error that is returned for read errors can be caused by many
events. The read returns EIO for transmission, framing, parity,
break, and overrun errors, or if the internal timer expires. The
internal timer starts when the second trigger character is sent by the
computer, and ends when the terminating character is received by the
computer. The length of this timer is determined by the number of
bytes requested in the read and the current baud rate, plus an
additional ten seconds.
The blmode driver was developed by HP.
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