delay is selected, then there is a risk that some of the transmitteddata is output at all.  If the device is extremely slow, like a plotter, the closing_wait may need to be larger.  .TP.B session_lockoutLock out callout port (/dev/cuaXX) accesses across different sessions.That is, once a process has opened a port, do not allow a process witha different session ID to open that port until the first process hasclosed it..TP.B ^session_lockoutDo not lock out callout port accesses across different sessions..TP.B pgrp_lockoutLock out callout port (/dev/cuaXX) accesses across different process groups.That is, once a process has opened a port, do not allow a process in adifferent process group to open that port until the first process hasclosed it..TP.B ^pgrp_lockoutDo not lock out callout port accesses across different process groups..TP.B hup_notifyNotify a process blocked on opening a dial in line when a process hasfinished using a callout line (either by closing it or by the serialline being hung up) by returning EAGAIN to the open.  The application of this parameter is for getty's which are blocked ona serial port's dial in line.  This allows the getty to reset themodem (which may have had its configuration modified by theapplication using the callout device) before blocking on the open again..TP.B ^hup_notifyDo not notify a process blocked on opening a dial in line when thecallout device is hung up..TP.B split_termiosTreat the termios settings used by the callout device and the termiossettings used by the dialin devices as separate.  .TP.B ^split_termiosUse the same termios structure to store both the dialin and calloutports.  This is the default option..TP.B callout_nohupIf this particular serial port is opened as a callout device, do nothangup the tty when carrier detect is dropped..TP.B ^callout_nohupDo not skip hanging up the tty when a serial port is opened as acallout device.  Of course, the HUPCL termios flag must be enabled ifthe hangup is to occur..TP.B low_latencyMinimize the receive latency of the serial device at the cost ofgreater CPU utilization.  (Normally there is an average of 5-10mslatency before characters are handed off to the line discpline tominimize overhead.)  This is off by default, but certain real-timeapplications may find this useful..TP.B ^low_latencyOptimize for efficient CPU processing of serial characters at the cost of paying an average of 5-10ms of latency before the characters are processed.This is the default..SH CONSIDERATIONS OF CONFIGURING SERIAL PORTSIt is important to note that setserial merely tells the Linux kernelwhere it should expect to find the I/O port and IRQ lines of aparticular serial port.  It does *not* configure the hardware, theactual serial board, to use a particular I/O port.  In order to dothat, you will need to physically program the serial board, usually bysetting some jumpers or by switching some DIP switches.This section will provide some pointers in helping you decide how youwould like to configure your serial ports.The "standard MS-DOS" port associations are given below:.nf.RS/dev/ttys0 (COM1), port 0x3f8, irq 4/dev/ttys1 (COM2), port 0x2f8, irq 3/dev/ttys2 (COM3), port 0x3e8, irq 4/dev/ttys3 (COM4), port 0x2e8, irq 3.RE.fiDue to the limitations in the design of the AT/ISA bus architecture,normally an IRQ line may not be shared between two or more serialports.  If you attempt to do this, one or both serial ports willbecome unreliable if you try to use both simultaneously.  Thislimitation can be overcome by special multi-port serial port boards,which are designed to share multiple serial ports over a single IRQline.  Multi-port serial cards supported by Linux include the ASTFourPort, the Accent Async board, the Usenet Serial II board, theBocaboard BB-1004, BB-1008, and BB-2016 boards, and the HUB-6 serialboard.The selection of an alternative IRQ lineis difficult, since most of them are already used.  The following tablelists the "standard MS-DOS" assignments of available IRQ lines:.nf.RSIRQ 3: COM2IRQ 4: COM1IRQ 5: LPT2IRQ 7: LPT1.RE.fiMost people find that IRQ 5 is a good choice, assuming that there isonly one parallel port active in the computer.  Another good choice isIRQ 2 (aka IRQ 9); although this IRQ is sometimes used by networkcards, and very rarely VGA cards will be configured to use IRQ 2 as avertical retrace interrupt.  If your VGA card is configured this way;try to disable it so you can reclaim that IRQ line for some othercard.  