Installation instructions
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R2
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Chapter 1: Introduction
1 .2 .4 G EN ERA L I N F O RMA T I O N A B O UT MO DB US PRO T O C O L
To enable communication between computerized systems, a protocol is required to
define the rules of communication. Modbus is one of the most common protocols
used for transmitting information between electronic devices over serial lines.
Mas t er & s l av es
The device requesting the information is called the Master. The devices supplying
information are called the slaves.
In a standard Modbus network, there is one Master and up to 247 slaves, each with
a unique address from 1 to 247.
Modbu s mes s ages
Modbus is transmitted over serial cables between devices. The data is sent as series
of ones and zeroes, called bits. Each bit is sent as a voltage. Zeroes are sent as
positive voltages and ones are sent as negative voltages.
When the master requests data, the first byte it sends is the slave address. This way
the slave will know whether to ignore the message or not, right after receiving the
first byte.
Error c hec k
CRC (Cyclic Redundancy check) is 2 bytes added to the end of every Modbus
message for error detection. Every byte in the message is used to calculate the CRC.
When receiving a message, the receiving device calculates the CRC and compares it
to the CRC from the sending device. If even one bit in the message is received
incorrectly, an error will be reported.
1 .2 .5 S ERI A L C O MMUN I C A T I O N
NOTE: Using twisted or shielded cables is recommended with RS232 and RS485.
R S 2 32
An asynchronous point-to-point standard that enables a variety of ways to
communicate with PCs. The transmissions seldom exceed 100 feet/ 30 m.
R S 4 8 5
A asynchronous multi-point standard using differential signaling for applications
where it's important to reduce wiring expenses, and achieve longer line lengths.
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IM-LX-RX101-EN-v01
