Author: Peter Below

How to set a printer to duplex mode?

Answer:

You typically switch a printer to duplex mode by changing its TDeviceMode(API: 
DEVMODE)record .But first you should test whether the installed printer driver 
supports this: 

1   uses
2     printers, winspool;
3   
4   function PrinterSupportsDuplex: Boolean;
5   var
6     Device, Driver, Port: array[0..255] of Char;
7     hDevMode: THandle;
8   begin
9     Printer.GetPrinter(Device, Driver, Port, hDevmode);
10    Result := WinSpool.DeviceCapabilities(Device, Port, DC_DUPLEX, nil, nil) <> 0;
11  end;


If it does you can try to switch the duplex mode on before you call  
richedit.print: 

12  var
13    Device, Driver, Port: array[0..80] of Char;
14    DevMode: THandle;
15    pDevmode: PDeviceMode;
16  begin
17    // Get printer device mode handle.
18    Printer.GetPrinter(Device, Driver, Port, DevMode);
19    if Devmode <> 0 then
20    begin
21      // lock it to get pointer to DEVMODE record
22      pDevMode := GlobalLock(Devmode);
23      if pDevmode <> nil then
24      try
25        with pDevmode^ do
26        begin
27          dmDuplex := DMDUP_VERTICAL;
28          dmFields := dmFields or DM_DUPLEX;
29        end;
30      finally
31        // unlock devmode handle.
32        GlobalUnlock(Devmode);
33      end;
34    end; { If }
35  end;