Communication Protocols in Process Controllers

The Technologic Range of Process Controllers

In a previous article, The Role of Communications in Process Controllers, we looked at why communications facilities are increasingly important for a modern process controller. Here we provide an overview of the comms facilities provided in the Tecnologic range of PLCs, noting what each protocol is best suited to and which products from our range support them.

The main protocols available are: RS232/485, Mobile network (GSM/GPRS/SMS), Ethernet, and VNC.

Most models in our PLC range are equipped with at least one of these options, often as an option so that manufacturers can easily choose their preferred comms method (to integrate with existing products, for instance).


Process Controller Protocols and suitablities

RS485 / RS232

RS232 and RS485 allow for signalling over short distances, up to 1200m, depending on bit rate. This makes it ideal for connecting together devices in serial within a fairly small area, such as a laboratory or factory floor. It is not designed for non-linear connectivity such as a star or ring configuration.

RS485 is available on many of our controllers and PLCs, including:

  • KR, KM and KX PID controllers
  • K85 DIN rail PID controller
  • TLK33 Peltier controller (optional)
  • Jazz PLC (optional)
  • Vision 350, 430, 700, 1210 and Samba series.



SMS (Short Message Service) allows text messages, incorporating one or more data values, to be sent via the mobile network and received either on a specified mobile device or sent to an email address. It is ideal for communicating meaningful alarms and system status updates to people who may or may not be within the vicinity of the originating PLC. It also allows messages to be easily sent to more than one destination at once (e.g. operators, supervisors, support team, or manufacturer for monitoring or maintenance).

As well as sending SMS messages for alarm, status or data logging purposes, SMS can also be used to send instructions to the PLC, for example to start or end a process.

SMS is available either as standard or as an option on many of our PLC’s including:

  • Jazz Series PLC
  • The entire Vision range, including Samba.

On Vision models, use of SMS is programmable via standard VisiLogic software as part of programming the overall process.


GSM Modem / GPRS

GSM (Global System for Mobile) communication enables data to be transmitted via the mobile network. It is the same system that is used for sending SMS, but using the general GSM protocol allows much larger amounts of data to be transmitted than can be accommodated in an SMS message. GPRS (General Packet Radio Service) is a packet-switched version of the GSM protocol.

This type of communication method enables a process controller to be controlled from a remote PC, or to send data to a remote PC. As well as allowing remote diagnostics, support, monitoring and logging, this facility means that it is possible to integrate multiple globally distributed sites belonging to the same company (for instance, integrating production processes with warehousing and order processing).

GSM/GPRS communication capability is available on the majority of PLCs in our Vision range. It can be programmed within VisiLogic directly, using a special GPRS function block – so no additional software is required to implement communications aspects.



Ethernet allows the use of either the TCP/IP or Modbus protocols to enable communication between PLCs, or between a PLC and a remote PC.

It is available on the Vision range and on the UniStream range of PLCs.

In terms of process control, it offers manufacturers a great deal of flexibility in setting up complex processes involving multiple PLCs. For instance, a master controller can be programmed to control a number of other smaller PLCs. On the UniStream range, which have 2 Ethernet ports as standard, it is possible to “daisychain” several PLC’s together, each controlling a different stage of a complex process.



CANbus is a communication/wiring facility native to Unitronics PLC’s enabling communication between multiple process controllers and remote I/O devices. Data is transmitted over CANbus via either CANbus ISC (up to 8 PLC’s) or UniCAN (up to 63 PLC’s) protocols, both native and specific to Unitronics devices.

However, communication via CANbus is not limited to local Unitronics PLCs. There are a range of other communications options available including CANopen which allows communication with remote devices, and communication to a remote PC via RS232 for software downloads, data logging and other purposes.

In short, CANbus provides a fully featured comms facility which enables all likely communications requirements for PLC to PLC communication as well as remote communication.

One of the key advantages of using CANbus is that it does not require or assume that one of the PLC’s is the master controller. Any PLC within the network can send a message to any other, enabling a wide variety of configurations compared to other comms methods.

CANbus is available on all Vision PLCs (using UniCAN) and is fully supported within the VisiLogic software.



VNC (Virtual Network Computing) allows a screen to be viewed and controlled remotely, and is available on the UniStream product range. It allows a UniStream controller to be controlled by any remote device such as a remote PC, tablet or smartphone, greatly enhancing the ability of manufacturers to offer systems which integrate fully into a customer’s existing working practices and available technology.


Not sure which comms method to use for your application? Or which process controller to choose to best support your preferred communications protocol? Talk to our team of experts today who can talk through your requirements and help you make the right choice.


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