2.9BSD/usr/man/man4/du.4

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.TH DU 4 
.UC
.SH NAME
du, dp \- DU-11/DUP-11 201 data-phone interface
.SH DESCRIPTION
The
.I dp0
file
is a 201 data-phone interface.
.I Read
and
.I write
calls
to dp0
are limited to a maximum of 512 bytes.
Each
.I write
call is sent as a single record.
Seven bits from each byte
are written along with an eighth odd parity
bit.
The sync must be user supplied.
Each
.I read
call returns characters received from a
single record.
Seven bits are returned unaltered; the eighth bit
is set if the byte was not received in odd parity.
A 10 second time out is set and a zero-byte
record is returned if nothing is received in that time.
.SH FILES
/dev/dp0
.SH "SEE ALSO"
dn(4)
.SH DIAGNOSTICS
.I Errno
is set to ENXIO if an
.IR open (2)
fails because the device is already in use by another process.
.SH BUGS
The name
.I dp0
is a historical dreg.