Answer: During the lifetime of the Sigma-5 there have been a few improvements made to the system, resulting in 3 main versions of the system. From 1983 to approx. 1989 the Single-Field version was delivered to customers. From 1989 to 1995 the Dual-Field system was delivered and from 1996 till now the Dual-Field system without R(esistance)-outputs is delivered to customers. The main differences are the catheter-connection plug (2 versions, grey plastic Viking-type and metal Lemo-type) and the output-connectors (round Din-type and D25-type) used.