But WirelessHART has its advantages too, says Phinney. "WirelessHART is easier to understand and implement than ISA 100. It has progressed further in terms of implementation and product variety."
One way to think about it is that from and end-user perspective, it's rather like deciding whether to get an iPhone or an Android phone. Both will do what you want them to do. But an iPhone has many more apps and is nearly plug-and-play. It also got out of the gate earlier and in a sense "owns" the marketplace. But it also is a closed environment. Android phones got out of the gate more slowly. They have a smaller app store. They don't have as "cool" a reputation. But their environment is a more open one and gives users more choices in terms of which systems (and phones) to use. But in either case, if your choice works for you, it's the correct one.
Practically speaking, "Some of the minutae are different, but functionally they are similar," says Mike Cushing, product marketing manager for pressure and temperature products at Siemens, which makes WirelessHART-compatible products. "ISA 100 is a larger umbrella for all kinds of communications. WirelessHART is more focused on instruments connecting to the control system. There are some technical differences, but they are not huge. ISA 100 is trying to cover all wireless applications in an operation, where WirelessHART is focused on instruments alone."
Wayne Manges, program manager at Oak Ridge National Labs, chairman of the ISA 100.11a standards committee, and a veteran wireless evangelist, says, "WirelessHART is simpler to deploy, but ISA 100 is more flexible."
He adds, "If you have a process or application that cannot stand more than 100-ms latency, you can't use WirelessHART. If you don't want to have to pick a lot of options, ISA 100 may not be what you want because it has options for better security, reliability and throughput. WirelessHART is meant to have no options. WirelessHART also has lower growth options. ISA can go up to 10,000 sensors. WirelessHART in that case would require intermediate networks and servers, etc. ISA is meant to be part of a whole suite of standards. If you want to do RFID, you have to use a separate system in WirelessHART."
Not everyone completely agrees with this assessment. Mark Nixon, research director at Emerson and a member of the Process Automation Hall of Fame, who has also been involved in the development of both Wireless HART and ISA 100.11a says, "Both WirelessHART and ISA100.11a provide mechanisms for allocating communication resources. In both cases, the user may decide to influence the number of hops between the device and the gateway by locating access points and backbone routers. In the case of WirelessHART, the standard allows for placing many access points. In the case of ISA100.11a, the standard allows for using multiple subnets. From the standpoint of latency, there is no technical basis to state that either WirelessHART or ISA100.11a is better."
He goes on to say, "The security specified by WirelessHART is very similar to ISA100.11a. The differences boil down to a couple of key things: ISA100.11a allows users to disable security and provides partial support for over-the-air provisioning (OTAP). I would argue that allowing security to be disabled creates more work for users—it forces users to have to check security settings. It also creates a security concern for users in that a) they may have missed enabling security, and b) it could open a vulnerability allowing hackers to disable key security settings."
He continues, "With WirelessHART the mesh is always on. ISA100.11a allows for the mesh features to not be enabled. Since mesh networks are inherently more reliable, for ISA100.11a to have the same reliability as WirelessHART, mesh capabilities must be enabled."
Nixon also argues that ultimately WirelessHART does have better throughput than ISA100.11a.
If you have wondered why the wireless standards war has been so long and protracted, now you know. The good news is that for those of us without a scorecard, there is a way through the technical maze.
Manges, one of whose briefs is to work with the Dept. of Energy and the Advanced Manufacturing Office to encourage the use of wireless and make it available for use with the smart grid, says, "In a way, you don't need to reach a standards agreement. It really doesn't matter who wins. The technology is there to work together. Protocols are designed to co-exist with others without hurting one another. WirelessHART and ISA are not interoperable, but they can co-exist."
And he offers a final word of advice. "If you're trying to decide which protocol to use, the worst answer is to do nothing. Use what you have. It's an enabler. The sooner you get into it, the sooner you can implement change."