The data file can be read and written to access the corresponding serial port. Reads will block until at least one character is available.

The ctl file accepts textual commands, via write (see sys-read(2)), that control or condition the serial port for subsequent IO:

bn

Set the baud rate to n.

cn

If n is non-zero, cause a hangup if the remote drops DCD.

dn

Set DTR if n is non-zero; otherwise clear it.

en

If n is non-zero, cause a hangup if the remote drops DSR.

f

Flush the output queue.

h

Hangup both input and output queues.

kn

Send a break lasting n milliseconds.

ln

Set number of bits per byte to n. Legal values are 5, 6, 7, or 8.

mn

Obey modem CTS signal if n is non-zero; otherwise clear it.

nn

Set the output queue to non-blocking, if n is non-zero; otherwise mark it as blocking (the default). In non-blocking mode, if the output queue is full, new output is discarded rather than block the writer until the queue drains.

pc

Set parity to odd if c is o, to even if c is e; otherwise set no parity.

qn

Set the limit on input and output queues to n; both queues are initially set to 4k bytes.

rn

Set RTS if n is non-zero; otherwise clear it.

sn

Set number of stop bits to n. Legal values are 1 or 2.

xn

Enable XON/XOFF flow control if n is non-zero; otherwise disable it (the default).

Changes to port settings persist between a subsequent close and reopen.

The status file can be read for a report of the serial port status.