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### Connecting a no-NIC 8086 PC to LAN through its parallel port ###
Here's the story: I'm the lucky owner of an ancient Toshiba T1100 laptop. This
laptop is fitted with an 80C86 CPU, 256 KB of RAM, two floppy drives and no
Ethernet port. There is also no way to add an Ethernet adapter. Since the
computer doesn't have a hard disk, working on it can get slightly frustrating
sometimes (swapping those 720K floppies all the time is fun, but come on). I
dreamed about being able to connect this laptop to a big network drive where I
could store all my relevant stuff, so the floppy drive would be used only for
booting DOS.
As a network drive solution, I use EtherDFS. Problem is, I need to connect
somehow the Toshiba T1100 to my Ethernet LAN, and without an Ethernet adapter,
it's no trivial task.
*** PLIP ***
PLIP stands for "Parallel Line Internet Protocol". It's a protocol that allows
to send and receive packets through the parallel (LPT) port of a computer.
While the name suggests it operates at the layer 3 (IP) level, nothing could
be less true. PLIP is actually Ethernet-aware, meaning it sends and receives
full Ethernet frames. And that's perfect for what I needed to do.
*** The PLIP client ***
Russ Nelson from Crynwr wrote a PLIP packet driver for DOS. It's a network
packet driver that emulates an Ethernet card, and sends/receives frames
through the computer's parallel port. Its usage is pretty straight-forward,
assuming one knows what IRQ and I/O port the local parallel port runs at:
plip -c 0x60 7 0x378 54:52:00:11:22:33
Note: the PLIP packet driver is able to auto-detect the I/O port and IRQ of
the LPT port on newer computers (it did so splendidly on my 386SX), but
apparently not on 8086-class computers, where it requires those to be passed
as arguments.
*** The PLIP "server" ***
Okay, I have my brave 8086 computer sending Ethernet frames through its
parallel port, but what next? I still need to hook somehow its parallel port
to my Ethernet LAN. This is where the server part comes in. I thought,
"wouldn't it be convenient to have some sort of switch that could have some
Ethernet ports and a PLIP access port?". Obviously such hardware contraption
doesn't exist (or at least I am not aware of this), but still, there is a
solution: use another PC as a switch! There is this YAPCBR project from the
university of Bombay that does exactly what I needed: it turns a DOS PC into a
switch, operating on packet drivers running on said PC. YAPCBR stands for "Yet
Another PC BRidge", presumably in reference to the "PCBRIDGE" tool that does a
similar thing (I did not test PCBRIDGE, in my understanding it operates
directly on network cards, while I needed a solution that talks to packet
drivers).
Now, all I had to do was to start my 386SX desktop PC (whose Ethernet card is
connected to my LAN already), load the PLIP packet driver additionally to the
packet driver related to the 386SX's Ethernet network card, and finally
execute YAPCBR. The latter made sure to forward Ethernet frames between my
386SX's network card and the PLIP packet driver back and forth. Naturally, for
the whole trick to work, both computers had to be connected together with a
parallel cable (same kind as used for LapLink-style parallel transfers).
+---------------+ +--------------+
| TOSHIBA T1100 | | 386 SX |
| | Parallel cable | |
| LPT :--------------------: LPT ETH :--------> MY LAN
| | | |
| PLIP pkt drvr | | <- YAPCBR -> |
+---------------+ +--------------+
The 386SX PC
acts as a dual
port switch.
Once all this was set, I could enjoy having Ethernet connectivity on my
Toshiba T1100 laptop, meaning the EtherDFS network drive solution was able to
load. From now on I could access my multi-gigabytes network drive without
having to transfer files on diskettes.
Of course having to transform another PC into a PLIP switch isn't very
convenient. A more elegant solution would be to use a hardware "PLIP to
Ethernet" device, and it would appear that such device actually exists: it's
called "plipbox" and it's an open-hardware project from Chris (aka "Lallafa").
Plipbox is designed to provide Ethernet connectivity to Amiga computers, but I
suppose it would work just as well with a PC. I did not follow this path
though, since I'm already very happy with the YAPCBR solution. If you're
curious about Lallafa's plipbox, read more about it on Chris' website at
http://lallafa.de/blog/amiga-projects/plipbox/. Another solution could be to
buy a Xircom PE3-10BT adapter - it's basically an Ethernet adapter connected
to the LPT port, produced by Xircom during the nineties (and increasingly hard
to find nowadays).
=== Attachments ==========================================
plip112.zip
ypcbr102.zip
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