You can leave full duplex enabled. I've never seen a problem caused by half vs. full duplex recently.

Does that program actually run diagnostics? If so, what were the results?

Yes, the program does run diagnostics. On the adapter diagnostics, everything passed, including the loopback test. That test took a while to complete but it did complete. The network diagnostics test does not appear to be doing anything.

Diagnostics passing is good, but be careful with how you interpret the loopback test. If it's not actually putting bits on the wire and then reading them back, then it's not going to catch problems with the physical layer. Put another way, depending on the implementation it might not be a full loopback test.

(I still think your card is not sending packets; you need another machine directly connected to it to verify that though.)

Should I try using Wireshark or a similar tool to see if packets are actually sent?

Found it. I have to load the packet driver with /jitioff to have functionality.

Tested that. Did not make any sort of difference at all.

I suggested using tcpdump (or an equivalent) earlier.

The best configuration is to hook the PC directly to your tracing machine directly to eliminate other noise from the switch. Or, you could just run the two on an isolated subnetwork for a bit.

My favorite debug tool was a Linux box with two Ethernet ports. I would configure it to bridge the two ports, allowing me to see all of the traffic destined for the mTCP box transparently. This is a little fancy for this; I think just connecting the two machines and getting a trace will suffice.

Yup, a Linux box with Wireshark and an auto MDX ethernet port.

A little update to this - 15 minutes ago, I decided to install Arachne 1.97 on the 286. The D Link packet driver did not work at all, so I removed it. I let Arachne set up a NE2000 generic driver - and it actually worked! I was able to browse the web with Arachne just fine. Strange how the drivers D Link provides do not work with my configuration. I only had about 5 minutes of time before I had to go somewhere, and I decided to fire up some MTCP apps to see if they work. Once I opened DHCP, it got an IP address and lease instantly. I was also able to ping my default gateway with no dropped packets. I guess I can say that this issue has been resolved.

I'm glad to hear that it was not a hardware problem after all. A bad device driver can definitely make it seem like so.