DHU(4)		    UNIX Programmer's Manual		   DHU(4)



NAME
     dhu - DHU-11 communications multiplexer

SYNOPSIS
     /sys/conf/SYSTEM:
	  NDHU _d_h_u__u_n_i_t_s # DHU11

     /etc/dtab:
	  #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'')

DESCRIPTION
     A DHU-11 provides 16 communication lines.

     Each line attached to the DHU-11 communications multiplexer
     behaves as described in _t_t_y(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 _t_t_y(4) for
     the encoding.

     Bit _0_2_0_0 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 _m_k_n_o_d /_d_e_v/_t_t_y_S_2 _c _4 _1_3_0 would cause line ttyS2
     to be treated in this way.  Turning on bit 6 (adding 64) to
     the minor device number via _m_k_n_o_d /_d_e_v/_t_t_y_S_2 _c _4 _1_9_4 enables
     RTS/CTS flow control.

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

FILES
     /dev/tty[S-Z][0-9a-f]

SEE ALSO
     tty(4)

DIAGNOSTICS
     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.



Printed 11/26/99	  May 31, 1997				1






DHU(4)		    UNIX Programmer's Manual		   DHU(4)



     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.  Ker-
     nel printf's are not interrupt driven and caused more over-
     runs by blocking interrupts for lengthy periods of time.

NOTES
     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.











































Printed 11/26/99	  May 31, 1997				2