PLC communications has grown and changed over
the many years since the controller’s inception decades ago. Industrial needs
and technological advances have kept PLC communications evolving, and this post
will explore some past and present communication standards, and what the
future may hold.
The communication protocol is a set of rules
that govern data communication.
In other words, the
communication protocol acts as an agreement between two or more communicating
devices.
PLCs needs to communicate to a number of
different devices. First, there are devices which are part and parcel of the
PLC, such as remote I/O. Next, are other PLCs and control devices, such as
motor drives and servo controllers. Communication with some type of operator
interface device is also often required, as is Internet connectivity. Finally,
PLCs often communicate to server level PCs running various manufacturing
related applications.
All of these communications require two
things:
Physical connection or layer, which is the
wiring and connection components, and
Shared protocol, which is the common language
allowing each device to understand what the bits and bytes in the communication
messages mean.
Back in its infancy, PLC communications were
typically proprietary, with each supplier having their own closed connections
and protocol. This made communications relatively simple among a single
supplier’s products, but devilishly complex if you chose to venture outside the
realm.
Following are the
different types of communication protocols used in every PLC.
- Ethernet
- Profibus
- RS-232
- RS-485
- Multi-Point Interface (MPI)
- Point to Point (PPI)
- Data Highway (DH)
- Control Net
- Device Net
- USB Adapter
- PC Adapter
These are the foremost
protocols used for the PLC and other network connections. These protocols are
supported by different PLC software brands.
The communication
protocols are depended upon three fundamental parts as mentioned below.
1. Baud rate
2. Network length
3. Number of nodes
Baud rate is also
known as “Communication speed”.
The rate of data
transmission on the network is called Baud Rate.
In simple term, it is
calculated as “number of data transferred per second”.
The different types of
standard communication protocols support different speed (baud rate), distances
(network length) and the number of connecting devices (nodes).
- If you want to communicate with
the more number of nodes with more speed, Ethernet is the best choice.
- If your network has a length
more than 25 Kilo-meter, you should use Control Net protocol
