C: Modbus
IntelliBox-I/O 2100 User Guide
173
The figure above shows various specific styles of Modbus operations. Traditionally,
Modbus/RTU devices fall into two groups:
Modbus slave devices:
These are generally the workhorse devices. They perform their
tasks 24 hours a day, 365 days a year. Flow metering, temperature control, batch
loading, and running entire automated assembly lines are examples of such tasks. The
slave devices are called slaves because as far as data communications is concerned,
they function as passive servers. Modbus slave devices passively sit and wait for a
remote Modbus master device to ask them to report existing data values (read) or accept
new data values (write).
Modbus master devices:
These are generally higher-level computers, devices in which
data and software are very important. The most common examples of Modbus master
devices are the “Human-Machine-Interface” (HMI) computers, which allow human
operators to monitor, adjust, and maintain the operations of field devices. Modbus master
devices are clients that actively go out and read from and/or write to remote Modbus
slave devices to monitor or adjust slave behavior. The IntelliBox does not support Master
devices connected to the serial ports, unless the Masters are being polled like a slave
device.
Examples
Modbus/TCP Master Talking to Modbus/TCP Slave
Devices A, B, D, and E are new Modbus/TCP devices, which are improved over
Modbus/RTU (see more about Modbus/RTU limitations below). All four devices can
function concurrently as both Modbus master and Modbus slave. Both computers A and
B can treat controller D as a slave, polling data in real time. Yet controller D can also act
as a master and poll data from controller E, which can in turn also act as a master to
write alarm data directly up to computers A and B to alert the operators to the alarm
condition. Traditional Modbus/RTU requires slave devices, even with severe alarm
conditions, to sit patiently and wait for a remote master to poll the specific data that
caused the alarm condition.
It is revolutionary for such a simple and flexible protocol as Modbus to offer such
functionality. Therefore, Modbus/TCP offers exciting new design options for industrial
users, which the IntelliBox-I/O extends to traditional Modbus/RTU serial devices.
Modbus/TCP Master Talking to Modbus/RTU Serial Slave
Devices C, F, and G are traditional Modbus/RTU slave devices. Device C uses a point-to-
point electrical interface like RS232. This allows only a single Modbus/RTU master to talk
to device C. However, the IntelliBox makes device C appear on the Modbus/TCP network
as a full Modbus/TCP slave device. All Modbus/TCP enabled devices, A, B, D, and E,
can actively share access to slave device C. A limitation in traditional Modbus/RTU
implementation expects devices to be dedicated as either master or slave devices, so
device C can only act as a Modbus slave.
Devices F and G are different from device C. They share a single RS485 multi-drop line
that strictly limits them to act as slaves to a single Modbus/RTU master. However, all
Modbus/TCP enabled devices A, B, D, and E can actively share access to both slave
devices F and G. IntelliBox manages and coordinates the shared access. In fact, the
IntelliBox allows up to sixteen concurrent Modbus masters (or thirty-two if an additional
TCP Server is also used) to share access to the slaves.
Summary of Contents for I/O 2100
Page 1: ...Part Number 900 474 Revision B March 2012 IntelliBox I O 2100 User Guide ...
Page 36: ...5 Getting Started IntelliBox I O 2100 User Guide 36 Figure 5 1 Lantronix DeviceInstaller ...
Page 86: ...9 Services Settings IntelliBox I O 2100 User Guide 86 Figure 9 7 HTTP Configuration Page ...
Page 95: ...10 Security Settings IntelliBox I O 2100 User Guide 95 Figure 10 1 SSH Server Host Keys Page ...
Page 123: ...12 Advanced Settings IntelliBox I O 2100 User Guide 123 Figure 12 3 Email Configuration Page ...
