RS485 data acquisition: a new chapter

MOXA Inc
By Steve Huang and Wayne Chen, Moxa Inc
Wednesday, 02 October, 2013



RS485 and ethernet are two commonly adopted communication interfaces used in data acquisition and control systems. Ethernet I/O devices are generally considered more sophisticated, RS485 I/O devices may be sufficient for budget-conscious system integrators. Nonetheless, there are still ways device manufacturers can improve on the lasting legacy of RS485 data acquisition devices.

The biggest drawback of using RS485 remote I/O devices instead of alternative technologies, such as ethernet I/O devices, is probably the communication speed. For example, RS485 networks only offer data transmission speeds from 1.2 to 115.2 Kbps whereas ethernet networks can transmit data at rates of 10/100 Mbps.

Despite their physical limitations and maintenance difficulties, RS485-based remote I/O devices remain some of the most commonly used data acquisition devices in both commercial and industrial applications today, even with so many new data acquisition technologies becoming available.

RS485 limitations

A number of practical limitations and time-consuming maintenance problems persist with RS485 devices. As technology continues to advance and industrial data acquisition devices become more and more sophisticated, system integrators tasked with choosing between RS485 and ethernet I/O devices for their data acquisition networks often cite the following issues for turning away from RS485.

Physical limitations

Although RS485 data acquisition devices can be used to build multipoint communications networks, the standard architecture is limited to 32 drivers and up to 32 receivers on a single (2-wire) bus. Furthermore, RS485 data acquisition devices have no intrinsic method of avoiding data packet collisions. As a result, only one node in the entire network can transmit data to the bus at any given time, and all other nodes must wait in receiving status. If two or more nodes transmit data to the bus at the same time, all transmissions will fail.

Difficult troubleshooting

The process of configuring or updating RS485 data acquisition devices can be excruciatingly time-consuming because engineers must adjust each communication parameter one by one. Replacing RS485 devices or expanding an existing RS485 network usually requires an engineer to bring a personal computer to various field sites to configure parameters or upgrade firmware for each field device. In a network comprising hundreds of devices spaced up to 1.2 km apart from each other, one can only imagine the hassle and cost of hiring an engineer to haul a personal computer from one field device to the next.

Serial communication is often sufficient

Despite the traditional shortcomings discussed above, many system integrators continue to choose RS485 I/O devices over faster ethernet-based alternatives due to the former’s simple design, ease of control, and low cost of deployment and maintenance. In addition, RS485 can establish peer-to-peer communications between devices up to 1220 m apart before repeaters are needed, whereas ethernet switches and I/O devices are limited to only 100 m before repeaters are required to re-amplify communication signals.

Compared to ethernet I/O devices, RS485 I/O devices are still relatively inexpensive and thus attractive options for organisations that lack economies of scale and cannot afford the more expensive ethernet option. Many small and medium-sized enterprises, especially those in developing countries, opt for RS485 remote I/O devices because the low deployment and material costs make replacing broken machines an economical alternative to investing limited resources in training skilled maintenance professionals or building an expensive ethernet network. However, it is important to note that such cases are not mission-critical, which explains the low priority placed on preventing data loss, establishing network redundancy to ensure continuous connection or reducing system downtime.

There may be times when a cheap and simple RS485 data acquisition device is enough, but as the world becomes smarter and enterprises become more sophisticated, so must network devices used in modern data acquisition systems. As a result, savvy system integrators are not only on the lookout for easy-to-use and cost-effective solutions, but also network devices that are more dependable, easier to deploy and maintain, and reliable enough for backup or redundancy.

Some new ideas for RS485 data acquisition

A smarter RS485 data acquisition device should not only be equipped with all the basic benefits of RS485 communication, but also address the standard’s traditional limitations. Although not much can be done about the standard’s physical limitations, innovative device manufacturers can still find ways to improve on the tried and true foundation of RS485 serial communications.

Easier maintenance through software configuration

Instead of manually turning hundreds of dials and switches across a large field site, using software to configure all the RS485 devices on the network would greatly reduce deployment and maintenance effort. This software should enable quick access to all the RS485 data acquisition devices’ status information and settings in a user-friendly graphic interface. In addition, the software configuration tool should also provide an easy method for updating firmware remotely for all the data acquisition devices on the RS485 network.

Figure 1: A user-friendly software interface can make configuring hundreds of network devices easy and less time-consuming.

Figure 1: A user-friendly software interface can make configuring hundreds of network devices easy and less time-consuming.

The ideal software configuration tool would also allow users to set up their RS485 data acquisition device’s communication parameters from a remote location. So, instead of turning hundreds of dials to configure COM ports and device IDs during initial set-up for all the data acquisition devices on the same RS485 network, simply configure and duplicate the baud rate and mode for each network device through the graphical user interface. Converting manual hardware configuration to user-friendly software configuration will not only ease the effort of maintenance engineers, but also simplify the process of upgrading traditional device configurations and firmware.

Deployment and maintenance by USB

Although RS485 is a relatively simple technology, RS485 networks are not necessarily easy to maintain. In particular, companies are still required to train a specialised engineer who must carry a personal computer to the field site when remote access is insufficient. A simpler way to troubleshoot and repair field devices when field visits are necessary would not only allow system integrators to reduce labour costs, but also alleviate the maintenance effort of field engineers.

One solution is to add a USB port to the serial data acquisition device. This would serve as an alternative method for upgrading and configuring RS485 remote I/O communication devices and firmware if remote maintenance was not feasible. All the engineer needs to do is upload the configurations onto a USB drive, bring the USB drive to the field site, plug it into the field device’s USB port and all the configurations and firmware updates will upload to the field device automatically.

Redundant RS485 ports

Even though packet loss and transmission failures are common issues encountered by RS485 networks, these problems can be easily prevented. System integrators for mission-critical applications generally deploy some type of backup system or redundant network to ensure that devices operate continuously without failing. If an RS485 data acquisition device were equipped with dual RS485 ports, system integrators could easily set up dual wiring or dual computer redundancy to back up their network.

For wiring redundancy, when the central computer detects that one of the RS485 connections is not responding, it can quickly switch over to the other RS485 line to guarantee continuous communication between the field sensor and the central computer. Alternatively, the additional RS485 connection on the I/O device could connect field devices to a secondary computer on a local network to provide system operators with a secure backup in case the primary system goes down.

Figure 2: Dual RS-485 ports open up more network topologies for backup and redundancy.

Figure 2: Dual RS485 ports open up more network topologies for backup and redundancy.

Although dual RS485 ports are nothing new, an additional RS485 port enables maintenance engineers to test or repair an I/O device more quickly when one of the RS485 ports is damaged. So even though RS485 remote I/O devices are already considered a low-cost technology, the addition of dual RS485 ports can help system integrators save even more down the road without compromising on reliability.

RS485 data acquisition - the next chapter

Thanks to their affordability and simplicity, modern RS485 serial devices are here to stay because they not only provide all the basic benefits of RS485 communication, but also address some of the common limitations of traditional RS485 remote I/O devices. In particular, time-consuming maintenance can be greatly reduced by introducing user-friendly improvements like software configuration, uploading configurations or firmware via USB, dual RS485 ports and built-in repeaters. These are all practical solutions that can help enterprises around the world integrate RS485 I/O devices into an existing network, ease maintenance effort, reduce costs and generate business growth at the same time.

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