The IBM 3705 Communications Controller was a simple computer which attached to an IBM System/360 or System/370. Its purpose was to connect communication lines to the mainframe channel. It was a first communications controller of the popular IBM 37xx series. It was announced in March 1972. Designed for semiconductor memory which was not ready at the time of announcement, the 3705-I had to use 1.2 microsecond core storage; the later 3705-II used 1.0 microsecond SRAM. Monolithic System Technology components, similar to those in S/370, were used.
The 3705 normally occupied a single frame two feet wide and three feet deep. Up to three expansion frames could be attached for a theoretical capacity of 352 half-duplex lines and two independent channel adapters.
The 3704 was an entry level version of the 3705 with limited features.
Read more about IBM 3705 Communications Controller: Purpose, Architecture, Peripherals, Communication Scanners, Maximum Communication Line Connection Estimates, Channel Adapters, Related Machines, Demise