Direct Commands to Printer – Passthrough/Escape

 Technical Information Database

TI1497D.txt - Direct Commands to Printer - Passthrough/Escape

Category   :Printing
Platform   :All-32Bit
Product    :All32Bit,   

Description:
Although Delphi's TPrinter unit makes it easy to interface
to a given printer, there are times when you may need
to drop down to the printers level and send device specific
escape sequences. Under 16-bit versions of Windows, this
was as easy as opening the printer port, but under Windows NT,
directly accessing the hardware is is illegal. One solution
is to use the Windows "PASSTHROUGH" escape to send an escape
sequence directly to the printer. In order to use the
"PASSTHROUGH" escape, it must be supported by the printer
driver. Be forwarned that not all printer drivers will support
this feature.

It's worth noting that the "PASSTHROUGH" escape is documented
as obsolete for thirty-two bit applications. It should be a
number of years before this escape goes by the way, since
it is used in many commercial applications.

The example code presented is not targeted to any specific
printer model. You will need to know the correct escape
sequences to send to the printer you are interfacing to.
Note that you must still call the BeginDoc and EndDoc methods
of TPrinter. During the BeginDoc call, the printer driver
initializes the printer as necessary, and during the EndDoc
call, the printer driver will uninitialize the printer and
eject the page. When you do make your escape call, the printer
may be set for the current windows mapping mode if the printer
supports scaling internaly. Technically, you should not do
anything that would cause the printer memory to be reset,
or eject a page with an escape sequence. In other words,
try to leave the printer in the same state it was in when
you made your escape. This is more important on intellegent
printers (Postscript printers), and not important at all on
a standard TTY line printer, where you can do just about
anything you like, including ejecting pages.

Code Example:

You will need to declare a structure to hold the buffer you are
sending. The structure of the buffer is defined as a word containing
the length of the buffer, followed by the buffer containing the data.

Before making the escape call to pass the data, we will use
the escape "QUERYESCSUPPORT" to determine if the "PASSTHROUGH"
escape is supported by the print driver.

Finally, be aware that your data will be inserted directly into
the printers data stream. On some printer models (Postscript),
you may need to add a space to the start and end of your data
to separate your data from the printer drivers data.

(Postscript is a Registered Trademark of Adobe Systems Incorporated)


unit Esc1;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{ add the printers unit }
uses
   Printers;

{$R *.DFM}

{ declare the "PASSTHROUGH" structure }
type TPrnBuffRec = record
  BuffLength : word;
  Buffer : array [0..255] of char;
end;


procedure TForm1.Button1Click(Sender: TObject);
var
  Buff : TPrnBuffRec;
  TestInt : integer;
  s : string;
begin

{ Test to see if the "PASSTHROUGH" escape is supported }
  TestInt := PASSTHROUGH;
  if Escape(Printer.Handle,
            QUERYESCSUPPORT,
            sizeof(TestInt),
            @TestInt,
            nil) > 0 then begin

  { Start the printout }
    Printer.BeginDoc;

  { Make a string to passthrough }
    s := ' A Test String ';

  { Copy the string to the buffer }
    StrPCopy(Buff.Buffer, s);

  { Set the buffer length }
    Buff.BuffLength := StrLen(Buff.Buffer);

  { Make the escape}
    Escape(Printer.Canvas.Handle,
           PASSTHROUGH,
           0,
           @Buff,
           nil);

  { End the printout }
    Printer.EndDoc;
  end;
end;

end.


Reference:
 

4/22/99 12:48:27 PM