Specifications

2-26
Guide to Printers and Printing
Native, 8–port, 16–port, and third–party controllers
Native port (S1 or S2), 8–port, and 16–port controllers do not provide hardware support for
terminal–attached printers, and the hardware support for third–party controllers is unknown.
As a result, print files must be split into small data blocks. The mc5 control sequence
precedes each data block, which is in turn followed by the mc4 control sequence. When the
terminal receives the mc5 control sequence, all subsequent data is routed to the AUX port
until an mc4 control sequence is received.
Data blocks sent to the terminal must be kept relatively small. Sending too many characters
to the tty at once may cause output to the printer to be mixed with the echo of what is typed
during the sending operation. A delay time between data block transmissions must also be
established to minimize data reception errors.
Native port, 8–port, 16–port, and third–party controllers have the following virtual printer
attributes for specifying block size and delay value:
y1 Indicates the maximum number of characters in a data block.
y2 Indicates the number of microseconds to delay between data block transmissions.
64–port controller
The 64–port controller provides hardware support for terminal–attached printers. The
64–port controller has the following virtual printer attribute:
y1 Sets the priority with which printing will be done over terminal activity. The larger the
number, the greater the priority the printer has over the terminal.
128–port controller
The 128–port controller also provides hardware support for terminal–attached printers. The
128–port controller has the following virtual printer attributes:
y1 Sets the maximum characters per second (CPS) rate at which characters are sent to
the print device. The rate should be just below the average print speed for your
printer. Consult your printer’s documentation for print speed.
y2 Sets the maximum number of print characters the device driver places in the output
queue. Reducing this number increases system overhead. Increasing this number
delays operator keystroke echo times when the terminal–attached printer is in use.
y3 Sets the device driver estimate of the size of the terminal–attached printer’s input
buffer. After a period of inactivity, the driver bursts the designated number of
characters to the printer. Consult your printer’s documentation for input buffer size.
Printer backend commands
The piobe command is the normal backend program run by the print spooling subsystem
when printing to a locally attached printer device. The piobe command is started via the
qdaemon process. It determines the data stream it is going to create by reading a flag or
querying the virtual printer database. The piobe process then passes the print file through a
pipeline of appropriate filters so that it generates the correct data stream. At the end of this
pipeline, the filtered file is passed to the pioout device driver interface program.
The pioout command is invoked in a pipeline by the piobe command. For locally attached
printers, the pioout command sends the print file to the appropriate printer device driver (for
example, /dev/lp1). However, for terminal–attached printers, the print files are sent to the
printer via the tty device driver (for example, /dev/tty0), after being modified by data
gathered from the terminfo and virtual printer databases. The terminfo database is queried
for the mc5 and mc4 terminal control attributes. The virtual printer database is queried for
the asynchronous controller–specific attributes.