These industrial communication protocols are used in the
automation industry to connect different devices and systems.
ModBus
What is MODBUS?
Modbus is a serial communications protocol originally
published by Modicon (now Schneider Electric) in 1979 for use with its
programmable logic controllers (PLCs). Modbus has become a de facto standard
communication protocol and is now a commonly available means of connecting
industrial electronic devices.
What is it used for?
Modbus is typically used to transmit signals from
instrumentation and control devices back to a main controller or data gathering
system.
Modbus is typically used to transmit signals from
instrumentation and control devices back to a main controller or data gathering
system.
For example, a system that measures temperature and
humidity and communicates the results to a computer.
Modbus 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).
How
does it work?
Modbus is transmitted over serial lines between
devices. The simplest setup would be a single serial cable connecting the serial
ports on two devices, a Master and a Slave.
The data is sent as a series of ones, and zeroes called bits. Each bit is sent as a voltage. Zeroes are sent as positive voltages and ones as negative. The bits are sent very quickly. A typical transmission speed is 9600 (bits per second).
Communication between ModBus device:
MODBUS devices communicate using a master-slave
technique in which only one device (the master) can initiate transactions
(called queries).
The other devices (slaves) respond by supplying the
requested data to the master.
A slave is any peripheral device (I/O transducer,
valve, network or other measuring device), which processes information and
sends its output to the master.
Masters can address individual slaves, or can initiate
a broadcast message to all slaves.
ModBus frames:
The messages exchanged between the master and the slave
are called frames.
There are two types of Modbus frames: Protocol Data
Unit (PDU) and Application Data Unit (ADU).
The PDU frames: function Code+ data.
The ADU frames: Add+FC+data+Error check.
The FC -> action to perform and the data ->
information to be used for this action.
Profibus
What is PROFIBUS?
PROFIBUS (Process Field Bus) is a fieldbus which is being
used for high speed cyclic data communication in the world of automation.
PROFIBUS has two different applications:
For each application PROFIBUS can use a different
protocol. PROFIBUS DP (Decentralized Peripherals) is the
high-speed version, which is mostly used for factory automation (i.e.
logistics, production areas, etc). PROFIBUS PA (Process
Automation) is mostly used in industries such as water treatment, oil, gas,
chemicals, etc.
PROFIBUS PA runs
at a fixed transmission speed of 31.25 kbps, where
PROFIBUS DP can be configured to run at a maximum speed of 12 Mbps.
PROFIBUS
communication is half duplex, which means that only one device is
communicating at the time. PROFIBUS DP is based on the RS485 protocol
and PROFIBUS PA is based on the MBP-IS protocol, which is a
bus-powered protocol.
PROFIBUS
DP networks are most commonly installed as a large daisy chain,
creating a fault sensitive network. PROFIBUS PA can either be wired as a daisy
chain or as a star topology, but by using intelligent/advanced junction boxes,
PROFIBUS PA can also be installed as a ring topology.
Profinet
What is PROFINET?
PROFINET is a fieldbus system which is based on Ethernet.
PROFINET is one of the many industrial ethernet systems out there, which is
often compared to PROFIBUS.
PROFINET is a full duplex system.
PROFINET networks are often designed as a star topology, or as several stars
branching off a large ring backbone by using managed PROFINET switches. Because
it’s ethernet based, PROFINET can operate on a much higher transmission
speed than PROFIBUS.
With PROFINET, the cycle times are
also adjustable for each device, whereas PROFIBUS has a fixed cycle time.
PROFINET is applied in a wide variety of industries, such as automotive, oil
and gas, logistics, etc.
EtherCAT
What is EtherCAT?
EtherCAT uses the same physical and data link layers as
Ethernet.
With EtherCAT, the master sends data, possibly only a single frame for
the entire node network that will pass through each node.
The master is the boss and is the only node allowed to send a frame. The
frame is then forwarded, downstream, by each node in the network. This approach helps to assure real-time operation and avoids
delays.
EtherCAT networks do not require external switches in
the network.
Each
EtherCAT device typically has two Ethernet ports, the first port being the
receiving port or previous node’s cable and the second port connected to the
next node in the network.
Another advantage of EtherCAT comes in the form of network topology. Many of the different network topologies may be used within an EtherCAT
network. Because it utilizes Ethernets full duplex layers, the EtherCAT slave
will automatically return the frame, to the master, with an open port detected
downstream, essentially, self-terminating.
EtherCAT’s Network Topology: Star,
Tree, Line, Bus.
Ethernet
What is the function of
Ethernet in PLC?
Ethernet enables devices to communicate with each
other via a protocol, which is a set of rules or common network language.
Ethernet describes how network devices format and transmit data so other
devices on the same LAN or campus network can recognize, receive and process
the information.
Ethernet is a wired
system that started with using coaxial cable and has successfully progressed to
now using twisted pair copper wiring and fiber optic wiring.
