By Dick Caro, CMC Associates and ISA SP 100 committee member
For the first time, the ISA SP100 standards committee was presented with some details about WirelessHART at its May 2007 meeting in Austin, Texas. The WirelessHART specification was scheduled to be voted on by the HART Communications Foundation’s membership in June 2007.
Product plans are already in place for the WirelessHART protocol. Products from major suppliers of process control instrumentation are expected to be announced soon after the WirelessHART specification is available and field-tested. In addition to field instruments that support WirelessHART, “small modules” that enable a wired HART instrument to communicate data via the WirelessHART network will be available.
While protocol specifications for WirelessHART are very close in many ways to the planned protocol for SP100.11a Release 1, they’re not identical. Both protocols have been developed independently, with WirelessHART having a narrow focus and SP100.11a having a more general, broader focus. Both have decided to use the same radio technology—IEEE 802.15.4:2006 chips that are also used for ZigBee. This choice of a similar commodity radio chip is like the choice of using commodity Ethernet chips for Foundation fieldbus HSE, Profinet and Modbus/TCP. However, the protocols used for the upper communications layers aren’t the same, which makes it impossible for either network to carry signals from the other.
The committee’s User Working Group debated the effect of the information about the incompatibility between WirelessHART and SP100.11a. Users have always stated a need for one wireless data communications standard in their plants, and this situation would result in two incompatible protocols installed in the same plant areas and operating in the same radio frequency band. There seemed to be only three possible alternatives: request that WirelessHART adopt the SP100.11a protocol; request that SP100.11a adopt a protocol compatible with WirelessHART; or do nothing and allow both protocols to be installed and manage the co-existence issues, if any.
The users debated what they should recommend to the SP100 meeting. It’s useful to follow their line of reasoning as they investigated each of their three alternatives:
- Alternative 1 – WirelessHART is already being balloted, and vendors are already prototyping products based on the technology used for this specification. While it would make technical sense to ask WirelessHART to use SP100.11a protocol, there’s actually nothing to recommend it, since these protocol layers of SP100 are not yet completed. Such an action would delay the release of WirelessHART, and that would likely not be acceptable to the majority of the members of the HART Communications Foundation—instrument vendors wanting to release a wireless standard as soon as possible.
- Alternative 2 – It might be technically feasible for SP100 to use the same middle level protocols that are planned for WirelessHART, but the HART Communications Foundation hasn’t released this intellectual property to SP100, nor have any of the details of their protocol been revealed. It’s also unknown if the WirelessHART protocols would meet the broader, user-driven needs of the SP100.11a standard, since WirelessHART was developed by instrument vendors with no official user committee for input or direction. People knowledgeable about both protocols say they’re different enough to not allow either to receive or transmit messages from the other.
- Alternative 3 – The situation in which both WirelessHART and SP100.11a instruments are installed in the same plant will most often result in both networks working normally. If excessive interference occurs, it can be resolved by assignment of channels used by both protocols for their channel-hopping in such a way that they will not overlap. The user may give up efficiency and scalability of the frequency spectrum in doing this, but the impact will be local to this application alone.
How can both of these networks operating in the same 2.4 GHz frequency band avoid interference with each other? Analysis of the proposed Data Link Layer (DLL) services document for SP100.11a shows a well-conceived method for avoiding interference from other devices sharing the same 2.4 GHz spectrum in the plant including WirelessHART, ZigBee and WiFi. ZigBee doesn’t use channel-hopping; therefore, SP100 will dynamically avoid any ZigBee channel in frequent use. SP100’s 15-16 channels are designed to avoid overlap with any WiFi channels that may interfere. Although WirelessHART and SP100 use the same channels, and both use channel-hopping, the channel-hopping sequences are not the same. This means that any interference will result in the SP100 device retrying that message on a different channel that isn’t likely to be the same as the next channel used by WirelessHART. For these reasons, WirelessHART and SP100 will most likely coexist without problems in most situations.
The User Working Group has now agreed to make no recommendations to SP100 or to WirelessHART regarding the potential competition for bandwidth in their plants. The situation, while not meeting the group’s basic requirement for a single wireless protocol, is tolerable and will usually cause only limited problems on an application-by-application basis. Early adopters who do experience any problems will complain to their suppliers, who are expected to make changes to resolve any problems that do actually appear, either by channel assignment or by future revisions of their products.
SP100 Universal Network
During the debate on WirelessHART, the point was made that SP100 is intended to be a single “universal” network for the wireless transport of information from all types of industrial wired protocols including HART, Foundation Fieldbus H1 and HSE, Profibus’ PA, DP and Profinet, and Modbus RTU and TCP.