History Data for a TBox RTU

TBox RTUs, manufactured by CSE Semaphore, can store periodic time-stamped process data internally using Sample Tables. Because the standard Modbus does not have facilities for transferring time-stamped data, the TBox implements a set of custom Modbus functions to accomplish the transfer. To enable these tables to be read into VTScada tags, enhancements have been made to the standard VTScada Modbus driver specifically to access the TBox sample tables.

3 forms of history data are available: Sample Data, Digital Chronology and Analog Chronology.

• Sample data is read with a History Address of "STx", where x is the table number to read, starting at 1.
• Digital Chronology is read with a History Address of "DCxxxx", where xxxx is the address of the digital.
• Analog Chronology is read with a History Address of "ACxxxx", where xxxx is the address of the analog.

The driver will read all the available history data in a compressed mode, and will then poll at the History Scan interval for new data, reading any that is available.

The total number of simultaneous sample-table or chronology reads on a TBox RTU is ten. Because Ethernet-connected RTUs can be accessed by multiple masters, VTScada reads only one sample table or chronology at a time even if multiple tables are read at the same rate. Using this scheme, if multiple tables need to be read at the same time they are automatically read in sequence.

Up to 64 simultaneous reads are possible, but a practical limit should be lower than this, particularly for RTUs that can be accessed by multiple masters. You are advised to keep the total number of reads to fewer than ten.