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<h1 class="SECT1"><a name="APPSOCKET" id="APPSOCKET">11.4.
AppSocket TCP/IP Protocol</a></h1>
<p>The AppSocket interface is supported by Tektronix and some
other printer vendors. It is similar to the Socket API, with a
couple of significant differences.</p>
<ol type="1">
<li>
<p>The printer has two ports for network connections: a TCP
port 9100 for TCP/IP stream connections and a UDP port for
UDP packet connections.</p>
</li>
<li>
<p>When a 0 length UDP packet or a UDP packet containing
only <tt class="FILENAME">CR/LF</tt> is sent to UDP port
9101, the printer will return a packet to the sender
containing print status information. This information
indicates the printers current status (busy, idle,
printing) and any error conditions.</p>
</li>
<li>
<p>The format, reliability, and repeatability of the UDP
format and information is totally undocumented, and the UPD
port facility should be regarded as, at best, an advisory
function of the printer.</p>
</li>
<li>
<p>To send a job to the printer, a connection to TCP port
is made. This connection will be refused while the printer
is busy or has a connection to another host.</p>
</li>
<li>
<p>When the TCP connection is established, the information
to be printed can be sent over the TCP connection. Bytes
sent on this stream will be placed in the input buffer of
the Print Engine and processed.</p>
</li>
<li>
<p>An end of job (EOJ) sequence indication in the data
stream will cause the printer to terminate the connection.
This is different than the Socket API, where the printer
will keep the connection open. This means that if the
PostScript CTRL-D (end of job) character is sent in a job,
then the connection will be terminated.</p>
</li>
<li>
<p>Some models of printers modify this behavior slightly
and will not terminate the connection, but will simply
ignore any data following the EOJ indication.</p>
</li>
<li>
<p>Some printers support bidirectional AppSocket
communication, and while the connection is open will return
error indications or status information.</p>
</li>
<li>
<p>Once all the data has been received and the job has
finished printing, the connection will be terminated by the
printer.</p>
</li>
</ol>
<p>The <b class="APPLICATION">ifhp</b> filter, one of the
helper programs for <b class="APPLICATION">LPRng</b>, is used
with <b class="APPLICATION">LPRng</b> to provide AppSocket
support. For details, please see the [<span class=
"CITATION">IFHP-HOWTO</span>] in the <a href=
"http://www.private/" target="_top"><b class=
"APPLICATION">ifhp</b> Distribution</a> and <a href=
"p450.htm">Tektronix P450 and Family</a> for details. The
following is a typical printcap entry for the AppSocket
protocol. The actual network connection to the printer is made
by the <b class="APPLICATION">ifhp</b> filter:</p>
<div class="INFORMALEXAMPLE">
<a name="AEN4975" id="AEN4975"></a>
<pre class="SCREEN">
lp:
# <b class=
"APPLICATION">LPRng</b> opens a dummy connection for consistency
:lp=/dev/null
# we pass the ifhp filter options indicating that the
# Tektronics printer will need the appsocket protocol
# and to use port 35 at 10.0.0.1 to make the connection
# The ifhp filter may open and close the connection several
# times during the file transfer in order to ensure that
# the printer handles the job correctly.
:ifhp=model=tek,appsocket,dev=10.0.0.1%35
:filter=/usr/local/libexec/filters/ifhp
</pre>
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