Wednesday, July 20, 2011

Design of the Control System of PLC Driver

Right now, only VMEs under the operating system vxWorks can be employed as an IOC of EPICS. There are a lot of modules VME are offered commercially for processing analogue signals and normal digital. The drivers of EPICS are already offered for many modules. For that reason, it doesn’t need to worry about devices which need only the signals. Though, there are a lot of devices which require special handlings. PLCs are reliable and cheap in a sense that they remain operation under any trouble in networking or computers, and thus it is preferable to employ PLCs for some serious devices for example an ion-source. How PLCs create communication with IOCs is one of the problems that have to establish early. The answers are as follows;
• Using Ethernet TCP/IP. Along with PLCs offered commercially, presently only PLC of FA-M3 by Yokogawa support the protocols of data transfers we needed.
• Allocate an ID (identification number) to every PLC. It is hazardous to rely only on the address of IP. Check the ID to ensure you are communicating with a suitable PLC before any communication with a PLC and at any data transmit to a PLC.
• No entity commands for action. Only transfer data from/to PLC memory is supported. Any operation of device should be done while contents of memory in a limited address range.
• No straight write to memory of PLC. An IOC transfers 3-word data such as a content, an ID, and a memory address) to the PLC memory communication area. After that, a PLC ladder program transfers the content to a proper address after checking its ID.

The PLC EPICS driver prototype has been written and effectively used for the operation of ion-source, even though all the rules stated above are not fully applied yet in the prototype driver. Waveform digitizers choice is another vital issue that has to think for device control. Its driver of EPICS has been finished and tested partially. Since the network of Ethernet runs over the area, then chose Ethernet for digital data connection avoiding another field buses type so that all of the communication of data is united. By doing so, we can trace back all data throughout network which will considerably help us for diagnostics of operation. Also it lessens the stock items number which may decrease the operational cost. It is developing an interface board of Ethernet which will be employed primarily in the quadrupole magnets power supplies at the DTL to achieve the unified Ethernet connections.

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