Use the fields on the Modbus tab to define the Modbus-specific properties for a digital point. For information on those fields that are common to many simple drivers, see Configure Common Point Properties in the ClearSCADA Guide to Simple Drivers. Fields that are specific to the Simple Modbus driver are described below.
- Word Offset—Specify the location of the point data within the scanner’s register range. The lowest acceptable offset is zero.
- Bit Offset—Enter the lowest (starting) bit of the point. With a multi-bit point, you need to have the bits consecutive (for example, a 2-bit digital might use bits 5 and 6, where bit 5 would be the starting bit). The bits of a multi-bit point have to reside consecutively in the same Word in a register range—bits for one point cannot continue into another Word. The range is 0-15.
Example:
A scanner scans several analog and digital points on a Modbus PLC.
The scanner has this configuration:
Data Address: 10
Data Length: 12
Location: Holding RegistersOne of the digital points has this configuration:
Number of Bits:2
Word Offset: 5
Bit Offset: 4The configuration determines that the scanner scans registers 10 to 22 within the PLC. Within this block of registers, the digital point given above is located at register 14, starting at the fourth bit within that register, and continuing to the fifth bit. (Each register is one 16-bit Word.)
In order to read the value of this point, the scanner extracts bits 4 and 5 of register 14.
- Persistence—Specify the time that the driver waits before it reports a change of state. Enter the time in seconds.