DH(4)

NAME
dh - DH-11/DM-11 communications multiplexer

SYNOPSIS
/sys/conf/SYSTEM:
	NDH	dh_units	# DH11; NDH is in units of boards (16 each)
	NDM	dm_units	# DM11; NDM is in units of boards (16 each)
/etc/dtab:
	#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
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'')

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 dm_units dh-11's.

Each line attached to the DH-11 communications multiplexer behaves as described in tty(4). Input and output for each line may independently be set to run at any of 16 speeds; see tty(4) for the encoding.

Bit 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 mknod /dev/ttyh2 c 3 130 would cause line ttyh2 to be treated in this way. Bit 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: mknod /dev/ttyh2 c 3 194

The 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.

FILES
/dev/tty[h-k][0-9a-f]
/dev/ttyd[0-9a-f]
/dev/MAKEDEV    script to create special files
/dev/MAKEDEV.local    script to localize special files

SEE ALSO
tty(4), dtab(5), autoconfig(8)

DIAGNOSTICS
dh%d NXM. 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.

dh%d %d overruns. 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.