5
2 MODBUS FRAMING
2.1 Transmission Mode
The protocol uses the Modbus Remote Terminal Unit (RTU) transmission mode. In RTU mode, data is sent in 8-
bit binary characters. The 8 bit even parity or 8 bit no parity data format must be selected when configuring the
instrument communications. The data format is shown in the following table.
Table 2-1 RTU Data Format
Field
No. of bits
Start bit
1
Data bits
1
8
Parity (optional)
1
Stop bit
1
1
Least significant bit first
2.2 The RTU Frame Format
Frame synchronization is maintained in RTU transmission mode by simulating a synchronization message. The
receiving device monitors the elapsed time between receptions 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. The 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
CRC Check
T1 T2 T3
8 bits
8 bits
N * 8 bits
16 bits
2.3 Address Field
The address field contains a user assigned address (1-247) of the instrument that is to receive a message.
Address 0 is used in broadcast mode to transmit to all instruments (broadcast mode is available only for functions
06 and 16). In this case all instruments receive the message and take action on the request, but do not issue a
response. In the PM172, the broadcast mode is supported only for register addresses 287-294 and 301-302 (reset
energies and maximum demands) and 3404-3415 (reset/clear registers).
2.4 Function Field
The function field contains a function code that tells the instrument what action to perform. Function codes used in
the protocol are shown below in Table 2-2.
Table 2-2 Modbus Function Codes
Code
(decimal)
Meaning in Modbus
Action
03
Read holding registers
Read multiple registers
04
Read input registers
Read multiple registers
06
Preset single register
Write single register
16
Preset multiple registers
Write multiple registers
08
Loop-back test
Communications test
NOTE
Broadcast mode available only for functions code 06 and 16.
2.5 Data Field
The data field contains information needed by the instrument to perform a specific function, or data collected by
the instrument in response to a query.
IMPORTANT
Fields composed of two bytes are sent in the order high byte first, low byte second.
