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COMPUTERS
Adding An Extra
Parallel Port
Recently, we added a second parallel port to one of
our old PCs and while we were at it, we upgraded the
serial ports with faster UARTs as well. What we did
can be applied to other machines.
By GREG SWAIN
I
N ITS DAY, our old 25MHz 386 computer was a very
impressive machine. Purchased new just six short
years ago, it came with a 120Mb SCSI hard disc drive,
a fancy Radius graphics card and a whole 4Mb of RAM.
Of course, those specifications would be sneered at today
but at the time, it really was the latest and the greatest.
After a couple of years use in a desktop publishing
role, the 386 was relegated to more mundane tasks,
such as wordprocessing, running payroll software and
These are the two cards that we added to our
486. The X-2233 serial card from Dick Smith
Electronics (above) gave us upgraded serial
ports, while the old printer card at right gave
us a second parallel port so that we could
permanently connect two printers.
82 Silicon Chip
maintaining a subscriptions database.
And, earlier in the year, we added
an external fax/modem so that faxes
could be sent directly from the computer.
When it recently conked out, it
didn’t take long to discover why. A
distinctive burn mark in the middle
of one of the main chips on the multi-I/O board told the tale and, as we
subsequently discovered, the mother
board had failed as well. Strangely,
everything else in the machine proved
to be OK, including the RAM, the disc
drives, the Radius video card and the
power supply.
As a result, the old 386’s role was
taken over by a 50MHz 486 machine
that had been sitting unloved in a
corner of the office, itself the victim of
a recent upgrading. This replacement
machine came with a 250Mb hard
disc drive, 8Mb of RAM and Windows
3.11. We swapped its graphics card for
the Radius card from the defunct 386
(so that we could still use the Radius
monitor) and we were up and running.
After that, it was simply a matter of
reinstalling the necessary software and
copying our backup data files onto the
replacement hard disc.
Parallel ports
Having come this far, we decided
to take a closer look at the 486 to see
if it could easily be improved in any
way. The first thing that could be done
was obvious.
For various reasons, we wanted to
permanently connect two printers to
the 486, one a laser printer and the
other a dot matrix machine (don’t
laugh; dot matrix machines are still
good for printing out address labels
and multi-part forms). In the past, with
the old 386, we had simply swapped
printer cables when ever the alternative printer was to be used but that’s
a clumsy way of going about things to
say the least.
Perhaps even more importantly,
there’s now a whole raft of non-printer
devices designed to run from a parallel port. These include such things
as the Snappy video capture system
described last month, as well as a
huge range of mass storage devices
such as Iomega’s Zip and Jazz drives,
external CD ROMs and even scanners.
The problem is, how do you connect
these devices without disconnecting
the printer?
One solution is to use a printer
The existing serial ports were disabled by changing
a couple of jumper settings on the multi-I/O card. We
also reassigned the existing printer port from LPT1 to
LPT2, to avoid a conflict will the “new” printer card.
switcher but the most elegant way is to
add a second parallel port. That way,
both printers (or a printer and some
other peripheral) can be permanently
connected to the computer and the
various applications can be set up to
print to a preferred default printer.
A quick rummage through our
computer junk box soon turned up a
dedicated printer card. Just where it
originally came from is now a mystery
but it proves that if you keep something for long enough, it will eventually come in handy. More to the point,
there was no literature with the card
and in any case, there are no on-board
jumpers to configure.
Assuming that it still worked, it
would give us our required second
printer port. We’ll come back to that
shortly.
Faster serial ports
The other thing that could be done
was to improve the performance of the
serial ports. As with other machines of
its vintage, our 486 used 8250 UARTs
(universal asynchronous receiver
transmitters) in its serial port circuitry
and these are only good for about 9600
Fig.1: to check which type of UARTs your machine has go to the DOS prompt,
type msd to run Microsoft Diagnostics and click on Com Ports (or press C). This
particular machine uses 16550 UARTs but if yours uses the older 8250s, it will
need upgrading to take advantage of the latest fast modems.
November 1996 83
baud. Anything faster and you need
the later 16550 UARTs.
Because our fax/modem is capable
of operating at 14.4Kb in fax mode and
28.8Kb in modem mode, we decided
that an upgrade would be well worthwhile. The answer of course is to add
a new serial card with 16550 UARTs
to the motherboard.
This time, we weren’t so lucky with
our junk box but a quick check in the
Dick Smith Electronics catalog soon
turned up a suitable RS232 serial card
(Cat. X-2233; $39.95). It comes with
two 16550 UARTs, two serial port
connectors (9-pin and 15-pin) and a
manual with all the IRQ (interrupt
request) and address configuration
details.
By the way, you can easily check
which type of UARTs your machine
has. Just go to the DOS prompt, type
msd to run Microsoft Diagnostics and
click on Com Ports (or press C). The
UARTs used in the computer will be
listed at the bottom of the display –see
Fig.1.
Resolving hardw
Installing the hardware
Unfortunately, it’s not just a matter
of plugging in the new cards and
expecting everything to work. If you
do that, the existing parallel and serial ports will conflict with the new
ones. More precisely, you will get
IRQ conflicts and conflicts between
memory addresses which could lead
to problems.
