Sorry... I did try to search for earlier discussions... but couldn't find a good search string.
I have a simple TCP/IP server running in a Sparkfun Ethernet Pro... the product with "Pro Ethernet" in the silkscreen, sku: DEV-10536. (Nice little device!)
It works fine... when I have my ModernDevice "BUB" programming cable attached to the DTR/ RX/ TX/ 5v/ Gnd pin set. (The BUB uses the FT232R chip, the same as Arduinos, the FTDI cable ModernDevice sells, and the Spark Fun breakout boards.)
My program doesn't interact with the computer I use for programming it once the program in the Arduino starts running... at least I don't THINK it does! (I haven't started communications with the serial monitor, for instance.) I don't thing that anything is traveling via the BUB now except the power to operate the Ethernet Pro.
However... If I disconnect the BUB, and plug suitable a DC voltage into the connector on the Ethernet Pro, although LEDs wink merrily, the server no longer serves. I've re-booted the client software after the power swap. (Also pressed the Ethernet Pro reset button.) I didn't disconnect the device from the LAN during the power change over... those lines are pretty "hot swap" friendly, aren't they?
Anyone have any ideas? Why will it work if powered via programming cable, but not if powered from a wall-wart which powers a different Arduino just fine?
I've swapped the power provision a couple of times... same result each time. "5v" line at about 4.5 on both of my Arduinos this evening. Voltage from wallwart, when under load of Ethernet Pro, 7.5 volts.