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BSD 2.11 - man page for dhu (bsd section 4)

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DHU(4)											   DHU(4)

       dhu - DHU-11 communications multiplexer

	    NDHU dhu_units # DHU11

	    #Name Unit# Addr   Vector Br Handler(s)	 # Comments
	    du	  ?	160020 310    5  dhurint dhuxint # dhu11 terminal mux

       major device number(s):
	    raw: 4
       minor device encoding:
	    bits 0017 specify line on DHU unit
	    bits 0060 specify DHU unit
	    bit  0100 specify RTS/CTS flow control
	    bit  0200 specifies non-blocking open (``CD always on'')

       A DHU-11 provides 16 communication lines.

       Each  line  attached  to  the  DHU-11  communications  multiplexer behaves as described in
       tty(4).	Input and output for each line may independently be set  to  run  at  any  of  13
       speeds (50, 200 and 38400 baud are not available); see tty(4) for the encoding.

       Bit  0200  of  the  minor device number for DHU 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 mknod /dev/ttyS2 c
       4 130 would cause line ttyS2 to be treated in this way.	Turning on bit 6 (adding  64)  to
       the minor device number via mknod /dev/ttyS2 c 4 194 enables RTS/CTS flow control.

       The DHU-11 driver normally uses input silos and delays receiver interrupts by 20 millisec-
       onds rather than taking an interrupt on each input character.



       dhu(%d,%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 believed to be a carryover from the VAX driver
       when it was ported.  It is not serious.

       dhu%d %d overruns.  The character input silo overflowed before it could be serviced.  This
       message	is  printed  only  at  line close time rather than on each overrun error.  Kernel
       printf's are not interrupt driven and caused more  overruns  by	blocking  interrupts  for
       lengthy periods of time.

       The  driver  currently  does not make full use of the hardware capabilities of the DHU-11,
       for dealing with XON/XOFF flow-control or hard-wired lines for example.

       The 4 40-way cables are a pain.

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