GE Industrial Solutions POWER LEADER ModBus Monitor - Manual (Page 57)

POWER LEADER Modbus Monitor
Appendix A – Modbus RTU Protocol
51
Register Groupings
Device registers are divided into the following
categories. All registers are defined as read only except
for coils.
Œ
Fixed-Value registers: These contain information that is
very unlikely to change, such as rating plug value, serial
number, and factory configuration options. These registers
have addresses starting at 0000. Use the Read Holding
Registers (Function Code 03) command to read this data.
Œ
Actual value registers: These hold dynamic information
such as metering values and have addresses starting at
1000. Use the Read Input Registers (Function Code 04)
command to read this data.
Œ
Event registers: These store event-specific data for
abnormal behavior such as a circuit breaker trip and have
addresses starting at 2000. Use the Read Input Registers
(Function Code 04) command to read this data.
Œ
Setpoint registers: These store configuration information
or programmed parameters that are user programmable
and have addresses starting at 3000. Use the Read Holding
Registers (Function Code 03) command to read this data.
Œ
Coils: These read/write registers are used to implement
supervisory control through remote commands and are
addressed sequentially. Use the Read Coil Status (Function
Code 01) and Force Single Coil (Function Code 05)
commands to read this data and perform remote control
respectively.
Data Conventions
All registers are composed of 16 bits (2 bytes) per the
Modbus RTU protocol standard. All integer values less
than 256 are stored in the lower byte of the register.
All data is transmitted with the high byte first except for
the CRC-16 which is transmitted low byte first.
Format for Floating Point and Long Integer
Data Types
Registers in the Modbus Protocol are 16-bit quantities.
Since long integers and floating point data formats are
32-bit quantities, it is necessary to store these quantities
in 2 registers. The following information describes the
register, byte and bit ordering of floating point and long
integer formats transmitted by the GE Modbus Monitor.
Floating Point Number Format
Floating point values, in the GE Modbus Monitor, follow
the IEEE Single Precision Floating Point Standard. This
standard is readily available in computer communication
text books. The following summarizes the IEEE format.
IEEE Floating Point Number - Single Precision (32 bits)
bit
31 bits
30...................23
bits
22....................................0
Sign Exponent Fraction
The order of the bytes of floating point numbers
transmitted by the Monitor via Modbus protocol is
rearranged (register x is transmitted before register x +
1). Floating point values will be transmitted as 1st byte,
0th byte, 3rd byte and 2nd byte from the Modbus
Monitor. This format is compatible with standard DDE
servers.
Modbus Monitor Transmission of Floating Point Value
in Modbus Protocol
bits
15...............8
bits
7..............0
bits
31..............24
bits
23..............16
1st byte 0th byte 3rd byte 2nd byte
Register x Register x + 1
Long Integers
Long Integers are transmitted in MSB/LSB format, as
shown below.
Modbus Monitor Transmission of Floating Point Value
in Modbus Protocol
bits
31..............24
bits
23..............16
bits
15...............8
bits
7..............0
3rd byte 2nd byte 1st byte 0th byte
Register x Register x + 1