History of Process Control Signals

The Evolution of Process Control Signal Standards have evolved over the years, starting with the 3-15 psi standard.

Process Control Signals

• Year 1940-1975: 3-15 psi standard

• Year 1955-1990: 4 to 20 mA signal

• Year 1990-2005: HART protocol for smart instruments

• Year 2000 – to date: Field bus

• Year 2007: Wireless HART

There are also other communication methods, but they have not gained widespread acceptance. With many standards there is typically a slow transition period as plant engineers and managers test period does gain widespread acceptance.

However, once the benefits of the new standard become tested and proven, more plant will install new state-of-the-art standard because of its benefits and economic cost


4~20mA or pulse or frequency output is popular.