The Automationdirect DL405 series PLCs with D4-450 CPU’s were chosen for some reasons. First, this PLC series can be used for all of the upgrades. This reduces software development, maintenance and spare parts costs by reducing the number of separate hardware platforms that are in use at the facility. Second, the hardware is inexpensive. Automationdirect hardware is approximately half the cost of a similarly equipped Allen Bradley system. A PLC system provides greater capability at a smaller cost than a similarly equipped CAMAC system for a small number of channels. As the channels number increases, however, that saving decreases. Third, sufficient documentation is available to communicate with the hardware using Linux, including the source code and a user supplied Linux port of their software development kit. The source and documentation are a “work in progress”, but we had no difficulties in getting the system up and running. Fourth, the system can be used with either the PC or a dedicated PLC CPU as the controller. In the first case, the PLC can be used as I/O simply. In the second case, local intelligent can be provided for equipment ramping, protection or PID control. Fifth, the D4-450 CPU’s can generate exceptions if the expected scan time is exceeded, providing a watchdog against failure of the equipment protection modules. Sixth, interrupts inputs are available. These can be used for synchronization purposes, allowing the use of both the PLC and the CAMAC systems during ramping if driven by a timing signal. Seventh, PID control is locally available in the CPU.
Using PLC has some drawbacks. Analog voltage input and output modules are limited to 12-bit conversion, and are only available with up to four channels in a module. Memory of each CPU is limited to approximately 28K words, so complicated ramps may have to be used. The programming interface of PLC is Automationdirect DirectSoft Software.