SCC(4) BSD Kernel Interfaces Manual SCC(4)
scc -- Zilog 8530 Serial Communications Controller interface
scc* at ioasic? offset ?
The scc driver provides support for the Zilog 8530 Serial Communications Controller (SCC)
via the IOASIC found on DECstation 5000 models in the /100, /20, and /240 series (supported
Each SCC device has two serial ports. The DECstation 5000 model 20 provides one SCC device.
Other models provide two, but one port of each device is dedicated to mouse and keyboard
Input and output for each line may set to one of following baud rates: 50, 75, 110, 134.5,
150, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 38400, or 57600.
Speeds up to 230400 are supported by the chip and the motherboard, but speeds higher than
57600 do not work reliably without an external clock signal.
The mapping from units to serial-hardware outlets is idiosyncratic. The even ports are
wired serial connectors and the odd-numbered ports are reserved for mouse and keyboard.
On machines with one port like Personal DECstations, the single device is ttya.
On the 5000/1xx and 5000/2xx, the first serial port (default serial console) is ttyc and the
second port is ttya.
intro(4), ioasic(4), ttys(5), MAKEDEV(8)
The scc driver first appeared in 4.4BSD.
The scc driver was also used for the IOASIC SCC found in DEC Alpha model 3000 TurboCHANNEL
based systems; NetBSD/alpha has since been converted to use the machine-independent
The IOASIC provides internal DMA channels that can be programmed to transfer up to 4096
bytes of data into, or out, of an SCC without further software intervention. This feature
of the IOASIC is not yet supported.
The mapping from device-special files (major and minor number) to chip and port is arguably
backwards. ULTRIX tries to hide the hardware mapping, but NetBSD does not. Users wanting
to use ULTRIX compatible tty names /dev/tty0 and /dev/tty1 for the scc comm-port lines
should make links or device-special files which match their hardware setup.
BSD April 24, 2003 BSD