[NA-User] LPT Tutorial For W5WZ et al

["David A. Pruett" <k8cc@comcast.net>]

Sounds like Scott is well on his way to solving his LPT dilemma.  However, 
let me use a smidgen of bandwidth to explain how LPT ports are addressed by 
a PC.  There are some tricks to this, and it all goes back to the original 
IBM PC (i.e., this is not an NA issue).

The PC recognizes three legitimate addresses for LPT ports.  (The numbers 
given here in hexidecimal, or base 16, which is standard computer 
nomenclature.)  These addresses are 3BC hex, 378 hex, and 278 hex.

When the PC boots up, the operating system (actually, the BIOS in this 
case) goes looking for LPT ports IN THE ORDER GIVEN ABOVE.  The first port 
it finds is called LPT1, the second LPT2, and the third, LPT3.  These 
addresses are stored in memory so that DOS-based programs like NA can 
access them.

The fact that the computer assigned LPT numbers based on order often causes 
confusion when ports are added or removed.  For example, in the situation 
given above, 3BC is assigned LPT1, 378 is assigned LPT2 and 278 is assigned 
LPT3.  However, if the computer is shut down and the 3BC port is removed, 
the 378 port (which *was* LPT2) now becomes LPT1 and 278 (which *was* LPT3) 
now becomes LPT2!

One area where I've had to help people with this involves the fine W9XT 
Contest Card voice keyers, which look like LPT ports to the computer's 
operating system.  The card has a dip switch to change from 378 hex to 278 
hex.  I believe in the documentation this is called LPT1/LPT2, which is 
true only if there is no 3BC LPT port in the computer, otherwise the card 
will be LPT2 or LPT3.

Most LPT ports today are addressed as 378 hex or 278 hex.  3BC addressing 
was common for the LPT ports on the old "monochrome" video cards.  Some I/O 
cards still allow setting up the port for 3BC, which is very useful on a 
contesting computer.  For example, the 3BC port (LPT1) can be used for a 
printer, and all your standard software will talk to it.  The 378 port 
(LPT2) can be used for a W9XT card, and the 278 port (LPT3) for an SO2R box.

These comments apply only to LPT ports.  COM port are always addressed 
absolute, although the operating system (BIOS) does go look for them in 
order.  However, a port is a port - 3F8 is always COM1 and 2F8 is always 
COM2 - even if you remove one of the ports, the other one is still named 
the same.  Once you get beyond COM2, it gets a little fuzzier and you 
sometimes have to load drivers or use the NA utility SETCOM.EXE to make 
things work.

Also, these comments apply primarily to DOS and DOS windows under Windows, 
not running under Windows itself.  And if you're running Win2000 or WinXP, 
you can forget that you ever read this e-mail :-(

Dave/K8CC


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