I had a similar problem right away when I upgraded to my G5 2 yrs ago in my office's small network.....
Along with the configuration window in ArchiCAD, be sure to set your "Location" in the Systems Preferences" under "Network" with the same address, port and so forth.
Save the location as "HP500 Plotter" or similar name.
Now.....before you plot from ArchiCAD or Plotmaker; choose and set the 'Location' under the apple menu to your saved plotter name. Your file should be directly routed to your HP500.
When you are finished, remember to reset the 'Location' back to your original setting (or you may start to wonder why no one is emailing you).
Apparently some older HP plotters don't see signals that are 'automatically' routed. The location feature was Apple's solution to aid in router and peripheral compatibily.
BTW, Printing with my older HP plotter has been painfully slow for me, esp when I'm coordinating drawings for completing a project. I've stayed with Plotting for the HP.
When I have a big order, I either use Plotflow X or I email to my local offsite copy house.
HTH
Mark
Mac G5 Dual, OSX - 10.4.8
2 Gb Ram
HP 455CA Plotter, Epson R1800 Printer,
ArchiCAD 10 - Build 1010 USA full
ArchiCAD 9 - Build 2219 USA full
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Mark Wallace Architect
P.O. Box 26357
Collegeville PA, 19426
Mark R. Wallace AIA
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MacBook 2.53 Ghz, Intel Core i5, 8 Gb,
Mac OSX (Sierra 10.12.6,
ArchiCAD 22 USA Full, +21, & 20.