Subject: Testing printer status

15. *****
 Q: How can I test that the printer is ready?

 A: Strictly speaking there is no guaranteed way to detect the
printer status on a PC. As Brian Key Brian@fantasia.demon.co.uk
wrote "Any book dealing with the PC BIOS support of a printer will
quickly show you that there is no hardware definition which deals
with the printer power status.  It simply wasn't designed (and I use
the word loosely!) into the IBM hardware specs."

   The usually advocated method in Turbo Pascal is to test the
status of regs.ah after a call to interrupt 17 Hex (the parallel
port driver interrupt), service 02:
      regs.dx := PrinterNumber;  (* LPT1 = 0 *)
      regs.ah := $02;            (* var regs : registers, uses DOS *)
      Intr ($17,regs);           (* Interrupt 17 Hex *)
      status := regs.ah          (* var status : byte *)
But this is not a good method since the combinations of the status
bits which indicate a ready state can vary from printer to printer
and PC to PC. If you want a list of the status bits, see e.g. Ray
Duncan (1988), Advanced MS-DOS Programming, p. 587. For an example
of a code using interrupt 17 Hex see Douglas Stivison (1986), Turbo
Pascal Library, pp. 118-120. Also see Michael Yester (1989), Using
Turbo Pascal, pp. 494-495.
   The more generic alternative is to try to write a #13 to the
printer having the i/o checking off, that is, while {$I-} is in
effect, and testing the IOResult. But then you must first alter the
printer retry times default (and restore it afterwards). Else the
method can take up to a minute instead of an immediate response.
Also, you must have set the FileMode for LPT1 appropriately (and
restore it afterwards). Sounds a bit complicated, but you don't have
to do all this yourself. There is a boolean function "LPTONLFN Get
the online status of the first parallel printer" for this purpose in
my ftp://garbo.uwasa.fi/pc/ts/tspa3470.zip (or whatever version
number is the latest) Turbo Pascal units collection available by
anonymous FTP or mail server from garbo.uwasa.fi.

 A2: One potential, somewhat advanced solution is to use the Device
Driver Control (IOCTL) information. Here is the code.
  uses Dos;
  function PRNSTAFN : boolean;
  var regs   : registers;
      handle : ^word;
      f      : file;
  begin
    prnstafn := false;
    if swap(Dosversion) < $0200 then exit;  { At least MS-DOS 2.0 }
    Assign (f, 'prn');                 { Printer }
    Reset (f);
    FillChar (regs, SizeOf(regs), 0);  { Just to make sure }
    regs.ah := $44;                    { Function $44 }
    regs.al := $07;                    { Subfunction $07 }
    handle  := @f;                     { Establish a file handle }
    regs.bx := handle^;
    Msdos (regs);                      { Call interrupt $21 }
    Close (f);
    if regs.flags and FCarry <> 0 then exit;  { Is the carry flag set? }
    if regs.al <> $FF then exit;       { regs.al = $FF signals success}
    prnstafn := true;
  end;  (* prnstafn *)
  {}
  begin
    if PRNSTAFN then writeln ('Printer ready')
      else writeln ('Printer not ready');
    readln;
  end.

 A3: Another advanced method is using ports. This solution is based
on a posting by Joerg Kunze joerg@ang-physik.uni-kiel.de. A warning.
Do not experiment with the port parameters unless you know exactly
what you are doing. A serious loss of data might follow.
  function PRNON : boolean;
  var dr : word absolute $40:$08;
  begin
    prnon := port[dr+1] and $18=$18;
  end;
--------------------------------------------------------------------