I've just spent a frustrating evening trying to run ARC on my new MRX500 without any success. In fact, I thought I almost bricked my new unit while trying to upgrade the new firmware made available today.
A fellow AVS member was kind enough to lend me a Bytecc USB-RS232 adapter, which I believe is the same one that monoprice sells. I found the driver on the Bytecc website and it installed fine (it uses the Prolific driver). I proceeded to upgrade the firmware** and was successful. However, when I tried to run ARC, the software failed to initialize and returned a message saying it can't locate a valid processor. I went into Device Manager and checked the serial port settings: I tried a few different com ports (1-6), even changed the stop bits to 2 from the default 1, but I got the same message each time about being unable to locate a valid processor. I even found a newer version of the Prolific driver, but still no go. Is this because this Bytecc cable won't work with ARC and I have to get the Keyspan adapter or is it something else?
I figured this Bytecc one was successful in upgrading the firmware so it should be fine to run ARC, right? Has everyone who has bought one of these receivers so far been using the Keyspan adapter?
**My first attempt failed and I thought the receiver or my HTPC crashed. I was using my HTPC to run the firmware upgrade and this was being output to my projector screen. When the firmware was writing to the receiver video RAM and ROM, my screen went blank. I waited and waited and figured something went wrong, and so I finally unplugged the receiver. When I turned it back on again, the audio was fine, but there was no video! I clicked on the INFO button and saw there was no firmware version shown for the video on the receiver. I then went and tried the firmware upgrade a 2nd time, but this time using my netbook. This time, everything went fine. The readme text file does mention that this whole process takes about 10 mins, so I unplugged the receiver too quickly the first time when I thought something crashed. I think when it was writing to the video RAM and ROM, it obviously affected the output of video. Anyways, I don't think there was any harm done, but if this had been a computer motherboard I was flashing, I'm sure I would've bricked it.