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     sio — fast interrupt driven asynchronous serial communications interface


     For standard ISA ports:
           device sio

           In /boot/device.hints:

     For AST compatible multiport cards with 4 ports:
           options COM_MULTIPORT
           device sio

           In /boot/device.hints:

     For Boca Board compatible multiport cards with 8 ports:
           options COM_MULTIPORT
           device sio

           In /boot/device.hints:

     For Netmos Nm9845 multiport cards with 6 ports:
           options COM_MULTIPORT
           device sio

           In /boot/device.hints:

     For Hayes ESP cards:
           options COM_ESP
           device sio

     For single port PCI and PCCARD cards:
           device sio

           No lines are required in /boot/device.hints for these cards.

     For dual port PCI cards that share an interrupt:
           device sio
           options COM_MULTIPORT

           In /boot/device.hints:

     Meaning of flags:
           0x00001   shared IRQs
           0x00002   disable FIFO
           0x00004   no AST/4 compatible IRQ control register
           0x00008   recover sooner from lost output interrupts
           0x00010   device is potential system console
           0x00020   device is forced to become system console
           0x00040   device is reserved for low-level IO (e.g. for remote kernel debugging)
           0x00080   use this port for remote kernel debugging
           0x0??00   minor number of master port
           0x10000   PPS timestamping on CTS instead of DCD
           0x20000   device is assumed to use a 16650A-type (extended FIFO) chip


     The sio driver provides support for NS8250-, NS16450-, NS16550 and NS16550A-based EIA
     RS-232C (CCITT V.24) communications interfaces.  The NS8250 and NS16450 have single
     character buffers, the NS16550A has 16 character FIFO input and output buffers.

     Input and output for each line may set to one of following baud rates; 50, 75, 110, 134.5,
     150, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 28800, 38400, 57600, or 115200.  Your
     hardware may limit your baud rate choices.

     The driver supports `multiport' cards.  Multiport cards are those that have one or more
     groups of ports that share an Interrupt Request (IRQ) line per group.  Shared IRQs on
     different cards are not supported.  Frequently 4 ports share 1 IRQ; some 8 port cards have 2
     groups of 4 ports, thus using 2 IRQs.  Some cards allow the first 2 serial ports to have
     separate IRQs per port (as per DOS PC standard).

     Some cards have an IRQ control register for each group.  Some cards require special
     initialization related to such registers.  Only AST/4 compatible IRQ control registers are
     supported.  Some cards have an IRQ status register for each group.  The driver does not
     require or use such registers yet.  To work, the control and status registers for a group,
     if any, must be mapped to the scratch register (register 7) of a port in the group.  Such a
     port is called a master port.

     The driver supports controller based PCI modems.  The 3Com FaxModem PCI and the Advantec 56k
     Voice Messaging PCI FaxModem are the only cards supported.  WinModems, softmodems, hfc
     modems and any other modems that are not controller based are not supported.

     The flags keyword may be used on each device sio line in the kernel configuration file to
     disable the FIFO on 16550A UARTs (see the synopsis).  Disabling the FIFO should rarely be

     The flags keyword must be used for all ports that are part of an IRQ sharing group.  One bit
     specifies IRQ sharing; another bit specifies whether the port does not require AST/4
     compatible initialization.  The minor number of the device corresponding a master port for
     the group is encoded as a bitfield in the high byte.  The same master port must be specified
     for all ports in a group.

     The irq specification must be given for master ports and for ports that are not part of an
     IRQ sharing group, and not for other ports.

     In the synopsis, flags 0x701 means that the 8th port (sio7) is the master port, and that the
     port is on a multiport card with shared IRQs and an AST/4 compatible IRQ control register.

     flags 0xb05 means that the 12th port (sio11) is the master port, and that the port is on a
     multiport card with shared IRQs and no special IRQ control register.

