Printing with NetCom
Sometimes the NetCom Serial Device Servers are used together
with serial printers. These printers are available via a network to several stations for
printing (Figure 1). 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.
Internal structure of LPD implemented in NetCom
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 (Figure 2). 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.