9.9
Date Code 20120126 Instruction Manual SEL-2032 Communications Processor
Protocols
Modbus RTU Protocol
Figure 9.3 SEL-2032 Virtual Modbus Network
In addition to direct database access, the SEL-2032 provides access to data
from individual ports with SEL relays mapped as input (3X) registers. If you
have configured the SEL-2032 to collect data from an SEL relay, the relay
data are automatically mapped into registers providing a virtual environment
of Modbus slaves corresponding to the connected SEL relays.
Modbus Basics
Modbus is a protocol developed to allow master devices to collect data from
Modicon PLCs. Modbus is much less complex than protocols like DNP3 and
UCA2, but does not contain features like time-stamped data or file transfer. To
overcome the lack of features, some manufacturers have designed and
implemented proprietary extensions to Modbus that are only compatible with
other devices from that manufacturer. The SEL-2032 Modbus implementation
avoids proprietary extensions and therefore provides several basic functions
for reading data, writing data, and diagnostics. You can get more information
on the Modbus protocol at the Modicon web site www.modicon.com.
The key to implementing Modbus in a device that is not a Modicon PLC is to
map data within the device to data types available in a PLC and provide
mapping information to allow Modbus masters to manipulate the data and
perform control operations. The basic Modbus data types are shown in
Table 9.2.
SEL-2032
Port 1 Modbus Slave
D1
Region
Port 2 Modbus Slave
D1
Region
. . .
Port 18 Modbus Slave
D1
Region
Port 12
Table 9.2 Modbus Data Types (Sheet 1 of 2)
Modbus
Data Type
Type Examples
Modicon PLC
Data Type
SEL Data Type
Coil 0X 00001,
0099, 01023
Internal
binary storage,
binary outputs
Remote bits, breaker bits,
bits within database registers
Input 1X 10001,
1099, 11023
Discrete inputs Remote bits, breaker bits,
bits within database registers
Courtesy of NationalSwitchgear.com
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