The first open protocol wasn't developed in the 1990s, but 1986! American Auto-Matrix (AAM) had the first open protocol, named the Public Unitary Protocol (PUP protocol). It was developed to standardize communications protocol among controls manufacturers in the facility automation industry. This protocol is generally understood to form the basis of the current BACnet protocol, which has become popular of late.

AAM has been utilizing this open protocol since that time. Other companies have written interfaces to this protocol as well.

PUP Unitary Controllers include:

DX1™	SBC-ASC
IX1™	SBC-VAVra
GC1™	SBC-VAVtf
GX1™	SBC-STAT™
MC1™