When I download the reference designs or the files from the ML510 site, the PLB_Err LED is always on. Is this a problem?
The PLB_Err LED is not being driven by any signal from any of the reference designs, so this is just an general purpose I/O LED. This is not an issue with the board or with the reference designs; the LED is not being used.
If you want to drive the OPB_Err and/or the PLB_Err led when your ML510 design does not use these leds, you can add this to your MHS and UCF.
# RED RED
PORT opb_led = net_vcc, DIR = O
PORT plb_led = net_vcc, DIR = O
PORT opb_led = net_gnd, DIR = O
PORT plb_led = net_gnd, DIR = O
Net opb_led LOC=AL19;
Net plb_led LOC=AK24;