The trick is to reassign one of the
parallel ports from LPT1 to LPT2 and
to disable the existing serial ports.
Before doing that however, we ran
Microsoft Diagnostics (type msd at
the DOS prompt) to check on the IRQs
used by the existing serial ports (COM1
& COM2) and their addresses. This
showed that COM1 and COM2 used
IRQs 4 and 3 respectively, while their
memory addresses were 03F8-03FF
and 02F8-02FF respectively.
These settings are pretty much
standard and a quick check in the
manual soon revealed that these were
also the default settings on our new
serial card. If the latter had been different, it would have been necessary
to reconfigure the jumper settings to
match the existing ports.
For good measure, we also used
Microsoft Diagnostics to check on the
address and IRQ status of the current
parallel port (0378 and 5, respectively). This was mainly a precautionary
84 Silicon Chip
Clicking on Computer and then Properties
brings up the Computer Properties dialog
box below. This lets you view current IRQ
assignments and address allocations, so
that you can easily choose free resources
before adding new hardware.
ware conflicts in Win95
Provided you have all Plug and Play (PnP)
devices, Windows 95 will successfully allocate
resources to avoid device conflicts. The presence
of older (legacy) cards can lead to conflicts
however, which you’ll need to sort out yourself.
To bring up the System
Properties dialog
box, go to the Control
Panel and double-click
the System icon. The
system properties
dialog box will
immediately indicate
any resource conflicts.
To view the resources
assigned to a particular
item, select it in the
System Properties
dialog box, click the
Properties button
and then click the
Resources tab.
Any device conflicts
are indicated in this
panel. To change
resource settings,
deselect Use automatic
settings, select the
Resource type to be
changed (eg, Interrupt
Request) and click the
Change Setting button.
November 1996 85
The two additional cards were inserted into vacant slots above the multi-I/O
card. It’s a good idea to remove the connectors for the old COM ports if possible
to avoid confusion. If they can’t be removed, label them clearly with a sticker.
measure, in case we had to change
things later on.
Next, we pulled the power plug,
removed the back from the PC and
pulled the multi-I/O card. In addition to providing one parallel and
two serial ports, this particular card
also provides two IDE hard disc drive
controllers plus two floppy disc drive
controllers.
Because there was no way of config
uring the old printer card, the obvious
thing to do was to make it LPT1 and
reassign the port on the multi-I/O
card from LPT1 to LPT2. This simply
involved changing one of the jumpers
on the card. At the same time, we
changed the positions of two other
jumpers to disable the serial ports (it
really pays to keep the manuals that
come with computers).
And that’s really all there was to it.
We replaced the I/O card, plugged in
the additional serial and printer cards,
reconnected everything and found that
it all worked. Of course, you have to
remember to plug the mouse into one
of the new COM ports, since the old
COM ports no longer function. In fact,
it’s a good idea to remove the old COM
port connectors if possible to avoid
confusion, or at least label them with
a disabled sticker.
If you strike trouble, it’s probably
due to a resources conflict. To resolve
86 Silicon Chip
the problem, check the IRQ settings
and the memory addresses carefully
and try again. If your printer ports
don’t work, for example, try changing
the address on one of the cards to the
alternative setting.
Variations on a theme
All the foregoing is just one variation on a number of possible configurations although the basics still apply in
each case. For example, a single multi-I/O card could be used to provide
both the additional printer port and
the new serial ports – just remember
to disable the functions that aren’t
required on both the new card and the
existing card. As a matter of interest,
Dick Smith Electronics sell a multiI/O card with four serial ports and
three parallel ports (Cat. X-2573) for
$129 or you can buy a 2-port parallel
printer card (Cat. X-2548) for $49.95.
On late-model 486s (and Pentiums),
the I/O functions are integrated onto
the motherboard but these invariably
use 16550 UARTs anyway so you
won’t have to upgrade the serial ports.
If you do need to disable ports or
change address settings, this can be
done via the system BIOS. This should
only rarely be necessary if adding another printer port, however – in most
cases, you will be able to configure the
add-in card to avoid conflicts.
What about Windows 95? In this
case, Windows probably won’t notice
the new hardware when it boots and
you’ll have to run the Add New Hardware routine from the Control Panel.
You can then elect to have Win95 automatically detect the new hardware or,
if you are like me and are too impatient
to wait through several minutes of hard
disc rattling, you can manually select
the hardware to be added.
In the latter case, just say No to the
auto-detect routine, then select Ports
(COM & LPT) in the next dialog box
and finally Communications Port or
Printer Port as appropriate. Windows
95 will then assign resources to the
new hardware. If these resources conflict with those used by another device,
you can change them manually later
on – see panel.
At least, that’s the theory. We haven’t tried adding an additional printer
port on a system running Win95 but
the above procedure is routine.
Finally, if you have a computer
that’s already crammed with multiple
disc drives, a network card, a sound
card, a SCSI card and any other devices, check your system resources
carefully before trying to add more
hardware. There are only 16 IRQs
available to begin with and over half
of these are taken up by essen
tial
items before anything extra is added.
Once all the IRQs are gone, that’s it –
you can’t add new hardware unless
you’re willing to sacrifice something
SC
else.
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