Datablock or Poll Record
This explanation may slighltly vary based on vendor/manufacturer.
Typically, poll record is old driver term for the specific data you want to access in a device. OPC Servers use the term datablock to refer to the same data because typically you want to access entire sections or blocks of data at one time.
You can define the data you want to access by entering the range of memory addresses the device uses to store the data and the type of data you want to read, (for example, Integer, ASCII, or Float). The driver needs the data type so that it can store the information correctly for your Human-Machine Interface (HMI)/SCADA software.
The driver also needs two time values:
How frequently to read data (called the poll rate).
How long the driver reads the data from the device and updates the poll record/datablock when there has been no request for it from your HMI software (called the access time).
Suppose you want to read eight integer values from an Allen-Bradley device.
The device stores these values starting with address N7:4.
You want to read the data every 5 seconds and update the poll record/datablock for 15 seconds after requests have stopped.
Using this information, you would specify the following for your poll record/datablock:
Start address: N7:4
End address: N7:11
Data type: integer
Poll rate: 5 seconds
Access time: 15 seconds
The poll record/datablock reads the following location in the device:
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20
IIIII IIIII IIIII IIIII IIIII IIIIII IIIII IIIII