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, respectively.
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.
/dev/ttya
/dev/ttyb
/dev/ttyc
/dev/ttyd
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
.
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 zstty(4).
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.