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Print Drivers and Link Files |
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While it is possible to communicate with a printer directly, most output data is delivered through one or more interfaces, software liaisons between the source application and the printer.
Printer manufacturers generally supply their own interfaces (printer drivers) to make it easier for operating systems to communicate with their products. Windows supplies its own interface (print subsystem API) for directing access to the various printer drivers installed on the system. It also handles other tasks, including print manager that allows users to select from a list of available printers to which they may send their output, and spooler that acts as a buffer/queue for multiple print jobs so that printers can receive data at their own rate.
There are different printing interfaces within the PxPlus environment itself. PxPlus includes several ready-made interfaces (*WINDEV*, *WINPRT*, *PDF*, etc.) that allow you to print directly from an application. However, you can also assemble your own interfaces for tailoring print requirements to different environments:
A custom driver is a PxPlus program that is automatically loaded and run when the target device file is opened. They reside outside the main program in the PxPlus *dev directory (lib/_dev). Driver names are limited to 12 characters in length.
Most custom drivers are used to consolidate a set of Mnemonics or the escape sequences that apply to a specific file or printer. They are often used to initialize printers in Character-Based Printing, but they can also be used to redirect output from one device/file to another, prompt the user for additional information, and/or execute operating system commands.
The DEFPRT directive at the beginning of the code tells PxPlus that the channel is a printer.
Example:
The following printer driver, *dev/hplaser, defines the maximum columns and rows for the printer, then defines all of the mnemonics and initializes the printer:
0010 ! HP Laser Jet: 10 cpi, 6 lpi, Portrait
0020 DEFPRT (LFO)80,60
0030 MNEMONIC (LFO)'*C'=ESC+"E" ! Close printer mnemonic -- Resets all
0040 MNEMONIC (LFO)'FF'=$0C$ ! <formfeed>
0050 MNEMONIC (LFO)'CR'=$0D$ ! <cr>
0060 MNEMONIC (LFO)'LF'=$0D0A$ ! <cr><lf>
0070 MNEMONIC (LFO)'NP'=ESC+"&k0S":80,0 ! 10 cpi
0080 MNEMONIC (LFO)'SP'=ESC+"(s12H":96,0 ! 12 cpi
0090 MNEMONIC (LFO)'CP'=ESC+"&k2S":132,0 ! 16.66 cpi
0100 MNEMONIC (LFO)'LT'=ESC+"&l12D":0,120 ! 12 Lines per inch
0110 MNEMONIC (LFO)'L8'=ESC+"&l8D":0,80 ! 8 lines per inch
0120 MNEMONIC (LFO)'L6'=ESC+"&l6D":0,60 ! 6 lines per inch
0130 MNEMONIC (LFO)'PM'=ESC+"&l0O" ! Portrait mode
0140 MNEMONIC (LFO)'LM'=ESC+"&l1O" ! Landscape mode
0150 MNEMONIC (LFO)'RM'=MNM('PM',LFO)+MNM('NP',LFO)+MNM('L6',LFO):80,60
0160 X$=MNM('PS',0); IF X$<>"" THEN MNEMONIC (LFO)'PS'=X$ ! Printer Start
0170 X$=MNM('PE',0); IF X$<>"" THEN MNEMONIC (LFO)'PE'=X$ ! Printer End
0180 X$=FIB(LFO); IF X$(19,1)="S" THEN LOCK (LFO,ERR=*NEXT)
0190 PRINT (LFO,ERR=0200)'*C','RM',
0200 END
The system variable LFO is used to identify the affected channel/file. It will contain the value of the last file opened, which, in the case of a device driver, is the channel for which the driver is responsible. Once created, *dev/hplaser could be accessed in an application as follows:
OPEN (1)"*WINDEV*;HP Laser Jet"
CALL "*dev/hplaser"
For general information on device drivers in PxPlus, see Device Drivers.
A link file is a device descriptor that contains a single line of code that simply points to a device or device driver.
Example:
[Pvxdev]*WINDEV*;Generic/Text Only;FILE=ABC.PRN hplaser
Where:
The format of the device descriptor is as follows:
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Location |
Length |
Contents |
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1 |
8 |
"[Pvxdev]": Device descriptor ID |
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9 |
60 |
True path to the device |
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69 |
12 |
Device type (name of driver) |
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71 |
184 |
Reserved for future (blank) |
When PxPlus opens a link file, it redirects to the path specified in bytes (9,60). This is the actual file to be opened. Once the file is opened, PxPlus will automatically CALL the device driver specified in bytes (69,12).
The link file name serves as an "alias" that can be used in place of where the associated file or device information appears in an application's OPEN statement.
Example:
If the HP Laser Jet device and driver described above were defined in a link named "LP", the device could then be accessed from the application as follows:
OPEN (1)"LP"
The application now sends data to the printer defined via "LP". If "LP" is always used in the OPEN statement, the contents of the link file itself can be changed, tailored to any device or driver that will be used with the application. If device information changes, the application will direct output to the new destination, provided that the contents of the "LP" link file is updated, the device exists, and the custom driver is located in the PxPlus *dev directory.
Link files are simply plain text files that may be produced using any text editor. PxPlus also supplies a prompt-driven utility (*UCL) that simplifies the creation and maintenance of link files. This utility allows you to choose between three slightly different link header types (File, Device or Attached Printer):
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[Pvxlnk] |
To point to a file without calling a program. Example: If you moved the GLDetail file to another directory, you could create a link file using the original name (GLDetail), then with the file name portion of the link, point to the new location of the actual GLDetail file. |
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[Pvxdev] |
To open a file, as well as call a program. Commonly used to associate printers and printer drivers. |
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[Pvxapr] |
Specifically for pointing to a printer that is physically attached to the computer. This issues an additional escape sequence wrapped around every line. PxPlus opens a file, calls the program in question and automatically prints the 'PS' mnemonic to the printer. It will also issue a 'PE' mnemonic when it attempts to close the file. |
*UCL prompts for the device information that will appear on the descriptor line. The following examples illustrate some device information that could be written to Link files via the *UCL utility for various printer setups:
Example:
Link File Name: LP
Other File to Open: LPT1
Program to Call: epson
Link File Name: /MYAPP/P1
Other File to Open: >lp -d queuename -c -s 2>/dev/null
Program to Call: hplaser
Link File Name: Googles
Other File to Open: /tmp
Program to Call: spooler
Link File Name: WINDOWS
Other File to Open: [wdx]*winprt*
Program to Call: myprog
You can access an entire page and print to it. However, you can only print to the current page at any one time. To complete a page, either use a form feed or close the channel. For proper spooler operation, do not include leading or trailing form feeds within the print job. Spoolers always assume the paper is at top-of-form when a job begins.