The PLCs have been extensively used in industry to automate a variety of control a d monitoring functions in many years. They are flexible, versatile and are built to withstand harsh factory conditions. However their role was confined to industries due to their slow response, slow processing power and lack of communication standards. Because of recent evolution towards PLCs with fast processors, a variety of compatible I/O products and communication adapters that support TCP/IP network, the PLCs have made a quantum leap into the arena of distributed control systems for large accelerator facilities.
The control system at the NSLS has a two-level distributed architecture consisting of HP/c8000 series workstations for the high-level (the operator level) and 80 VME-based microprocessors subsystems for the lower level. The communication is high speed Ethernet. The micros are responsible for the control and monitoring of the storage ring hardware. The equipment to be controlled, dictates the type of I/O boards for a micro. The I/O peripherals include ADC, DAC and digital I/O cards in the micro crate.
Though fast and reliable, VME I/O cards are very expensive and may not be required when controlling and monitoring devices with slow response. PLC was considered as an alternative. The hardware engineers also preferred to replace the traditional digital circuit hardware with PLC.