.\"
.\"	@(#)dhv.4	1.0 (2.11BSD) 1997/5/31
.\"
.TH DHV 4 "May 31, 1997"
.UC 2
.SH NAME
dhv \- DHV-11 communications multiplexer
.SH SYNOPSIS
.ft B
.nf
/sys/conf/SYSTEM:
	NDHV	\fIdhv_units\fP	# DHV11

/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
	dhv	?	160440	310	5	dhvrint	dhvxint	# dhv terminal mux
.DT

major device number(s):
	raw: 24
minor device encoding:
	bits 0007 specify line on DHV unit
	bits 0070 specify DHV unit
	bit  0100 specifies RTS/CTS (``hardware'') flowcontrol
	bit  0200 specifies non-blocking open (``CD always on'')
.fi
.ft R
.SH DESCRIPTION
A DHV-11 provides 8 communication lines.
.PP
Each line attached to the DHV-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 13 speeds (50 and 200 baud are not available).  While
38400 is available the underlying hardware is not fast enough to handle it and
there will be pauses/gaps between characters.
.PP
Bit
.I 0200
of the minor device number for DHV 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 "4, 130" via
.I "mknod /dev/ttyS2 c 4 130"
would cause line ttyS2 to be treated in this way.  Turning on bit 6 in the
minor device number via
.I "mknod /dev/ttyS2 c 4 194"
would enable RTS/CTS flow control.
.PP
The DHV-11 has an input silo but does \fBnot\fP have the programmable
receiver delay that the DHU (and DHQ) have.  Thus system services more
interrupts (i.e. gets fewer characters per interrupt on average) with a
DHV-11 than with a DHQ (in DHU mode).
.SH FILES
/dev/tty[S-Z][0-9a-f]
.SH "SEE ALSO"
tty(4)
.SH DIAGNOSTICS
\fBdhv%d,%d NXM\fR.  No response from QBUS on a DMA transfer
within a timeout period.  This error has never been observed on a PDP-11 and
is a carryover from the VAX driver when that was ported to 2BSD.
.PP
\fBdhv%d diag %o\fR.  Diagnostic information from the DHV11.  This has never
been observed.  The DHV-11 hardware manual will be required to decode the 
value printed out.
.PP
\fBdhv%d: %d overruns\fR.  The character input silo overflowed
before it could be serviced.  This message is printed only when the line is
closed.  By only printing this when the line is closed further silo overruns 
are avoided (kernel printf statements are not interrupt driven).
.SH NOTES
The DHV lacks the receiver delay that the DHU (and DHQ) have.  Thus it is 
extremely easy (indeed it's almost certain at higher data rates) for a 
DHV-11 to enter an interrupt per character mode and use 70-80% of the cpu.