9 Print Server Operation
9 Print Server Operation
Sometimes the Serial Device Servers are used together with serial printers. These printers are
available via a network to several stations for printing. So far there have been two operation modes
to achieve this. First the serial port can operate as a TCP Raw Server, and the station just sends
the data to print via a TCP connection. As second option a computer running Windows could
install the driver for virtual serial ports. The printer is then controlled via this Com port. In
both these solutions the buffering of data occurred on the client station. Beginning with Firmware
version 2.2 the NetCom
+
Devices offer a true Print Server mode, using the Line Printer Daemon
protocol as of RFC1197. Here a print server (lpd) is a station with one IP Address and a single
defined port to accept commands and data for printing. Several printers may be attached to the
print server. Each printer has a separate data queue for management of print jobs. The data of
the jobs is saved in this queue, instead of the client as before.
9.1 Printer Queue
The basic function of an lpd is to accept the data for printing, store it in a spooler queue, and
send it to the printer when this is ready for printing. This is done for several queues in parallel.
Each printer is identified by the name of the queue, where it is attached to. The NetCom
+
Device
Servers allow to configure a custom name for each queue, while the default name is »lpd« plus the
number of the serial port (lpd1, lpd2, . . . ). This name is set in the properties of the serial port.
When the lpd is running on a separate computer, the hard disk is used to save the data of the
queues. The NetCom
+
Servers neither have a mass storage device, nor huge amounts of memory.
Each queue accepts at least one job with a size of up to 250 KB print data. If the job has more
data, memory is either assigned dynamically to save the job, or the data is spooled through a ring
buffer. Data is printed while the client still sends data. The amount of available dynamic memory
depends on the number of ports in a NetCom
+
Device Server, and the operations active on these
ports.
9.2 Printer Reset
Before a new job is sent to the printer, this printer should be in a well known state. On a parallel
printer port this is easy to achieve. There is a defined signal to send a »reset« command to the
printer.
Such a definition is not available for serial printers. Instead there is a reset command, which users
may send via the serial line. Typically this command is specific to the manufacturer or even to
the printer model. So the NetCom
+
allows to specify this command by entering an ‘InitString’ for
each queue.
9.2.1 Init String Definition
The Initialization of the printer typically involves ASCII control codes, ordinary ASCII characters
and some binary data. On some models it may also be necessary to provide a certain state of
the modem control signals RTS and DTR, applied with special timing. The ‘InitString’ in the
NetCom
+
Device Serves offer all these options.
September 2016
NetCom Plus User Manual
105
Summary of Contents for NetCom Plus 111
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