DH(4)							     Kernel Interfaces Manual							     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  indepen-
       dently 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 con-
       trol. 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.

3rd Berkeley Distribution					   May 31, 1997 							     DH(4)