It was over 35 years ago that the first DCS systems were installed in glass industry processes. In those early days of automation, there was a clear spilt between DCS and PLC systems, with both types being designed for their specific tasks. DCS took care of the delicate continuous processes like melting and conditioning control.
On the contrary, PLCs took care of digital control. Distributed control was developed to control analogue processes that required high precision, while programmable logic control had to run at high cycle speeds to be able to detect fast events in an instant. PLCs were suited to relatively small dedicated applications, whereas distributed control was suitable for controlling the whole furnace, conditioning and annealing systems and sometimes, even the batch control.
Control failure was much more critical in DCS than PLC systems because of the impact it could have on the most expensive glass manufacturing plant assets. Therefore, redundancy and hot swappable input and output (I/O) boards were a vital requirement, arriving relatively early in DCS systems, compared to late or not at all, in PLCs. When higher processor speeds and larger memory sizes became available at an achievable price, some PLC suppliers tried to enter the DCS market by adding specific functionalities to their product line with sometimes more but unfortunately predominantely less success. The subsequent result was that DCS system suppliers focused on high value businesses like oil, gas and chemical industries.