PQube User Manual 2.1
Page 71 of 113
Appendix 1: Modbus
A brief introduction to Modbus
Modbus is an industry-standard protocol you can use to transfer meter readings from your PQube
to a Modbus client program that runs on your computer.
Your PQube, like all Modbus-enabled meters, has a list of Modbus “registers”. Each register holds
16 bits of data. Often, the registers are used in pairs to hold a 32-bit floating point number; but
sometimes, a single register holds a 16-bit integer. It depends on the type of data that makes
sense for each particular meter – floating point numbers include fractions and very large values,
but take up twice as much space, while integers are whole numbers and use half the space. For
each PQube meter, you can find the type of data, “Float”, “Integer”, “Long Long”, etc. in the
PQube Modbus Registers table below.
The registers are numbered. By Modbus convention, you pick a “base address” for your registers –
7000 is the default base address for your PQube. Then each register, or pair of registers, has a
“register offset” that tells you how far from the base address it is.
For example, the PQube’s frequency meter is a 32-bit floating point number. Looking in the
PQube Modbus Registers table, you see that frequency is in Modbus registers 26 and 27.
(Remember that the registers are 16-bits, so two registers are required for a 32-bit number.) If you
need the actual physical address of the registers, you must add the base address – the physical
address of these registers is 7026 and 7027. But most Modbus client programs do this for you.
Also, most Modbus client programs will allow you to refer to this as a floating-point meter located
at register 7026.
Another example: the PQube’s meter reading for the month on its internal clock-calendar is an
integer (1,2,3,4,5,6,7,8,9,10,11, or 12). Looking in the PQube Modbus Registers table below, you
can find out that the register address is 135.
PQube® Modbus registers
The Modbus registers in the PQube, listed below, are offsets from a base address. By default, the
base address is 7000. If you want, you can change the base address my modifying your PQube’s
SETUP.INI text file.
The Modbus registers in your PQube are in actual units, including all multipliers for PT’s, CT’s, etc.
Most other meters require you to multiply by scaling factors, but your PQube takes care of this
itself.
For example, 480.0V is presented as an IEEE floating point number: 480.000 . 24kV is presented as
an IEEE floating point number: 24000.000 . 59.6 kWh is presented as an IEEE floating point
number: 59600.00 . 213.567 MWh is presented as an IEEE floating point number: 213567000.000 .
If no data is available, your PQube uses a float value of NaN (not a number) to indicate “no data”,
and your PQube uses an integer value of 32,767 (7FFF hex) to indicate “no data”.
NOTE: Under normal conditions, your PQube updates its registers approximately once per second.
However, if your PQube needs to generate files for events and trends, or send an email, then you
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