RS-232 Extends Its Grasp Via Wireless
The marriage of wireless to RS-232 provides a no-risk path for extending embedded networked systems.
LONNY STANFORD, MAXSTREAM
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For decades, asynchronous serial communication via RS-232 and RS-485 has prevailed as a method for connecting low-bandwidth electronic devices. And its popularity continues today. As the practical limits of using wired RS-232 systems are reached, designers have looked toward wireless technologies to extend transmission range and provide networking for their embedded systems. Related costs, engineering experience and time-to-market needs influence the path a designer will take when designing wireless into their projects.
Radio frequency (RF) discrete transistor-based radio designs and chipsets offer the lowest hardware costs to a designer and require extensive RF hardware engineering and significant RF firmware development to implement the solution appropriately into a product or system. Circuit boards populated with all essential RF components needed for a wireless link (modules) offer the greatest ease-of-use and deployment by providing a completed wireless solution that can be plugged into a product or system design. Table 1 lists RF experience, development time and time-to-market expectations for a designer to consider before choosing to develop with a transistor, chipset or module.
Benefits of Wireless RS-232
System designers should consider the many benefits of integrating wireless transceiver RS-232 modules into their projects. Theres no doubt that high-data throughput technologies, such as Ethernet, WiFi (802.11b) and Bluetooth have become popular in business and industrial environments. Such technologies let designers leverage existing infrastructuresbase stations and access points, utilize high-bandwidth flexibility in transmission schemes, and provide adequate networking within RS-232 systems up to 300 feet of range. Yet for some applications, those alternatives are too limited in range and the added cost and complexity make them undesirable.
In contrast, proprietary wireless RS-232 systems that specialize in long range and low data throughput can be very cost effective while providing the following benefits:
- Ease-of-use and deployment in new and legacy systems
- Extended range capabilities
- Speed and flexibility of installation
- Extension of existing product lines
Ease-of-Use & Deployment
Whether designing wireless into new or legacy systems, wireless RS-232 links call for minimal or no configuration or programming effort. Where configuration is needed, standard modem AT or binary commands are usually available. Many manufacturers of wireless transceiver modules take into account the varying needs of OEMs and have designed simple interfaces to their transceivers.
Wireless transceiver modules often support addressing and networking configuration needed for common point-to-point and point-to-multipoint topologies. The easiest modules to use and integrate go one step further by offering completely transparent communication between devices. Transparent wireless RS-232 communication appears as wired communication to the system, as the wireless link passes data in the same form as it was received. Where transparent communication is not available, transceiver manufacturers try to minimize the software requirements needed outside of the wireless link to pass the systems data.
Of particular benefit for RS-485 systems is the transparent peer-to-peer networking that is available in some wireless transceiver modules on the market. Peer-to-peer networking is best understood as a roaming master system where each module assumes the role of the master during the time it has data to transmit, then drops back to a slave role when the data transmission is complete. Any wireless transceiver module will synchronize all modules within range, transfer the data, and then release the communication channel. Each module can initiate communication with any other module within range.
Peer-to-peer networking modules have distinct advantages over master-slave systems in that they require no networking configuration, have extremely fast startup times (no need to acquire network synch) and allow for transmitter only functionality, flexible (ad-hoc) networking topologies, power saving modes, multi-drop bus support and the absence of constant synch pulses. The only difference between some wireless RS-485 networks and an RS-485 2-wire multi-drop bus is that wireless networks introduce a small amount of latency into the system.
Depending on the wireless transceiver modules, the range of RS-232 communication can be extended up to 20 miles or more in pure line-of-sight conditions. More typically, designers can expect two to seven miles in line-of-sight conditions and 300 feet to 1/4 mile in urban or industrial environments. In long-range applications, wireless communication links offset the expenses and challenges inherent in running long cables. When the range of a system fluctuates, as in mobile or remote applications, the wireless benefit of portability becomes valuable.
Another key benefit of wireless networks is their easier installation. Wireless links can be up and running in a fraction of the time it takes to deploy a wired link, especially when working with long range or legacy systems. Removing the expensive hardware and labor costs associated with wired installations also makes wireless solutions more affordable.
Handheld devices, radio frequency identification (RFID) devices, vehicle applications and other mobile designs can be moved to various distances and locations using wireless RS-232 while maintaining reliable communication links. Where transparent communication is available, introducing a wireless link can be as simple as a plug-and-communicate scenario. The ease of installation also makes wireless transceiver modules available to an increasing number of product designs.
Choosing Wireless RS-232 Solutions
Selecting the appropriate wireless RS-232 solution for connecting low-bandwidth electronic devices requires several considerations. These considerations may include range related issues, power requirements, 900 MHz versus 2.4 GHz transmission bands, interference immunity and time-to-market issues.
Transmission range in a system is determined by link margin calculations. Figure 1 shows the overall link margin of a system that includes transmission power output, antenna gain, receiver sensitivity and path loss. Such path loss is due to cable and antenna attenuation, air content and obstacles preventing line-of-sight conditions. Achieving long range with wireless transceiver modules requires an effective combination of output power, antenna gain and receiver sensitivity. Each of these specifications can have dramatic effects on the link margin of a wireless link path.
