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1.Eastron SDM530-MT Smart Meter Modbus Protocol Implementation V1.1
1.1 Modbus Protocol Overview
This section provides basic information for interfacing the Eastron Smart meter to a Modbus
Protocol network. If background information or more details of the Eastron implementation is
required please refer to section 2 and 3 of this document.
Eastron offers the option of an RS485 communication facility for direct connection to SCADA
or other communications systems using the Modbus Protocol RTU salve protocol. The
Modbus Protocol establishes the format for the master’s query by placing into it the device
address, a function code defining the requested action, any data to be sent, and an error
checking field. The slave’s response message is also constructed using Modbus Protocol. It
contains fields confirming the action taken, any data to be returned, and an error-checking field.
If an error occurs in receipt of the message, SDM530-MT will make no response. If the
SDM530-MT is unable to perform the requested action, it will construct an error message and
send it as the response.
The electrical interface is 2-wire RS485, via 2 screw terminals. Connection should be made
using twisted pair screened cable (Typically 22 gauge Belden 8761 or equivalent). All "A" and
"B" connections are daisy chained together. Line topology may or may not require terminating
loads depending on the type and length of cable used. Loop (ring) topology does not require
any termination load. The impedance of the termination load should match the impedance of
the cable and be at both ends of the line. The cable should be terminated at each end with a
120 ohm (0.25 Watt min.) resistor. A total maximum length of 3900 feet (1200 meters) is
allowed for the RS485 network. A maximum of 32 electrical nodes can be connected, including
the controller. The address of each Eastron can be set to any value between 1 and 247.
Broadcast mode (address 0) is supported.
The format for each byte in RTU mode is:
Coding System: 8-bit per byte
Data Format:
4 bytes (2 registers) per parameter.
Floating point format ( to IEEE 754)
Most significant register first (Default). The default may be changed if
required -See Holding Register "Register Order" parameter.
Error Check Field: 2 byte Cyclical Redundancy Check (CRC)
Framing:
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity (or no parity)
1 stop bit if parity is used; 1 or 2 bits if no parity
Data Coding
All data values in the SDM530-MT smart meter are transferred as 32 bit IEEE754 floating point
numbers, (input and output) therefore each SDM530-MT meter value is transferred using two
Modbus Protocol registers. All register read requests and data write requests must specify an
even number of registers. Attempts to read/write an odd number of registers prompt the
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