When designing controllers and monitoring systems for use in factories and other industrial locations or civil engineering projects there is often a need to communicate with the outside world in some manner. Even if the equipment normally operates autonomously it invariably needs some interface for set up and diagnostic functions.
So the question is. What is the most popular communication interface for designers of industrial electronic equipment?
Increasingly bulk communications are handled by Ethernet as more industrial locations are equipped with the cables and fibres but there are always going to be situations where ethernet is not practical. USB is suitable for short distances and compatible with lap top computers but will not work with distances over a couple of feet.
Step in RS485 . As the rugged brother of RS232, RS485 is simple, inexpensive to implement can handle a wide variety of protocols and can cover long distances. It is good for collecting data and remote control providing the data rates are not too large. It can plug into USB virtual serial ports on laptops via USB powered converters such as the USB485i
Using RS485 to link multiple devices in the field can be accomplished with multi-drop networks
All the embedded microcontroller systems and processor cards used in industrial designs incorporate standard serial ports so all the designer has to do is add an RS485 transceiver chip.
There are a few important things to consider when including an RS485 port in your design. Perhaps the two most important thing are the electrical environment in which the system is to operate and the distance the signals have to travel.
RS485 networks can be up to 1Km long so if the total network distance is over 1Km the network has to be separated into shorter lengths linked by repeaters such as the R485i. Longer distances are often linked to other problems. If the nodes are far apart they could also be susceptible to damage by electrical storms.
The designer needs to protect the device, and other components in the system, from damage by potential differences across the network caused by electrical storms or electrical faults. Lightning strikes can cause short but huge potential differences between network nodes causing equipment to see damaging voltages at its connections.
The size of the voltages seen depends on the proximity of the lightning strike to a network node and is a very interesting subject in itself but for now lets just say the closer the strike to a node then the bigger the voltage.
How do you protect against this? The first thing to do is to apply electrical isolation to the each node on the network. Opto-isolation is a common technique and is used in interface devices such as USB485i , PCI248H2i , and MEVs new PCI Express combined RS232 and RS485 card the PCIe249i. Optical isolation is good for about 1 kV of isolation. So if the lightning strike is not too severe then all is good. In many cases this sort of isolation is all that is needed to protect the network but there are some situations where it is not!
Cement works in the hills, steelworks in Yorkshire, engine plants and bridges linking countries have two things in common. The distances between nodes are physically large and they are all exposed to extreme weather. The very nature of their remoteness makes them susceptible to local lightning strikes as they are often the tallest things in the area.
MEV have twenty years of experience of solving RS485 communication problems and we have come across sites where extra protection is necessary. We have seen opto-isolaters blown to smithereens. In these installations we protect the network with our R485i RS485 repeater. The repeater extends the network distances and has opto-isolation but to the isolation we add a combination of transient absorbers and series protection elements to the RS485 lines. The Transorbs ( a type of fast zener diode) break down if the voltage exceeds a safe limit and the series elements, a type of PTC thermistor, act as resettable fuses becoming high impedance if excessive current flows, limiting the energy dissipated by the transorb and protecting it and the rest of the system from damage.
The high level of protection offered by this device means that we have never had a unit fail in the field and has led to the R485i being used in, the cement works, steelworks and bridges mentioned earlier, by Boeing on its production line, in the O2 arena, Eden project and many other rugged locations.
So the answer to the question. ‘When will RS485 become obsolete?’ I think the answer is ‘Not in my lifetime’.
If you would like to know anything more about the issues here please contact Dave Cooper