The HART Protocol provides two simultaneous communication channels on the same wire: 4-20mA “current loop” analog and a HART digital signal. While the analog signal continues to provide primary values to and from field instruments, the digital signal provides additional device information. This is a very robust method with roots in the Bell 202 Frequency Shift Keying (FSK) standard, which originally superimposed a digital communication signal “on top of” the 4-20mA current loop to bring Caller ID technology to the field of telephony