.\" 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	6.4 (2.11BSD) 1997/5/31
.\"
.TH DH 4 "May 31, 1997"
.UC 2
.SH NAME
dh \- DH-11/DM-11 communications multiplexer
.SH SYNOPSIS
.ft B
.nf
/sys/conf/SYSTEM:
	NDH	\fIdh_units\fP	# DH11; NDH is in units of boards (16 each)
	NDM	\fIdm_units\fP	# DM11; NDM is in units of boards (16 each)

/etc/dtab:
.ta .5i +\w'#Name 'u +\w'Unit# 'u +\w'177777 'u +\w'Vector 'u +\w'Br 'u +\w'xxxxxxx 'u +\w'xxxxxxx 'u
	#Name	Unit#	Addr	Vector	Br	Handler(s)		# Comments
	dm	?	170500	310	4	dmintr		# dm11 dh modem control
	dh	?	160020	320	5	dhrint	dhxint	# dh11 terminal mux
.DT

major device number(s):
	raw: 3
minor device encoding:
	bits 0017 specify line on DH unit
	bits 0060 specify DH unit
	bit  0100 specify RTS/CTS (``hardware'') flowcontrol
	bit  0200 specifies non-blocking open (``CD always on'')
.fi
.ft R
.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.
If there are fewer dm-11's than dh-11's, the dm-11's are assumed
to be paired with the first \fIdm_units\fP dh-11's.
.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 0200
of the minor device number for DH lines
may be set to say that a line is not properly
connected, and that the line should be treated as hard-wired with carrier
always present.  Thus creating the special character device node "3, 130" via
.I "mknod /dev/ttyh2 c 3 130"
would cause line ttyh2 to be treated in this way.  Bit
.I 0100
of the minor device number enables RTS/CTS (also called ``hardware'') flow
control. It is enabled by adding 64 to the minor device number:
.I "mknod /dev/ttyh2 c 3 194"
.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
.ta \w'/dev/MAKEDEV.local  'u
/dev/tty[h-k][0-9a-f]
.br
/dev/ttyd[0-9a-f]
.br
/dev/MAKEDEV	script to create special files
.br
/dev/MAKEDEV.local	script to localize special files
.DT
.SH "SEE ALSO"
tty(4),
dtab(5),
autoconfig(8)
.SH DIAGNOSTICS
\fBdh%d NXM\fR.  No response from UNIBUS on a dma transfer
within a timeout period.  This has never been observed on a PDP-11 and is
a carryover from the VAX driver when it was ported.
It is not serious.
.PP
\fBdh%d %d overruns\fR.  The character input silo overflowed
before it could be serviced.  This message is only printed at line close time.
It is not serious but does indicate that the system was not able to keep up
with the data flow.