How to get data from Rayleigh Instruments RI-78-80-C kWh power meters
Modbus RTU Interface
The Rayleigh Instruments RI-78-80-C has a single 48bit Modbus RTU word available to be read (kWh)
It does not respond with an exception when an incorrect register is requested (or a single 16bit part of the 48bit word requested)
Only half of each 16bit registers within the 48bit word is used
Thus Register address 0, 1 and 2 can have a value 0-255 (00 00 to 00 FF) only
Reg2 0 to 255 (255 = 255/100 kWh)
Reg1 0 to 255 (255 = 255 x 256/100 kWh)
Reg0 0 to 255 (255 = 255 x 256 x 256/100 kWh)
So if Reg0 = 3, Reg1 =167 and Reg2 = 59
Then the reading would be(3 x 256 x 256 / 100) + (167 x 256 / 100) + (59/100)
= 2394.19 kWh
To read the three registers, Modbus Register Groups must be used within SmartStruxure, this ensure a single read is done for the 3 registers comprising the 48bit value.
Then a program can be used to calculate the actual meter value.
This device will not appear as online in SmartStruxure as the Device Online mechanism requires a response from a 16bit register read
(this device does not respond with an exception when an incorrect register is requested and has no 16bit registers to read)
Click here for a SmartStruxure example of Modbus Objects and Script.
The below example script could also be atapted for Continuum Plain English Program
Numeric Input ModReg0, ModReg1, ModReg2
Numeric Output kWReading
kWReading = ((256*256*ModReg0) + (256*ModReg1) + ModReg2)/100