PLC Communications

Communication systems in PLC

As with many functions within a PLC, we can find communication systems built directly into the central processing unit (CPU) or install additional modules to expand the system and increase its flexibility.

Programming Interface

In some PLCs, it is necessary to immediately distinguish between programming and networking. Sometimes this is achieved through the same interface port, so there's really no difference. However, a dedicated programming port usually appears in one of two ways.

A USB port (usually Type B, mini, micro, or Type C) is a point-to-point connection used exclusively in a PLC to load and upload programs to an IDE (Integrated Drive Electronics) host computer. Although extremely simple and reliable, it requires the computer to be physically located near the control cabinet if you ever need to edit or monitor the PLC.

An alternative, and often on slightly older hardware, is the 9-pin DB9 serial interface. We cannot immediately say that it is responsible for the programming process, since this port also carries a common RS-232 interface for communication systems such as Modbus. However, since it is also a point-to-point interface, it is not found in large networks. A variation of the DB9 connector is the 6-pin RJ11 connector, which is a narrow version of the CAT5/6 connector.

Built-in network interface

“Embedded” is the term we use when something is built directly into the computing unit of the device, in this case the PLC CPU. Most CPU modules have at least one network interface, although some have several. Sometimes it even seems that the designers were competing to see who could squeeze more network ports onto the front surface of the module.

These built-in ports typically include one or more of the following options: Ethernet, PROFINET, RS-485, and RS-232. Ethernet and PROFINET look like an RJ45 connector, common to all known Ethernet networks. RS-232, as noted earlier, can appear as a DB9 connector, regular screw terminals, or can be mounted directly into a simpler 4-wire harness shared with RS-485.

Of these options, RS-232 is the only one designed for a single point-to-point connection. This limits its functionality in network technologies, but it is often found among legacy equipment, so it remains.

The other three types of networks are point-to-multipoint, meaning they can communicate with many devices, each with a unique address. RS-485 is a slightly older standard and is now widely used for communication via Modbus, PROFIBUS and other proprietary networks such as Data Highway (DH485) from Allen-Bradley.

Industrial Ethernet and PROFINET are variations of the original Ethernet standard. Most large companies tend to choose one of the two protocols and stick with it. However, some PLCs allow the controller to be run as a scanner or host with any protocol, sometimes even at the same time, since the physical port is the same. If your automation process relies on many brands and generations of equipment interacting with the same controller, you are probably better off selecting appropriate modules that can be dedicated to each task.

Modular Network Interface Cards

PLC manufacturers use two different strategies. Either all communications are integrated into the CPU module, or the CPU remains very simple and we use additional cards to support network interfaces. The second strategy is usually used when working with a large system with more than 2-3 network points.

You can purchase modules compatible with the previously mentioned systems, including Ethernet, PROFINET, RS-232 and RS-485, but there are many others. Here are a few examples, although there are many more:

Fiber Optic

Typically this is an Ethernet module with a physical fiber network. You won't find many processors with built-in fiber optics.

EtherCAT

Created by Beckhoff Automation, this protocol is a faster version of Ethernet specifically designed for control automation technologies (hence the name CAT). Some controllers include it as a built-in protocol.

DeviceNet

This CAN-based network was founded by Allen-Bradley but has now been made open-source for wider integration. It uses flat or round stranded cable.

HART

The Bus Addressable Remote Transmitter is a digital protocol operating on a 4-20 mA analog signal, allowing HART devices to be installed into an existing analog system. However, HART requires a special module; A simple analog output module cannot transmit the HART (Highway Addressable Remote Transducer) protocol.

IO-Link

The increasingly popular protocol allows data and configuration to be transferred over a common M12 cable, and simple discrete devices can exist alongside IO-Link smart devices, allowing greater flexibility at the field level.

Many other protocols can be found in modules and built into the CPU for specialized applications.

Installation and configuration of the module

The process is largely the same for all PLC models. Modular cards are inserted into the chassis or connected using a wiring harness. The IDE always contains a hardware manager that checks the configuration of all modules and embedded networks. They can be scanned or manually added offline.

Most networks require an address or host number, which is easy to set up. Once booted, the network card starts up and can scan its own network for new devices or add these devices manually. The exact steps vary depending on the IDE, but it is a very consistent process.

Troubleshooting PLC Communication Systems

Most technicians don't like troubleshooting network problems because it is usually much more complex than for I/O signals. High-speed data cannot be tracked using simple handheld tools, and applications are much more software dependent than, say, discrete on/off updates. But as controllers become more capable of handling a variety of tasks, they must be flexible enough to interface with more hardware, and that means network protocols.

Adding advanced communication modules to the communication system of programmable logic controllers is an important building block of modern PLCs. To some extent, modules, interfaces and communication systems that provide network connectivity are helping to transform traditional PLCs into automation controllers (PACs).