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

DHV(4)					       Kernel Interfaces Manual 				       DHV(4)

dhv - DHV-11 communications multiplexer
/sys/conf/SYSTEM: NDHV dhv_units # DHV11 /etc/dtab: #Name Unit# Addr Vector Br Handler(s) # Comments dhv ? 160440 310 5 dhvrint dhvxint # dhv terminal mux major device number(s): raw: 24 minor device encoding: bits 0007 specify line on DHV unit bits 0070 specify DHV unit bit 0100 specifies RTS/CTS (``hardware'') flowcontrol bit 0200 specifies non-blocking open (``CD always on'')
A DHV-11 provides 8 communication lines. Each line attached to the DHV-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 and 200 baud are not available). While 38400 is available the underlying hardware is not fast enough to handle it and there will be pauses/gaps between characters. Bit 0200 of the minor device number for DHV 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 in the minor device number via mknod /dev/ttyS2 c 4 194 would enable RTS/CTS flow control. The DHV-11 has an input silo but does not have the programmable receiver delay that the DHU (and DHQ) have. Thus system services more interrupts (i.e. gets fewer characters per interrupt on average) with a DHV-11 than with a DHQ (in DHU mode).
dhv%d,%d NXM. No response from QBUS on a DMA transfer within a timeout period. This error has never been observed on a PDP-11 and is a carryover from the VAX driver when that was ported to 2BSD. dhv%d diag %o. Diagnostic information from the DHV11. This has never been observed. The DHV-11 hardware manual will be required to decode the value printed out. dhv%d: %d overruns. The character input silo overflowed before it could be serviced. This message is printed only when the line is closed. By only printing this when the line is closed further silo overruns are avoided (kernel printf statements are not interrupt driven).
The DHV lacks the receiver delay that the DHU (and DHQ) have. Thus it is extremely easy (indeed it's almost certain at higher data rates) for a DHV-11 to enter an interrupt per character mode and use 70-80% of the cpu. 3rd Berkeley Distribution May 31, 1997 DHV(4)

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