DC(4)                 OpenBSD Programmer's Manual (PMAX)                 DC(4)

NAME
     dc - dc659, dc7085 DZ11-on-a-chip serial communications interface

SYNOPSIS
     dc0 at ioasic?
     dc0 at mainbus
     dc1 at mainbus
     dc2 at mainbus

DESCRIPTION
     The dc driver provides support for Digital's dc7085 four-port serial com-
     munications chip.  The dc driver supports dc7085 hardware on attached to
     the following parent devices:

           mainbus  baseboard device on DECstation 2100, 3100, and 5100, and
                    optional expansion devices on the 5100.
           ioasic   baseboard device on DECstation 5000/200.
           vsbus    baseboard devices on the VAXstation 2000 and 3100.

     The dc device is in essence a single-chip clone of the DZ-11.  The DZ-11
     hardware design severely constrains performance.  One small input silo is
     shared across all input ports.  The dc hardware does not interrupt on mo-
     dem transitions; instead, they are detected via polling.  Only two lines
     on a baseboard dc device  have any modem control signals; the other two
     lines are wired for a keyboard and mouse.

     On some implementations (e.g., DECstation 3100), the hardware does not
     bring out modem flow-control signals to the bulkhead. Only modem carrier
     transition are provided. This is adequate for Unix dialin and dialout de-
     tection, but without hardware flow control, the dc device is prone to
     overruns when used for serial-line protocols.

     The DECstation 5000/200 dc hardware provides full modem control on both
     bulkhead RS-232 ports and is less prone to overrun.

BUGS
     The 19200 speed on all dc devices is actually implemented as 19800, pre-
     sumably for bug-compatibility with the original DZ-11.  This is a hard-
     ware feature and cannot be corrected in software.  Many serial devices at
     19200 fail to interoperate with the dc at its 19800 speed setting.

     The DECstation 5000/200 can set a bit in the system register to allow the
     dc to run at 38400, but this applies to all four lines and disables the
     19200 speed on all four lines. This feature is not currently supported.

     The dc driver should be merged into a single driver supporting the origi-
     nal DZ-11, DZ-32, and Qbus DZV-11 and DZQ-11 clones.

HISTORY
     The dc driver first appeared in 4.4BSD/pmax. This manual page first
     appeared in NetBSD 1.2.

OpenBSD 2.6                    February 20, 1998                             1
