ICS ELECTRONICS 4809A, Instruction Manual

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To read a 32-bit value, two successive 16-bit registers are read and the
user's program then puts the values together to form the 32-bit result. An
example is reading a process variable from Registers 20 (MSW) and 21
(LSW). The 4899A can be used to read each register individually or to
read two successive registers. The commands are:
R? 20,1 'reads register 20
R? 21,1 'reads register 21
or
R? 20,2 'reads register 20 and 21
Both sequences return two numbers to the user. The MSW is returned
from Register 20, the LSW from Register 21. Multiply the MSW by 2^16
or 65536 and add it to the LSW. Divide the result by 1000 to scale it to
three decimal places.
Reading = ((MSW * 65536) + LSW)/1000
3.8.8 Floating Point Variables
Some new Modbus devices like Watlow's EZ PM series controllers use
two consecutive register to control a value or to read back a process vari-
able. The two registers hold an IEEE-754 32-bit floating point word. The
registers are read and written to in the low word-upper word order.
3.8.9 Floating Point Write
The WF command writes the num value in floating point format to two
consecutive registers starting with the low word register.
WF 2160, 75 'writes to registers 2160 and 2161
3.8.10 Floating Point Read
The RF? query reads a 32-bit floating point value from two sequential reg-
ister in low word-upper word order. The RF? does not require the number
of register to read since it is fixed at two registers.
RF? 360 'reads registers 360 and 361
3.8.11 Setting Modbus Device Timeouts