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 communcations 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 because it has one small input silo that is shared across all ports. The dc hardware does not interrupt on modem transitions; instead, they must be detected by polling the device. Only two ports on a baseboard dc device have any modem control signals; the other two ports are wired for a keyboard and mouse.

BUGS
On some implementations (e.g. DECstation 3100), the hardware does not bring out modem flow-control signals (RTS/CTS) to the bulkhead; only modem carrier transition are provided. This is adequate for UNIX dialin and dialout detection, but without hardware flow control, the dc device is prone to input silo overruns when used for serial-line protocols at high speed. The DECstation 5000/200 dc hardware provides full modem control on both bulkhead RS-232 ports and is thus less prone to input silo overrun.

The 19200 speed on all dc devices is actually implemented as 19800, presumably for bug-compatibility with the original DZ-11. This is a hardware "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 ports and disables the 19200 speed on all four ports. This feature is not currently supported.

The dc driver should be merged into a single driver supporting the original 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.