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4. In this case, we are accessing 32 words of global data, so set the bottom
portion of the MSTR instruction (“number of registers”) to 32. In the case
of MSTR global reads, this number cannot exceed 32 by definition.
5. Initiate the global read command by closing internal relay 1. The data
response will be seen in the MSTR Data Area (starting at register 40300).
16.6 MSTR Global Write Example
The following example demonstrates how to use the MSTR instruction to write
32 words of global data from a Modbus Plus master located at network address
1. Once written, any node on the Modbus Plus network will have access to this
global data. The same global data values will continue to be available until
another global data write occurs. Global data can only be cleared by
performing a global data write with a length of 0 words.
1. Set up a ladder logic program as shown in Figure 37.
2. Set the top portion of the MSTR Instruction to 40080. This will be the
starting address of the MSTR Control Block. Set the control block registers
to the following values:
Address Description Register Value
40080 MSTR function (global write) 5
40081 Error code Read Only
40082 Number of consecutive registers to write 32
3. Set the middle portion of the MSTR instruction to 40400. This will be the
starting address of the MSTR Data Area. This is the location of the data
that will be written to the master’s network global data.
Address Value
40400 Value to write to offset 0 in the master’s global data
40401 Value to write to offset 1 in the master’s global data
: :
40400+n Value to write to offset n in the master’s global data
: :
40431 Value to write to offset 31 in the master’s global data
4. In this case, we are writing 32 words of global data, so set the bottom
portion of the MSTR instruction (“number of registers”) to 32. In the case
of MSTR global writes, this number cannot exceed 32 by definition.
5. Initiate the global write command by closing internal relay 1. The data at
registers 40400-40431 will be written to the master’s (station address 1)
global data.