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DH(4)											    DH(4)

       dh - DH-11/DM-11 communications multiplexer

	    NDH  dh_units  # DH11; NDH is in units of boards (16 each)
	    NDM  dm_units  # DM11; NDM is in units of boards (16 each)

	    #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'')

       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 num-
       ber 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.

       /dev/MAKEDEV	   script to create special files
       /dev/MAKEDEV.local  script to localize special files

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

       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)
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