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

DHU(4)					       Kernel Interfaces Manual 				       DHU(4)

dhu - DHU-11 communications multiplexer
/sys/conf/SYSTEM: NDHU dhu_units # 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'')
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 charac- ter device node "4, 130" via mknod /dev/ttyS2 c 4 130 would cause line ttyS2 to be treated in this way. Turn- ing 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 milliseconds rather than tak- ing 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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