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2 MODBUS PROTOCOL
2.1 Introduction
This protocol provides data communication between a PC and multiple Powermeters and is a
subset of Modicon's MODBUS protocol. The protocol provides for one master and up to 247
Powermeter slaves on a common line. Although the protocol supports up to 247 slaves, certain
device restrictions may limit the number of slaves to less than 247*.
The protocol works in a master-slave mode. Only the master can initiate a transaction.
Transactions are either a query / response type, when a single Powermeter is addressed, or a
broadcast / no response type, where all the Powermeters are addressed. A transaction includes
a single query and a single response frame or a single broadcast frame. Powermeters
communicate in half duplex mode : Powermeter process only one query and do not receive a new
query while the pervious response is transmitted.
2.2 Transmission mode
The transmission mode is Remote Terminal Unit (RTU). In the RTU mode, data is sent in 8-bit
binary characters. The 8 bit even parity or 8 bit no parity should be set in the Powermeter. The
RTU transmission mode is defined in Table 6-4.
Table 2-1 RTU Transmission Mode
Field No. of bits
Start bit 1
Data bits c 8
Parity (optional) 1
Stop bit 1
c Least significant bit first
2.3 Framing
Frame synchronization is maintained in RTU transmission mode by simulating a synchronization
message. The receiving device monitors the elapsed time between reception of characters. If
three and one-half character times elapse without a new character or completion of the frame,
then the device flushes the frame and assumes that the next byte received will be an address.
Frame format is defined below.
The maximum query and response message length is 256 bytes including check characters.
RTU Message Frame Format
T1 T2 T3 Address Function Data Check T1 T2 T3
8 bits 8 bits N * 8 bits 16 bits
2.3.1 Address Field
The address field contains a user assigned address (1-247) of the Powermeter that is to receive
message. Address 0 is used in broadcast mode to transmit to all Powermeters (broadcast mode
is available only for functions 06 and 16). In this case all Powermeters receive the message and
take action on the request but do not issue a response.
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