I tried:

restarting the IOC
power cycling the dust monitor (Corey did this for me)
power cycling the Comtrol
removing the weather station from the Comtrol (I left it removed)
screwing in the DB9 connector from the Comtrol on the breakout box (it was connected but not screwed in)
checking the fuse in the breakout box (it looked fine)

The IOC can connect to the port, but it doesn't seem to be able to establish communication with the dust monitor. Corey looked at the dust monitor from inside the clean room and said that it is showing numbers on the front panel.