Blinks on power-up, reset, or on some error conditions. This is the large black rack-mountable chassis which contains the processor and special RS232 Serial Comport boards. RS-232 RS232 Serial Comport cable. This is normally contained in a cloth pouch with a battery. Backpack often refers to the entire subsystem which includes the remote master, cloth pouch, battery, and sensors.
Sensors come in different cable lengths.