SIO(4) BSD Kernel Interfaces Manual SIO(4)NAMEsio — fast interrupt driven asynchronous serial communications interface
For standard ISA ports:
For AST compatible multiport cards with 4 ports:
For Boca Board compatible multiport cards with 8 ports:
For Netmos Nm9845 multiport cards with 6 ports:
For Hayes ESP cards:
For single port PCI and PCCARD cards:
No lines are required in /boot/device.hints for these cards.
For dual port PCI cards that share an interrupt:
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 ker‐
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)
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
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. Fre‐
quently 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 sup‐
ported. 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 con‐
figuration file to disable the FIFO on 16550A UARTs (see the synopsis).
Disabling the FIFO should rarely be necessary.
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 con‐
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 corre‐
sponding 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 ini‐
tial-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 set‐
ting. 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
/dev/cuad? for callout ports
/dev/cuad?.lock corresponding callout initial-state and lock-state
/etc/rc.d/serial examples of setting the initial-state and lock-state
The device numbers are made from the set [0-9a-v] so that more than 10
ports can be supported.
sio%d: silo overflow. Problem in the interrupt handler.
sio%d: interrupt-level buffer overflow. Problem in the bottom half of
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
SEE ALSOstty(1), termios(4), tty(4), comcontrol(8)HISTORY
The sio driver is derived from the HP9000/300 dca(4) driver and is cur‐
rently under development.
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 can‐
not 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 prob‐
ably 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.
BSD August 30, 2006 BSD