Hello, Paul
I finally got it to work! Your answer above was not the solution I was looking for, but it did point me in the right direction to cut through the administrator jargon and to find the answer in the 300x 510x Administrator's Guide, pp 49-52.
(Your answer was going to continue to force me to continue to connect to the 300x through a parallel cable, which didn't gain me anything, and I didn't want to share the printer through the PC to the Macs either, when the 300x is a perfectly good server on its own.)
For anyone reading this who wants to know how to connect to the 300x from PCs over Ethernet, and from Macs over Ethernet WITHOUT using Appletalk, read on.
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For XP, of course, it was NOT necessary to install TCP/IP, as it was already there.
1. Verify that the Print Services for Unix is installed (required for LPR port availability):
a. Windows XP: Click Start, Control Panel, Network Connections.
b. Click the "Advanced" menu ans select "Optional Networking Components.
c. Select and enable "Other Network File and Print Services"
d. Click "Details" and verify that "Print Services for Unix" is enabled. If not, enable it. (I had to enable this for XP Home edition.)
e. Click "Okay" and then "Next".
2. Windows XP: Click Start, Control Panels, Printers and Faxes.
3. Select "Add Printer" from the sidebar. From the Add Printer Wizard welcome screen, click "Next"
4. Select "Local Printer" and disable automatic detection for Plug and Play printer installation. Click "Next".
5. Choose "Create a new port" and select "LPR Port". Click "Next".
6. In the "Add LPR compatible printer" window:
Enter the IP address of the Jetdirect 300x.
Enter (in lower case) "auto" for the name of the print queue. (if your printer supports auto language switching)
Then click "OK".
7. Select the Manufacturer and Printer model.
8. Choose to keep the existing driver. Click "Next".
9. Enter a printer name, and choose whether this printer will be the default printer. Click "Next".
10. Choose whether this printer will be available to other computers. If shared, enter a share name that identifies the printer to other users. Click "Next". (I did NOT choose to share the printer, as this PC tends to have continual virus and other problems and I don't want it to be on all the time.)
11. If desired, enter a location and other info. Click "Next".
12. Choose whether to print a test page, and click "Next".
13. Click "Finish" to close the wizard.
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After I had completed the above on the PC, it became obvious by analogy how to set up the OS 10.4 Macs using the "Printer Setup Utility".
1. Open "Printer Setup Utility" from Applications/Utilities.
2. Click "Add".
3. Choose "Line Printer Daemon - LPD" in the "Protocol" selection box.
4. Enter the TCP/IP address of the Jetdirect 300x in the "Address" field.
5. Enter "auto" (all lower case) in the "Queue" field.
6. Enter a name for the printer server/printer in the "Name" field.
7. Enter a location if desired (if you have more than one similar printer).
8. In the "Print Using" box, select the printer manufacturer, and then choose the model from the list of printers that then appears below. (to choose the correct PPD)
9. Click the "Add" button and you are done!
To check that the 300x server/laser printer was now truly set up as a TCP/IP printer, I turned off Appletalk, and I was finally, after 12 years, able to print to my printer without using Appletalk!!! Yay!