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+.\" Copyright (c) 1980, 1991 Regents of the University of California.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     from: @(#)dh.4	6.3 (Berkeley) 3/27/91
+.\"	$Id: dh.4,v 1.1 1995/10/18 08:44:34 deraadt Exp $
+.\"
+.Dd March 27, 1991
+.Dt DH 4 vax
+.Os BSD 4
+.Sh NAME
+.Nm dh
+.Nd
+.Tn DH-11 Ns / Tn DM-11
+multiplexer device interface
+.Sh SYNOPSIS
+.Cd "device dh0 at uba0 csr 0160020 vector dhrint dhxint" Op Ar flags
+.Cd "device dm0 at uba0 csr 0170500 vector dmintr" Op Ar flags
+.Sh DESCRIPTION
+A
+.Tn DH-11
+provides 16 serial communication lines;
+.Tn DM-11 Ns s
+may optionally be
+paired with
+.Tn DH-11 Ns s
+to provide modem control for the lines.
+.Pp
+An optional argument
+.Ar flags
+may be supplied with the device specification
+in the
+.Xr config 8
+file indicating
+that the line corresponding to
+bit number
+.Ar i
+is not properly
+connected, and should be treated as hard-wired with carrier
+always present.  Thus specifying
+.Ql flags 0x0004
+for
+.Li dh0
+would cause line
+.Pa ttyh2
+to be treated in this way.
+.Pp
+Normal I/O
+control parameters for individual lines are managed by
+.Xr ioctl 2
+calls.
+Line speeds may be initiated via
+.Xr getty 8
+and
+.Xr stty 1
+or may be communicated by other programs which
+utilize
+.Xr ioctl
+such as
+.Xr ifcongif 8 ,
+see
+.Xr tty 4  .
+.Pp
+The
+.Nm dh
+driver monitors the rate of input on each board,
+and switches between the use of character-at-a-time interrupts
+and input silos.
+While the silo is enabled during periods of high-speed input,
+the driver polls for input 30 times per second.
+.Sh FILES
+.Bl -tag -width /dev/tty[h-o][0-9a-f]x -compact
+.It Pa /dev/tty[h-o][0-9a-f]
+.It Pa /dev/ttyd[0-9a-f]
+.El
+.Sh DIAGNOSTICS
+.Bl -diag
+.It dh%d: NXM.
+No response from
+.Tn UNIBUS
+on a dma transfer
+within a timeout period.  This is often followed by a
+.Tn UNIBUS
+adapter
+error.  This occurs most frequently when the
+.Tn UNIBUS
+is heavily loaded
+and when devices which hog the bus (such as
+.Tn RK07 Ns s) are present.
+It is not serious.
+.Pp
+.It dh%d: silo overflow.
+The character input silo overflowed
+before it could be serviced.  This can happen if a hard error occurs
+when the
+.Tn CPU
+is running with elevated priority, as the system will
+then print a message on the console with interrupts disabled.
+It is not serious.
+.El
+.Sh SEE ALSO
+.Xr tty 4
+.Sh HISTORY
+A
+.Nm
+driver appeared in
+.At v6 .