Agilent Technologies E5900B User Manual page 102

Troubleshooting the Emulator
Problems with the Serial Interface
probe is turned off (or is not connected) will hang the PC. The only way to get
control back is to reboot the PC. Therefore, we recommend that you always
turn on the emulation probe before attempting to connect via RS-232.
E5900B / E5901B User's Guide Supplement 101