11.5 Modbus
11.5.1 What is it?
Modbus is a serial communication protocol used for transmitting information over serial lines
between electronic devices. The device requesting the information is called the Modbus Master
and the devices supplying information are Modbus Slaves. In a standard Modbus network, there is
one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can
also write information to Slaves.
Modbus has become a standard communication protocol in industry, and is now the most com-
monly available means of connecting industrial electronic devices. It is often used to connect a
supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition
(SCADA) systems. Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus
ASCII) and for Ethernet (Modbus TCP).
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11.5.2 What can I do with it?
Modbus-communication with SPA-2 allows reading of measurement values and device information
by a Modbus master. Additionally, the basic RS-485 port settings can be written to the SPA-2.
11.5.3 How do I wire it?
For Modbus communication the SPA-2 is wired according to the table below.
Modbus
Connector MAIN
Connection wire
Description
Common
BUS-1 G
GND
Common
D1 - B/B
BUS-1 B
RS-485 A
D1 - B/B
D0 - A/A
BUS-1 A
RS-485 B
D0 - A/A
Table 17:
Connection to a Modbus
NOTE
If the SPA-2 is operated with multiple Modbus devices within the same
network, termination resistors may be required. Please contact Sommer
Messtechnik for details.
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http://www.simplymodbus.ca/FAQ.htm
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