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@(#)dh.4 6.1 (Berkeley) 05/15/85

C 4 dh - DH-11/DM-11 communications multiplexer "device dh0 at uba0 csr 0160020 vector dhrint dhxint"

"device dm0 at uba0 csr 0170500 vector dmintr"

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.

Each line attached to the DH-11 communications multiplexer behaves as described in tty (4). Input and output for each line may independently be set to run at any of 16 speeds; see tty (4) for the encoding.

Bit i of flags may be specified for a dh to say that a line is not properly connected, and that the line should be treated as hard-wired with carrier always present. Thus specifying ``flags 0x0004'' in the specification of dh0 would cause line ttyh2 to be treated in this way.

The dh driver normally uses input silos and polls for input at each clock tick (10 milliseconds) rather than taking an interrupt on each input character.

/dev/tty[hi][0-9a-f]

/dev/ttyd[0-9a-f]

tty(4) dh%d: NXM. No response from UNIBUS on a dma transfer within a timeout period. This is often followed by a UNIBUS adapter error. This occurs most frequently when the UNIBUS is heavily loaded and when devices which hog the bus (such as rk07's) are present. It is not serious.

dh%d: silo overflow. The character input silo overflowed before it could be serviced. This can happen if a hard error occurs when the CPU is running with elevated priority, as the system will then print a message on the console with interrupts disabled. If the Berknet is running on a dh line at high speed (e.g. 9600 baud), there is only 1/15th of a second of buffering capacity in the silo, and overrun is possible. This may cause a few input characters to be lost to users and a network packet is likely to be corrupted, but the network will recover. It is not serious.