termio.7 (2010 09)

t
termio(7) termio(7)
processes.
The general terminal interface uses a system resource known as a
cblock to store data being transmit-
ted or received through a communications port. These cblocks are continuously used and freed for reuse
as data pass through the system. If too few cblocks are configured in the system, the cblock pool may be
temporarily or permanently exhausted, and data loss, system hangs, or reduced system performance can
result.
If cblock exhaustion is suspected, you can examine the system message buffer with
dmesg (see
dmesg(1M)) for messages indicating cblock exhaustion has occurred. Or, you can use
adb (see adb(1)) if
examining the corefile of a dump. The message format is
WARNING: cblock exhaustion occurred n times
where n indicates the number of times the operating system has requested a cblock and none could be
provided. If this message is observed, the kernel should be reconfigured to generate a larger number of
cblocks.
A cblock is 32 bytes in length. The default number of cblocks configured in the system is defined to be
8292.
This can be overridden by using the optional tunable system parameter
nclist to specify the desired
number of cblocks to be used in the system.
SAM or kctune (1M) may be used to change the nclist value.
DEPENDENCIES
Workstations
Built-in serial ports on workstation machines support the following additional baud rate settings: 57 600,
and 115 200. An RS-232-to-RS-422 converter may be required to achieve practical cable lengths at these
baud rates (because RS-232 only specifies up to 19 200 baud).
Timed delays are not supported.
Built-in serial ports on workstation systems have RTS and CTS flow control capability, configurable
receive FIFO trigger levels, and a configurable transmit limit. RTS/CTS hardware handshaking can be
enabled through a bit in the device file minor number, through an
ioctl() call (see termiox (7)), or
through the
stty command (see stty (1)).
The receive FIFO trigger level is configurable through two bits in the device file minor number. The
receive FIFO trigger level is used to set the level at which a receive interrupt is generated to the system.
Setting a smaller value for the receive FIFO trigger level enables the system to react more quickly to
receipt of characters. However, using a smaller trigger level increases system overhead to process the
additional interrupts. A higher receive FIFO trigger level reduces the system interrupt overhead for
heavy inbound data traffic at the cost of less time for the system to read data from the hardware before
receive FIFOs are overrun. When using RTS flow control, the receive FIFO trigger level also determines
the point at which the hardware lowers RTS to protect the receive FIFO. Use of a higher receive FIFO
trigger level also reduces XOFF flow control responsiveness because, under light inbound data flow condi-
tions, receipt of the XOFF character by the system is slightly delayed. Choice of the appropriate receive
FIFO trigger level should be based upon how the serial port is to be used. For most applications a receive
FIFO trigger level of 8 (c3,c2 = 10) is suggested.
Two bits in the device file minor number specify the transmit limit, the number of characters which are
successively loaded into the transmit FIFO. Setting a smaller transmit limit allows the transmitter to be
more responsive to flow control either from receipt of an XOFF character or de-assertion of CTS at the
cost of increased system interrupt overhead. Setting a larger transmit limit reduces interrupt overhead
but is not as responsive to flow control since the remainder of the transmit FIFO can be transmitted even
after the transmitter is flow controlled. When communicating with devices which have little tolerance for
data receipt after flow control, one must choose the transmit limit appropriately.
Device File Minor Number
Workstation device file minor numbers take the form:
0xIIC0HM
where:
II = Two hexadecimal digits (8 bits) to indicate the instance of the serial interface.
HP-UX 11i Version 3: September 2010 − 19 − Hewlett-Packard Company 19