How do I check that a printer tray exists before using srw.set_printer_tray

Question

Due to printer firmware issues I cannot know in advance whether the printer's tray number has a space in front of it.
Ex: '2' vs. ' 2'
I have introduced this code to production, but if the tray value passed in does not exist, then an error is thrown when setting the printer tray.
If I cannot determine this in advance, then I would like the code not to fail and simply skip the assignment. The printer tray selection is only a convenience to the end user. This is important becasue I have hundreds of printers to which a report may be printed.
function BetweenPage return boolean is
begin
IF :PLABLE_TRAY IS NOT NULL AND :PLABLE_TRAY <> '' THEN
     srw.set_printer_tray(:PLABLE_TRAY);
return (TRUE);
END IF;
return (TRUE);
end;
Thx in advance,
Gary

Answer 1

high-Q wrote:
It's an autonomous test station with various measurement instruments. I'm closing the bus after reciving the string from each instrument.
Again, that makes no sense.  Part of the command over the bus is the address.  So if you know all of your instruments' addresses, you just supply that when you iteract with that instrument.  There is no reason to constantly open and close your bus.
The problem with checking to see if data is available is that you have to send a querry to the instrument.  You are the master of the bus.  Nobody else can send data without you letting them.  So you have to keep you bus open and send querries to check to see if the oscilloscope is ready to send data.
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