◄Contents►  ◄Index►  ◄Back►
─────────────────────────────────────────────────────────────────────────────
                           ◄Knowledge Base Contents►  ◄Knowledge Base Index►
 
 Printer Error Can Hang; CALL INTERRUPT to Check Printer - Q37031
 
 Detecting printer errors with the ON ERROR GOTO statement is usually
 very slow. While the printer is waiting to time-out due to an error,
 the program may appear to be hung. Depending on the type of computer,
 MS-DOS or a Visual Basic for MS-DOS program may take from 20 seconds
 to more than two minutes before displaying a printer time-out error.
 This is expected behavior. The printer time-out period is determined
 by the ROM BIOS of your computer.
 
 A printer time-out error is one of the most likely reasons for a
 Visual Basic for MS-DOS hanging problem. (If you wait more than a few
 minutes without getting an error message, then a printer time-out
 error is probably not the problem.)
 
 Listed below are two alternatives to waiting for the printer time-out
 error: checking the status of the printer periodically throughout a
 program using ROM BIOS Interrupt 17h with function 2, or changing the
 ROM BIOS time-out value used for the printer port.
 
 More Information:
 
 The easiest way to check for printer errors is to send output to the
 printer and trap any errors with an ON ERROR GOTO statement. However,
 this method is slow and not always reliable, as in the case where you
 send out less than a full buffer to the printer. When a program sends
 out less than a full buffer to the printer, an error may go undetected
 by the program until the program fills the buffer later, at which
 point the program appears to hang until the printer times out.
 
 If you print a file from MS-DOS or a Visual Basic for MS-DOS program
 when the printer is offline, it may take up to several minutes for an
 error to be returned. The time allowed for a response varies from
 version to version of the computer's ROM BIOS.
 
 Using PEEK and POKE to Change Printer Time-Out Period
 -----------------------------------------------------
 
 You can change the time-out value used by the ROM BIOS to decrease or
 increase the amount of time it takes to generate a printer error. You
 can accomplish this in BASIC by using the DEF SEG, PEEK, and POKE
 statements to modify the ROM BIOS table entry for the desired printer
 time-out duration.
 
 The following code example shows how to change the printer time-out
 value for printer 1 in the ROM BIOS table. If your program changes the
 printer time-out value, then you should also restore the original
 time-out value before stopping the program.
 
    ◄See Example 1►
 
 Checking Printer Status with a ROM BIOS Interrupt
 -------------------------------------------------
 
 You can also check the status of the printer periodically throughout a
 program by using the ROM BIOS Interrupt 17h, with function 2. This
 interrupt returns the printer status in the AH register. Each bit
 returned in AH represents the following printer conditions:
 
     Bit     Condition
     ---     ---------
 
     Bit 7   Printer Not Busy (0 = Busy)
     Bit 6   Acknowledge
     Bit 5   Out of Paper
     Bit 4   Printer Selected
     Bit 3   I/O Error
     Bit 2   Unused
     Bit 1   Unused
     Bit 0   Timed-Out
 
 For example, to determine if the printer is out of paper, the
 interrupt could be called and bit 5 could be examined. To call MS-DOS
 interrupts from Visual Basic for MS-DOS, use the CALL INTERRUPT
 routine.
 
 The following is a sample BASIC program that uses the CALL INTERRUPT
 routine to check the printer status whenever the F1 key is pressed, or
 after a BASIC error:
 
    ◄See Example 2►