Month: July 2017

In today’s world, there are many different protocols, and some are better equipped to work in different applications. Modbus and profibus each have strengths, and there is some overlap as to what each of them is capable of. Some people, however, consistently prefer to use modbus over profibus.

Modbus was originally designed in the mid 70’s and was a proprietary protocol for Modicon. The company published it royalty free before finally making it an open protocol. Many companies began using it, and because of this, there are now several variations. Even though it has variants, it is best described as simple. It’s easy to implement and easy to use. This is one of its biggest strengths. The specification document is less than 100 pages, which indicates its simplicity. Modbus usually refers to one of three protocols – Modbus ASCII, Modbus RTU, or Modbus TCP/IP.

Modbus ASCII was the first modbus, and usually runs on the RS-232 or the RS-485 physical layer. There is one master and all slaves are polled on demand by the master. The message frame can be up to 253 bytes.
Modbus RTU is a variation of Modbus ASCII. The difference is the encoding of the data. RTU uses bytes to encode messages, which increases the throughput.
Modbus TCP/IP was added much later, and can be thought of as, essentially, a way of encapsulating an RTU packet within a TCP/IP packet. It’s simple to use, but is slower than other Ethernet products. It is still adequate for monitoring applications.

As Modbus is a simple master slave protocol, and the master has full control of the communication. The slave will only respond when asked. The master will record the outputs, and read the inputs from every slave during every cycle. The slave devices don’t join the network, and they only respond when spoken to, and remain idle when they are not being spoken to. There’s no requirement for a watchdog timer, and there’s no requirement for diagnostics for the slave’s health.

Modbus usually uses the RS232 or the RS485 physical layer, but it can also use other physical layers, like phone lines, or wireless. RS232 and RS485 were established physical layers during the Modbus original development, and it didn’t add any new requirements.

In the case of using phone lines and wireless, Modbus has excelled. Phone lines and wireless introduce delays in the messages, which can be problems, but Modbus either deals with this, or it adapts to work in these applications.

Modbus can be used from a controller, or monitor to a smart device, controller, or monitor to a smart device from multiple vendors and for remote monitoring of information from a smart device.

Profibus is not only more complicated to implement and use, but it also uses specialized characteristics which tie it to RS485. RS485 is used in many different industries but it will become obsolete. Modbus, on the other hand, is always going to grow and be utilized because of its simplicity, and because it can be adjusted to the Ethernet, without overly complicated processes.

However, it is possible to utilize both protocols to work together. Modbus is the data transport between a master controller/data concentrator and has a remote profibus station. Using both protocols allows the simple implementation and easy modem support of modbus, while also gaining some positives from profibus. Profibus can be used in hazardous and multi vendor applications, and modbus provides a link between a SCADA system and a data concentrator.

This is a digital computer which is specially adapted for the control of manufacturing processes such as assembly lines, lighting fixtures, robotic devices, amusement rides or activities that require high-reliability control, process fault diagnosis and ease of programming.

Programmable Logic Controllers (PLCs) have progressively become a fundamental part of the industrial environment. It is imperative to understand their basic functionalities as a technician involved with PLCs controlled processes.

PLCs are designed for multiple arrangements of input and output, resistance to impact and vibration, electrical noise immunity and extended temperature ranges.  The programs are written on a computer and then downloaded to the PLC. The Programs to control machine operation are stored either in non-volatile memory or battery backed up RAM.  A PLC allows for output results in response to input conditions within a set time.

A brief history of the PLC

Before the PLC, sequencing and control for manufacturing were composed of drum sequences, relays, and closed-loop controllers. The process of updating these was expensive and time-consuming because it required individual rewiring of relays.

PLCs were invented to replace the rewiring of relay based control panels in the American automotive industry when there was a change in production models.

GM Hydramatic, which is General Motors automatic transmission division, issued a request in 1968 for the hard-wires relays systems to be replaced. Bedford Associates of Bedford, Massachusetts gave the winning proposal.

Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product which they named MODICON (MOdular DIgital CONtroller). One of the very first 084 models built was decommissioned after almost twenty years of nonstop service. The automotive industry remains one of the largest users of PLCs to date.

Previously, the sequencing and control for automobiles manufacturing relied on hundreds of relays, dedicated closed-loop controllers, drum sequencers, and cam timers. Updating such facilities was costly and time-consuming, as technicians needed to manually and individually rewire the relays.

When digital computers were invented, they were used in in industrial processes to control combinatorial and sequential logic due to their general-purpose programmable devices. However, these early computers required programming specialists as well as rigorous operating environmental control for cleanliness, temperature and power quality. To overcome these challenges, the PLC was developed with several key attributes. It would tolerate a harsh environment; effortlessly support discrete input and output. Also, its operations could be monitored and would not require years of training to use.

Advantages of PLCs

Reduced Cost: PLC prices range from a few hundred to a few thousand. PLCs are cost efficient because they eliminate shipping and installation costs.

Flexibility: A single PLC can effortlessly run many machines.

Ease of Troubleshooting: With PLC control, any changes in sequence or circuit design are as simple as retyping the logic. PLCs allow for a fast and cost effective way of correcting errors. Moreover, a PLC program can be tested after it is developed before it is implemented in the field.

Space Efficient: A PLC requires fewer components than conventional hardware systems making them space efficient.

Visual observation: PLCs have a visual display on a screen which makes troubleshooting simple and fast.

Components of a PLC

All PLCs have the similar basic components. These include controller relay units for input and output, timers, CPU, a peripheral device, communication processor (CP), interface module (IM) and power supply. These components work jointly to carry information into the PLC, evaluate that information after which the information is sent back out to various fields.