Figure 7: Rs-485 Modbus; Communication Signal Lines; Rtu Message Frame Format - ABB 266 MODBUS User Manual

Communication interface
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MODBUS communications
The transmitter uses an 8 bit MODBUS
accomplished by using a 2-wire half-duplex RS-485 MODBUS
IMPORTANT NOTE: For a more detailed description of MODBUS
"Gould MODBUS
Division Andover, Massachusetts.
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"MODBUS over serial line specification and implementation guide V1.0" published by modbus.org
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MODBUS uses a master/slave communications concept. Slave transmitters respond only to requests made by the master.
Each slave is identified by an unsigned, one byte number ranging from 1 to 247 (inclusive). A slave sends a single response to a
master's request for data (Table 7).
— The packet field consists of the MODBUS
and the register group accessed.
— The error check field consists of a 16 bit cyclic redundancy check (CRC) calculated over the length of the packet field.
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Remote Terminal Unit (RTU) to communicate with host devices. Communication is
Assignment for RS-485 Bus
Cable entry
port
RS-485 Bus

Figure 7: RS-485 MODBUS

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Protocol Reference Guide" published January 1985, by Gould Inc., Programmable Control
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packet sent or received. The packet format varies with the function performed
Table 7: MODBUS
Packet
N x 8 bits
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hardwire link (Figure 7).
Terminal A (+) bus +
Terminal B (-) bus -
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communication signal lines

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protocol, see:
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RTU message frame format

Cyclic redundancy check
16 bits
266 MODBUS® multivariable transmitter User manual 2105216 Rev. AB | 11
2 System description