The VT200s were the last DEC terminals to provide a 20mA current loop serial interface, an older standard originally developed for the telegraph system but became popular on computers due to the early use of Teletype Model 33's as "ad hoc" terminals. A standard 25-pin D-connector was also provided for RS-232. Only one of the two ports could be in use at a given time. Later DEC terminals would replace both of these with their proprietary Modified Modular Jack (MMJ) connectors.