.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)dh.4	6.2 (Berkeley) 5/16/86
.\"
.TH DH 4 "May 16, 1986"
.UC 4
.SH NAME
dh \- DH-11/DM-11 communications multiplexer
.SH SYNOPSIS
.B "device dh0 at uba0 csr 0160020 vector dhrint dhxint"
.br
.B "device dm0 at uba0 csr 0170500 vector dmintr"
.SH DESCRIPTION
A dh-11 provides 16 communication lines; dm-11's may be optionally
paired with dh-11's to provide modem control for the lines.
.PP
Each line attached to the DH-11 communications multiplexer
behaves as described in
.IR tty (4).
Input and output for each line may independently
be set to run at any of 16 speeds;
see
.IR tty (4)
for the encoding.
.PP
Bit
.I i
of flags may be specified for a dh to say that a line is not properly
connected, and that the line should be treated as hard-wired with carrier
always present.  Thus specifying ``flags 0x0004'' in the specification of dh0
would cause line ttyh2 to be treated in this way.
.PP
The
.I 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
/dev/tty[h-o][0-9a-f]
.br
/dev/ttyd[0-9a-f]
.SH "SEE ALSO"
tty(4)
.SH DIAGNOSTICS
\fBdh%d: NXM\fR.  No response from UNIBUS on a dma transfer
within a timeout period.  This is often followed by a UNIBUS adapter
error.  This occurs most frequently when the UNIBUS is heavily loaded
and when devices which hog the bus (such as rk07's) are present.
It is not serious.
.PP
\fBdh%d: silo overflow\fR.  The character input silo overflowed
before it could be serviced.  This can happen if a hard error occurs
when the CPU is running with elevated priority, as the system will
then print a message on the console with interrupts disabled.
It is not serious.