manual page distributed with 4.1BSD

SCCS-vsn: share/man/man4/man4.vax/dh.4 4.1

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+.\" 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        4.1 (Berkeley) %G%
+.\"
+.TH DH 4 4/1/81
+.UC 4
+.SH NAME
+dh, dm \- DH-11/DM-11 communications multiplexer
+.SH SYNOPSIS
+device dh0 at uba0 csr 0160020 vector dhrint dhxint
+.br
+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
+If the Berknet line driver
+.IR bk (4)
+is included in the system, then the dh driver will use its input silos
+and poll for input at each clock tick (normally 1/50'th or 1/60'th of a
+second) rather than taking an interrupt on each input character.
+.SH FILES
+/dev/tty[hi][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 64 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.  If the
+Berknet
+.IR net (1)
+is running on a
+.I dh
+line at high speed (e.g. 9600 baud), there is only 1/15th of a second of
+buffering capacity in the silo, and overrun is possible.  This may
+cause a few input characters to be lost to users and a network
+packet is likely to be corrupted, but the network will recover.
+It is not serious.