One Ethernet network troubleshooting tool our Product Support Group recommends is Wireshark. It's been a helpful choice for many years, and it's free.
Our recent blog post explains where to get Wireshark and how to use it. You can pay a lot for network analysis tools—and in some cases you may need them—but many times, the free ones do the job.
You can use it to view network sessions between two devices, to see what’s actually going on at a packet level. You can even see the data that two devices exchange, to check whether it's in the right format.
The recommendations in the blog post apply to PCs, PACs, PLCs, and just about any industry-standard Ethernet network device.
Read the troubleshooting with Wireshark blog post.