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



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

SYNOPSIS
     /sys/conf/SYSTEM:
	  NDH  _d_h__u_n_i_t_s  # DH11; NDH is in units of boards (16 each)
	  NDM  _d_m__u_n_i_t_s  # 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 _d_m__u_n_i_t_s
     dh-11's.

     Each line attached to the DH-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 16 speeds;
     see _t_t_y(4) for the encoding.

     Bit _0_2_0_0 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 _m_k_n_o_d /_d_e_v/_t_t_y_h_2 _c _3 _1_3_0 would cause line ttyh2
     to be treated in this way.  Bit _0_1_0_0 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:
     _m_k_n_o_d /_d_e_v/_t_t_y_h_2 _c _3 _1_9_4

     The _d_h 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]



Printed 11/26/99	  May 31, 1997				1






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



     /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.







































Printed 11/26/99	  May 31, 1997				2