Removable Storage Media Add Flexibility to Modern Day PLCs

Jan. 14, 2008
Why can’t my PLC interface easily to off-the-shelf devices?

That question is being answered by a new wave of PLCs with standard interfaces to common peripheral devices. Without sacrificing the deterministic operation, reliability, and ease of use of the traditional PLC, modern day PLCs are now incorporating more and more PC features than ever before. One such feature becoming commonplace in this new generation of PLCs is an interface to removable storage media. Interfaces to memory devices such as USB memory devices and CompactFlash® are providing flexibility never before realized in PLC architectures.

Removable storage media bring flexibility to data collection and recipe storage applications in PLCs. For data collection, the PLC no longer needs to communicate directly to an external device, but can operate stand-alone, storing data to the removable memory device only to be retrieved at a later date. Data can still be collected by an external device, or can be retrieved by an onsite service technician. The technician can walk up to the PLC, remove one memory card replacing it with another, and walk away. Because these memory devices are capable of holding gigabytes of data, weeks, months, or even years of collected data can be stored to a single device. Similarly, an operator can change the recipe used in a process, or change the profile used in a motion application by swapping one storage device for another.

Data collection and recipe storage have always been challenging for PLCs because of their inefficiencies in sharing data with other devices. PLCs have long been used for data collection, and while the PLC may be good at collecting data, sharing that data with external devices is typically a cumbersome task. The PLC, being register-based, could only share information as blocks of raw unformatted data. Accurate management of these blocks of registers is critical, as the smallest addition or omission by the PLC significantly changes the meaning of the data to the external device. Similarly, downloading recipes that define process parameters to a PLC requires the same register management. The block of registers in the PLC that is receiving the recipe data needs to be identical to the incoming data from the host device. The slightest variance gives a whole new meaning to the process being controlled.