Cellular Modem?

The AT module passes undefined back if it times out while waiting for a response. The SIM900 module should really detect that where it can happen and throw an error though.

But... The charCode 255 looks a bit dodgy - could it be:

• that there's some baud rate mismatch on bootup
  
• that there's some bad connection on RX or TX wires
  
• the GSM module isn't sharing the same ground as the Espruino?
  
  

Also, I'm not sure how you're doing it, but is it possible that the GSM module needs a few seconds after power is applied for it to boot up correctly?