Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01

Question: I would like to get access to real time data from Citect to my Unix application. I need a fast response time as I want to develop a simulation process running on Unix with Citect as the interface. How can I do this? 

If you where sharing real time data between Windows applications you would use DDE or Network DDE to get the real time data (see Q1206). When you want to share real time data with platforms which don't support DDE then the best way is to use a DISK PLC. Sharing real time data via a DISK PLC is a very fast method of data transfer however it does involve some work in setting up. You will need to be a C programmer to extract the data from the DISK PLC to your application.

To allow sharing real time data via a DISK PLC, Citect and the other application must be able to share file services (NFS with Unix). How it works is Citect reads and writes to the DISK PLC while at the same time the other application also reads and writes to the DISK PLC. This allows Citect to write to a section of registers in the DISK PLC and the other application to read those registers to get the data. The reverse operation is used to transfer data back. You must configure Citect to read the data you want from the physical PLC's and then write it into the DISK PLC. A report is the best method for doing this. Then you get Citect to read the DISK PLC's and if the other application has changed any values, Citect will write them into the physical PLC.

To allow the other application to access the DISK PLC you must write your own code. The format of the DISK PLC is documented in Q1250.

This method of sharing real time data with the DISK PLC will not work with the DISK PLC driver supplied with Citect for Windows Version 3, as this DISK driver has support for redundant DISK PLCs which are incompatible with this operation. If you don't require redundancy of the DISK PLCs then you may use the DISK PLC from Citect for Windows Version 2.0. If you don't have this driver contact Citect Support.

A future product will allow sharing real time data to Unix via a simple socket API.