SGI rapidly developed its machines into workstations with its second product line — the IRIS 2000 series, first released in August 1985. SGI began using the UNIX System V operating system. There were five models in two product ranges, the 2000/2200/2300/2400/2500 range which used 68010 CPUs (the PM2 CPU module), and the later "Turbo" systems, the 2300T, 2400T and 2500T, which had 68020s (the IP2 CPU module). All used the Excelan EXOS/201 Ethernet card, the same graphics hardware (GF2 Frame Buffer, UC4 Update Controller, DC4 Display Controller, BP3 Bitplane). Their main differences were the CPU, RAM, and Weitek Floating Point Accelerator boards, disk controllers and disk drives (both ST-506 and SMD were available). These could be upgraded, for example from a 2400 to a 2400T. The 2500 and 2500T had a larger chassis, a standard 6' 19" EIA rack with space at the bottom for two SMD disk drives weighing approximately each. The non-Turbo models used the Multibus for the CPU to communicate with the floating point accelerator, while the Turbos added a ribbon cable dedicated for this. 60 Hz monitors were used for the 2000 series.