Ethernet Print Server Keeps iMac G5 from sleeping

Question

Hello All,
I have a small home network setup with a router to share my DSL Internet connection.
My iMac G5 has never had any sleep problems before. But, with the recent addition of a US Robotics Ethernet Print-Server, the iMac G5 no longer sleeps automatically.
It will sleep fine if I use the power button to sleep it. But, it will not sleep if I let the "Energy Saver" activate it.
The iMac G5 does turn off the display when it is supposed to (in the energy settings). So, it will sleep the display. Just not the computer when the Print Server is attached to the network.
If I unplug the print server from the Ethernet, then the iMac G5 sleeps just fine when the energy saver kicks in.
Further, if the time to sleep has elapsed, and the computer is still awake, then I can unplug the print server from the Ethernet and the iMac G5 will go to sleep a couple of seconds later.
Interestingly, the Mac Mini I have doesn't stay awake. It sleeps just like it is supposed to. It doesn't care if the print-server is attached or not.
The Mac Mini and the iMac G5 are both configured to communicate with the Print Server. It uses AppleTalk to share my printer (a Hewlett Packard LaserJet 5mp).
The iMac G5 is running OS X "Tiger" 10.4.3 and is updated to the current level (all updates). The Mac Mini is running OS X "Panther" 10.3.9 with all the available updates installed.
The system specs are all in my footer below my message.
Any ideas?
It is obviously the print-server keeping my system awake. If I un-plug it from the network then everything behaves perfectly normal.
Is there a setting somewhere to keep the iMac G5 system from listening to other devices on the network somewhere?
There is no bi-directional setting in the Print-Server. So, that shouldn't be the issue (plus the Mac Mini doesn't care).
The Print Server is a U.S. Robotics USR7500 Ethernet Print Server.
The website for my Print Server is at:
http://www.usr.com/products/networking/networking-product.asp?sku=USR7500
Hopefully there is a setting somewhere that I can adjust in OS X Tiger to fix this.
The Wake on LAN setting is not turned on. So that's not it.
I also just checked, and the scheduled sleep will work. So, it is just the "inactivity" setting that is giving me trouble.
Like I said though, if I unplug the Ethernet Print-Server, then everything works exactly as it should.
I'll look forward to any ideas you might be able to offer.

Answer 1

Update:
Hello All,
I found out what was causing the machine to not sleep.
My built-in Firewall was configured to block UDP communications.
I discovered, quite by chance, that the Print Server was sending a UDP signal to my Mac every 30 seconds or so. I have no idea why, especially since the Print Server should be a receiver not a sender.
As an experiment, I disabled UDP blocking in the FireWall, and my machine went to sleep just like it should.
Any ideas as to why the Print Server would be sending a UDP signal?
Here's an excerpt from my firewall log:
===== Display starts at offset 196869 within file of length 327941. ====
===== Use File->Reload (Cmd-R) to display more. ====
.1.206:138 192.168.1.255:138 in via en0
Jan 3 22:32:46 hunters-imac-g5 ipfw: 35000 Deny UDP 192.168.1.206:138 192.168.1.255:138 in via en0
Jan 3 22:33:16 hunters-imac-g5 ipfw: 35000 Deny UDP 192.168.1.206:138 192.168.1.255:138 in via en0
Jan 3 22:33:46 hunters-imac-g5 ipfw: 35000 Deny UDP 192.168.1.206:138 192.168.1.255:138 in via en0
Jan 3 22:34:16 hunters-imac-g5 ipfw: 35000 Deny UDP 192.168.1.206:138 192.168.1.255:138 in via en0
and it keeps going like that for 12 hours or so.
The print server is setup to allow for TCP/IP, NetBEUI, and AppleTalk communication.
I'd like to hear any ideas as to how to get rid of this UDP so I can re-enable the blocking feature in the Firewall.
I do need to keep features enabled that are required to allow my wife's portable PC with Windows XP to print to the server as well.
Thank you in advance.