It's not necessary for Linux and most other Operating systems.The only other available IRQ lines are 3, 4, and 7, and these areprobably used by the other serial and parallel ports.  (If your serialcard has a 16bit card edge connector, and supports higher interruptnumbers, then IRQ 10, 11, 12, and 15 are also available.)On AT class machines, IRQ 2 is seen as IRQ 9, and Linux will interpret itin this manner.IRQ's other than 2 (9), 3, 4, 5, 7, 10, 11, 12, and 15, should.I notbe used, since they are assigned to other hardware and cannot, in general,be changed.  Here are the "standard" assignments:.nf.RSIRQ  0      Timer channel 0IRQ  1      KeyboardIRQ  2      Cascade for controller 2IRQ  3      Serial port 2IRQ  4      Serial port 1IRQ  5      Parallel port 2 (Reserved in PS/2)IRQ  6      Floppy disketteIRQ  7      Parallel port 1IRQ  8      Real-time clockIRQ  9      Redirected to IRQ2IRQ 10      ReservedIRQ 11      ReservedIRQ 12      Reserved (Auxillary device in PS/2)IRQ 13      Math coprocessorIRQ 14      Hard disk controllerIRQ 15      Reserved.RE.fi.SH MULTIPORT CONFIGURATIONCertain multiport serial boards which share multiple ports on a singleIRQ use one or more ports to indicate whether or not there are anypending ports which need to be serviced.  If your multiport boardsupports these ports, you should make use of them to avoid potentiallockups if the interrupt gets lost.In order to set these ports specify.B set_multiportas a parameter, and follow it with the multiport parameters.  Themultiport parameters take the form of specifying the .I portthat should be checked, a .I maskwhich indicate which bits in the register are significant, and finally, a.I matchparameter which specifies what the significant bits in that register must match when there is no more pending work to be done.Up to four such port/mask/match combinations may be specified.  Thefirst such combinations should be specified by setting the parameters.BR port1 ,.BR mask1 ,and.BR match1 .The second such combination should be specified with.BR port2 ,.BR mask2 ,and.BR match2 ,and so on.  In order to disable this multiport checking, set .B port1to be zero.In order to view the current multiport settings, specify the parameter.B get_multiporton the command line.Here are some multiport settings for some common serial boards:.nf.RSAST FourPort    port1 0x1BF mask1 0xf match1 0xfBoca BB-1004/8  port1 0x107 mask1 0xff match1 0Boca BB-2016    port1 0x107 mask1 0xff match1 0                 port2 0x147 mask2 0xff match2 0.RE.fi.SH Hayes ESP Configuration.B Setserialmay also be used to configure ports on a Hayes ESP serial board..PPThe following parameters when configuring ESP ports:.TP.B rx_triggerThis is the trigger level (in bytes) of the receive FIFO. Largervalues may result in fewer interrupts and hence better performance;however, a value too high could result in data loss. Valid valuesare 1 through 1023..TP.B tx_triggerThis is the trigger level (in bytes) of the transmit FIFO. Largervalues may result in fewer interrupts and hence better performance;however, a value too high could result in degraded transmitperformance. Valid values are 1 through 1023..TP.B flow_offThis is the level (in bytes) at which the ESP port will "flow off"the remote transmitter (i.e. tell him to stop stop sending morebytes).  Valid values are 1 through 1023.  This value should begreater than the receive trigger level and the flow on level..TP.B flow_onThis is the level (in bytes) at which the ESP port will "flow on"the remote transmitter (i.e. tell him to resume sending bytes) afterhaving flowed it off.  Valid values are 1 through 1023.  This valueshould be less than the flow off level, but greater than the receivetrigger level..TP .B rx_timeoutThis is the amount of time that the ESP port will wait afterreceiving the final character before signaling an interrupt.  Validvalues are 0 through 255.  A value too high will increase latency,and a value too low will cause unnecessary interrupts..SH CAUTIONCAUTION: Configuring a serial port to use an incorrect I/O port can lock up your machine..SH FILES.BR /etc/rc.local.BR /etc/rc.serial.SH "SEE ALSO".BR tty (4),.BR ttys (4),kernel/chr_drv/serial.c.SH AUTHORThe original version of setserial was written by Rick Sladkey(jrs@world.std.com), and was modified by Michael K. Johnson(johnsonm@stolaf.edu).This version has since been rewritten from scratch by Theodore Ts'o(tytso@mit.edu) on 1/1/93.  Any bugs or problems are solely hisresponsibility.