2 System description
266 MODBUS® multivariable transmitter User manual 2105216 Rev. AB | 11
Communication interface
MODBUS
®
communications
The transmitter uses an 8 bit MODBUS
®
Remote Terminal Unit (RTU) to communicate with host devices. Communication is
accomplished by using a 2-wire half-duplex RS-485 MODBUS
®
hardwire link (Figure 7).
Figure 7: RS-485 MODBUS
®
communication signal lines
IMPORTANT NOTE: For a more detailed description of MODBUS
®
protocol, see:
"Gould MODBUS
®
Protocol Reference Guide" published January 1985, by Gould Inc., Programmable Control
Division Andover, Massachusetts.
"MODBUS
®
over serial line specification and implementation guide V1.0" published by modbus.org
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
®
packet sent or received. The packet format varies with the function performed
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.
Table 7: MODBUS
®
RTU message frame format
RS-485 Bus
Cable entry
port
Assignment for RS-485 Bus
Terminal A (+) bus +
Terminal B (-) bus -
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