High-speed Communications for Data-Intensive Applications
Controllers are increasingly called upon to be Communicators. Not only are many subsystems now typically connected to a controller via device-level networks, Ethernet or serial communications, but HMI and MES systems impose an increasing communications burden on what used to be isolated control systems.
A frequent challenge in this environment is transferring data at a speed high enough to keep up with the process, particularly where the data is being used in a higher level system to control the process in real time. In Ethernet networks, there are two technical issues that can unnecessarily constrain the transfer rate in such circumstances:
- The minimum packet size using any Ethernet-based protocol is 72 octets, plus a 12 octet interframe gap. So, if you are only transmitting a single 32-bit number per transaction, the minimum packet size imposes a 2000% overhead.
- Each transaction takes a finite amount of time to process in both the sending and receiving system. Usually this time swamps the amount of time wasted by the minimum packet size. If you are using typical industrial protocols able to transfer only one type of data per transaction a real problem arises if you have a large amount of disparate data to transfer. In such a case, many packets are required to accomplish the transfer, and the above two factors combine to greatly increase the time required to transmit each batch.
The Scatter/Gather Packet
A solution would be to allow the designer to select a variety of different data sources to be collected together and sent in one packet, which is exactly what was implemented in the CTC 5000 Series and some 2700 Series controllers.
The CTC DLL library or CTC OPC server is used to implement this capability, called Scatter/Gather. The Scatter/Gather function allows you to define up to 50 selected items that you want to track in the controller, which may include analog values, registers, servo positions and other numeric resources. This data is then returned in batches of 50 items, reducing turnaround time compared to sending each resource individually. You can also set an automatic transfer interval, making Scatter/Gather a comprehensive high-speed data collection tool for a variety of applications.