     Which port is the master port depends on the card type.  Consult the hardware documentation
     of your card.  Since IRQ status registers are never used, and IRQ control registers are only
     used for AST/4 compatible cards, and some cards map the control/status registers to all
     ports in a group, any port in a group will sometimes do for the master port.  Choose a port
     containing an IRQ status register for forwards compatibility, and the highest possible port
     for consistency.

     Serial ports controlled by the sio driver can be used for both `callin' and `callout'.  For
     each port there is a callin device and a callout device.  The minor number of the callout
     device is 128 higher than that of the corresponding callin port.  The callin device is
     general purpose.  Processes opening it normally wait for carrier and for the callout device
     to become inactive.  The callout device is used to steal the port from processes waiting for
     carrier on the callin device.  Processes opening it do not wait for carrier and put any
     processes waiting for carrier on the callin device into a deeper sleep so that they do not
     conflict with the callout session.  The callout device is abused for handling programs that
     are supposed to work on general ports and need to open the port without waiting but are too
     stupid to do so.

     The sio driver also supports an initial-state and a lock-state control device for each of
     the callin and the callout "data" devices.  The termios settings of a data device are copied
     from those of the corresponding initial-state device on first opens and are not inherited
     from previous opens.  Use stty(1) in the normal way on the initial-state devices to program
     initial termios states suitable for your setup.

     The lock termios state acts as flags to disable changing the termios state.  E.g., to lock a
     flag variable such as CRTSCTS, use stty crtscts on the lock-state device.  Speeds and
     special characters may be locked by setting the corresponding value in the lock-state device
     to any nonzero value.  E.g., to lock a speed to 115200, use “stty 115200” on the initial-
     state device and “stty 1” on the lock-state device.

     Correct programs talking to correctly wired external devices work with almost arbitrary
     initial states and almost no locking, but other setups may benefit from changing some of the
     default initial state and locking the state.  In particular, the initial states for non
     (POSIX) standard flags should be set to suit the devices attached and may need to be locked
     to prevent buggy programs from changing them.  E.g., CRTSCTS should be locked on for devices
     that support RTS/CTS handshaking at all times and off for devices that do not support it at
     all.  CLOCAL should be locked on for devices that do not support carrier.  HUPCL may be
     locked off if you do not want to hang up for some reason.  In general, very bad things
     happen if something is locked to the wrong state, and things should not be locked for
     devices that support more than one setting.  The CLOCAL flag on callin ports should be
     locked off for logins to avoid certain security holes, but this needs to be done by getty if
     the callin port is used for anything else.


     /dev/ttyd?       for callin ports
     /dev/ttyd?.lock  corresponding callin initial-state and lock-state devices

     /dev/cuad?       for callout ports
     /dev/cuad?.lock  corresponding callout initial-state and lock-state devices

     /etc/rc.d/serial  examples of setting the initial-state and lock-state devices

     The device numbers are made from the set [0-9a-v] so that more than 10 ports can be


     sio%d: silo overflow.  Problem in the interrupt handler.

     sio%d: interrupt-level buffer overflow.  Problem in the bottom half of the driver.

     sio%d: tty-level buffer overflow.  Problem in the application.  Input has arrived faster
     than the given module could process it and some has been lost.


     stty(1), termios(4), tty(4), comcontrol(8)


     The sio driver is derived from the HP9000/300 dca(4) driver and is currently under


     Data loss may occur at very high baud rates on slow systems, or with too many ports on any
     system, or on heavily loaded systems when crtscts cannot be used.  The use of NS16550A's
     reduces system load and helps to avoid data loss.

     Stay away from plain NS16550's.  These are early implementations of the chip with non-
     functional FIFO hardware.

     The constants which define the locations of the various serial ports are holdovers from DOS.
     As shown, hex addresses can be and for clarity probably should be used instead.

     Note that on the AST/4 the card's dipswitches should not be set to use interrupt sharing.
     AST/4-like interrupt sharing is only used when multiple AST/4 cards are installed in the
     same system.  The sio driver does not support more than 1 AST/4 on one IRQ.

     The examples in the synopsis are too vendor-specific.