The simplest approach would be to boost the output power and employ high-gain antennas to acquire the desired range. However, many regulatory agencies place limits on transmission power output and total antenna gain allowed in a wireless link. Moreover, many applications require compact size, portability, low power consumption and low cost from their wireless solutions. Improving receiver sensitivity proves a more cost effective means for increasing range with out the overhead of high-powered and/or cumbersome antenna solutions.
The more link margin that is available, the more range a designer is able to achieve. It is easiest to calculate link margin in dB. As receiver sensitivity becomes more negative it introduces more dB into the link margin. Every -6 dB of receiver sensitivity effectively doubles communication range in line-of-sight conditions (-10 dB in urban or indoor environments). The industrys receiver sensitivity average for wireless transceiver modules is -93 dBm. Figure 2 shows an example of where a wireless transceiver module with -105 dBm of receiver sensitivity will increase the link margin by 12 dB (more than the industrys average module) allowing the wireless link to communicate at four times the range in line-of-sight conditions and over twice the range in urban or indoor environments.
Low power consumption can be maintained in a system by employing greater receiver sensitivity. For example, a 1-Watt wireless transceiver module has 10 dB more power than a 100 mW module. However, if the 1-Watt wireless transceiver module has receiver sensitivity of -93 dBm it will have less range than a 100 mW module with a receiver sensitivity of -109 dBm, because the 100 mW module has 6 dB more link margin than the 1-Watt module. This effectively doubles the range while using the current consumption of a 100 mW module.
900 MHz vs. 2.4 GHz
Wireless RS-232-enabled devices typically utilize wireless transceiver modules that operate in the license-free Industrial Scientific and Medical (ISM) radio frequency bandwidths of 900 MHz and 2.4 GHz. Both of these bandwidths can benefit OEMs in different ways.
Wireless transceiver modules operating in the 900 MHz bandwidth offer up to twice the transmission range and penetrate obstacleswalls, buildings, trees, and so onbetter than 2.4 GHz transceivers. While 900 MHz signals outperform 2.4 GHz signals, the 900 MHz band is only available in North America, South America, Australia, New Zealand and Israel. On the other hand, wireless transceivers operating in the 2.4 GHz band are suitable for license-free communications throughout most of the world.
Designers that want to take advantage of 900 MHz performance, in the approved regions of the world noted above, would best be served by designing a mechanically and software-compatible system where 900 MHz and 2.4 GHz transceivers could be swapped based on the country of deployment. Where range is critical, it is disadvantageous to select one transceiver with inferior signal performance (2.4 GHz) for the sake of design consistency. For worldwide deployment of products, OEMs have the option to select manufacturers that offer 900 MHz and 2.4 GHz swappable transceivers.
Where transmission obstructions and interference may be encountered in different environments, some wireless transceivers are designed to penetrate obstructions and block interference to acceptable levels. These abilities allow wireless RS-232 to be more flexible than wired systems when used in portable applications.
By design, spread spectrum communications offer the added benefit of interference immunity. Some wireless transceiver modules provide added interference rejection, or blocking, achieved using proprietary filtering and communication across a narrower band of hopping frequencies.
Another set overlapping issue affects the speed with which a product can go through the development phase, onto production and into the marketplace. Among those are the development costs and regulatory agency approvals in each of the world regions where the product will be deployed. Also important are the trade-offs of a module vs. a chipset solution.
Module vs. Chipset Trade-offs
When time and RF engineering experience is of abundance, a designer may opt to use RF chipsets to save on RF component costs. Using chipsets, the designer actually develops the hardware and software workings of the product. While the individual chipsets offer functionality, the designer must dictate how those chips will work in concert with the software the designer will develop. Such efforts arent for the faint of heart, as completed designs must also pass rigid FCC and other agency requirements for deployment in the various regions of the world. The regulatory approval process can add months or years to development.
Wireless transceiver modules offer a faster time-to-market alternative to chipsets that allow designers at all levels of RF experience to integrate a completed wireless system into their products. Many modules are manufactured as a drop-in solution where designers create a compatible interface on their processor board and supply serial data to the appropriate pins. Modules offering the easiest integration allow the designer to send raw UART data into the module and expect that same data out on the receiving end of the wireless link.
Modules may carry FCC, ETSI, CE and other approvals allowing the designer to deploy products in various regions of the world with minimal additional approvals. The completed RF design and agency approval of many wireless transceiver modules make them a popular choice in the fast-paced world of wireless product design.
For its part, MaxStream has a line of wireless transceiver RS-232 modules (Figure 3) that are approved by the FCC and other agencies for use throughout the world. Integrating these solutions is as simple as sending serial data from a microcontroller (UART) or comm port. MaxStream modules handle all of the complexities of spread spectrum transmission and reception. The units can communicate at short or long range while consuming low power and maintaining high levels of interference rejection over transparent peer-to-peer, point-to-point, point-to-multipoint and multi-drop networks.