Q: Connecting LaserWriter directly to Mac Pro

You could try using Internet Sharing to make the connection using the second port.
System Preferences > Sharing \[√] Internet Sharing

Your printer will need an IP address. If it cannot get one working through the MacPro, you may need to assign it one manually.

My reference to Ethernet addresses above was a broad general comment. Many users find that setting the printer's Ethernet Address initially can be a major issue. If your printer already has an Ethernet Address, you are all set in this regard.

Do you have your own Router with Ethernet ports on it, or are you cabled directly to your ISP's interface (cable modem, DSL modem, or similar) ?

A Switch accepts good packets from all sources, and momentarily stores them and then forwards them on the cable most likely to connect to the right destination. It easily accommodates mixed speeds.

A Hub simply repeats the signals presented at each input port at all the other output ports. It knows nothing about packets, and cannot adjust for different speeds.

Your LaserWriter is definitely 10Base-T. Your computer can likely do 1000Base-T but is probably running 100Base-T, the most common speed for a DSL modem.

If that box is actually a Hub, it will not be able to adjust to the mixed speeds. If it is a Switch, there is some hope. An older Switch may need special attention to straight vs crossover cables. The lights on the Switch can help you see whether a valid connection has been made or not.

The Sharing setup is usually used to connect a stray Wireless device or two into a Wired network, or connect a stray Ethernet device into a wireless network, using your Mac as a Switch. But since you have a second Ethernet port, you should be able to connect your second Ethernet port into your existing Ethernet network.

Your query sent me into the Apple knowledgebase after more Mac Pro information. I turned up this:

Maximizing Ethernet Performance
Your computer performs best when the Ethernet port to which it is connected is set to auto-negotiate. Ask your network administrator to verify this setting.

Which reminds me that the LaserWriter will not be able to auto-negotiate a speed when Sharing port 2, so you would manually have to set the speed to 10Base-T in:
System Preferences >Network > Ethernet port 2 > Ethernet pane

Does the IP address of the printer have to be in the same scheme as the IP of the router and the IP of the Mac? In other words xyz.xyz.xyz.1 and xyz.xyz.xyz.36 or so?

What does "self assigned" IP adress under the IPv4 tab mean?

Those two questions are intertwined, and include the Subnet Mask as well.

An Ethernet device can communicate freely with any device on the same subnet, i.e., that matches the Subnet mask hi-order bits. Most private Networks are using something like 192.168.0.zzz with a subnet mask of 255.255.255.0. That means any device that matches in the top three bytes can communicate directly, and all other require a Router to intervene for each packet.

When an Ethernet device first wakes up, it does not know what to use for an Ethernet Address, so it self-assigns an arbitrary address in the range 169.254.xxx.yyy with a Subnet Mask of 255.255.000.000 and broadcasts a request for a Router to give it a better Ethernet Address.

That's great!

So is this correct? You got it to work by:

• Making sure the printer had an IP Address
• "Internet Sharing" your (main) Ethernet connection onto the second Ethernet port.
• Assigning a Manual IPv4 Address "in the same range" (but avoiding .255) to the second Ethernet port and setting the Subnet Mask to 255.255.255.0
• ?? manually setting the speed to 10Base-T ?? or did it take care of that itself?