10
Instruction Leaflet
IL012002EN
Effective March 2013
Instruction Leaflet for Modbus Power Monitoring/
Metering Module (PM3) for FD, JG, and KD/LG
Circuit Breakers and Motor Circuit Protectors
EATON CORPORATION
www.eaton.com
3. Modbus RTU Communications
Modbus is a popular application-layer messaging protocol. The
protocol takes place at the data link layer of the Open Systems
Interconnection (OSI) model. Modbus serial line (Modbus RTU) is a
Master-Slave protocol. A Modbus transaction consists of two mes-
sages: a request from the master, and a reply from the slave.
The Modbus Add-on Module is a Modbus slave. The Modbus physi-
cal layer transmitter and receiver conforms to the RS485 standard.
A Modbus serial line message frame, or Protocol Data Unit, con-
tains:
•
An Address Field;
•
A Function Code;
•
Data; and
•
CRC.
Valid addresses are 1 – 247 decimal.
The function code indicates what kind of action to perform.
Examples are:
•
01 – Read Coils;
•
04 – Read Input Registers;
•
06 – Write Single Register;
Messages consist of 11-bit characters. Characters consist of:
•
1 Start Bit;
•
8 Data Bits (least significant bit first);
•
1 Parity Bit (even parity is the default; other modes may be sup-
ported); or
•
1 Stop Bit (2 stop bits if no parity is used).
The least significant bit is transmitted first (see Table 4).
•
Modbus Slave, line terminator selectable by dip switch
•
Baud rate (9600, 19.2k) selectable by dip switch
•
Parity (even, odd, none) selectable by dip switches
3.1. Modbus RS485 Network
The following simplified rules apply to a given system consisting of
a cable link between master and slave devices. For more complex
configurations please refer to standard Modbus RTU wiring specifi-
cation rules for the RS485 network
The recommended Modbus cable has twisted-pair wires (24 AWG
stranded 7x32 conductors with PVC insulation) having an aluminum/
mylar foil shield with drain wire.
•
The maximum system capacity is 4,000 feet of communications
cable and 247 devices on the Modbus RTU network.
•
Make sure that there is twisted-pair wire that is recommended for
Modbus RTU network use. Use shielded twisted-pair wire to con-
nect each slave to the Modbus RTU network, daisy-chain style.
The polarity of the twisted pair is critically important.
3.2. Modbus Address Switches
Two rotary hexadecimal switches are used to set the Modbus Slave
Address for the unit. Valid addresses are from 0x01 to 0xF7 (1 to
247).
3.3. Exception Codes
Under certain circumstances, the PM3 will return an exception code.
If the function in the query is not supported by the PM3, exception
code 01 is returned in the response.
If the data (object) register is illegal, exception code 02 is returned in
the response.
If the data value in the query is illegal, exception code 03 is returned.
In certain circumstances, an exception code 05 (ACK) is returned.
If the PM3 cannot perform the requested function, exception code
07 (NAK) is returned.
If only a partial register is used in the query, exception code 84 is
returned.
