Section 5: Modbus Definitions; Introduction; Modbus Register Data Types; Float_Ieee_754 - Siemens 7SR45 Argus User Manual

Section 5: Modbus Definitions

5.1

INTRODUCTION

This section describes the MODBUS-RTU protocol implementation in the relays. This protocol is used
for communication with a suitable control system.
This protocol can be set to use any or all of the relays hardware interfaces (USB, RS485) where fitted.
The relay can communicate simultaneously on all ports regardless of protocol used.
The Station Address of the port being used must be set to a suitable address within the range 1 - 247
to enable communication. This can be set by the Communications Menu: xxxxx Stn Addr setting.
Definitions with shaded area are not available on all relay models.
5.2

MODBUS Register Data Types

5.2.1

FLOAT_IEEE_754

The float data type conforms to the IEEE 754 floating point definition. This specifies that 32
bits of data will be formatted as a sign bit in the most significant bit (MSB) followed by an 8 bit
exponent then a 23 bit mantissa, down to the least significant bit (LSB).
MSB
Sign Exponent
5.2.2

FP_32BITS_3DP

The FP_32BITS_3DP is a 32 bit integer fixed point value, containing 3 decimal places of
information. It is used to send a real value to 3 decimal places as an integer. For example, if
the value in a device is 123.456 it will be sent as 123456. As it is an integer, negative numbers
are sent as 2's complement.
FP_32BITS_3DP & MODBUS
In this MODBUS implementation the 32 bit value is stored in 2 16 registers in Big-Endian
format. As an example, if we take the hex representation of 123456, we have 1E240h.
Assume this is stored in the registers 30001 and 30002, it would look as follows:
Address
30001
30002
On reception these two registers should be interpreted in the correct order as a 32 bit integer.
5.2.3

UINT32

The UINT32 is a signed 32 bit integer. As it is an integer, negative numbers are sent as 2's
complement.
UINT32 & MODBUS
In this MODBUS implementation the 32 bit value is stored in 2 16 bit registers in Big-Endian
format. As an example, if we take the hex representation of -123456, in 2's complement, we
have FFFE1DC0h. Assume this is stored in the registers 30001 and 30002, it would look as
follows:
Address
30001
30002
On reception these two registers should be interpreted in the correct order as a 32 bit integer.
© 2018 Siemens Protection Devices Limited
Value
1
E240
Value
FFFE
1DC0
Chapter 4 - 7SR45 Data Communications Definitions
Mantissa
LSB
Chapter 4 Page 12 of 20