Two similar wireless protocols—WirelessHART, promulgated by the HART Communication Foundation (www.hartcomm.org) and ISA 100.11a, promulgated by the International Society of Automation (www.isa.org)—are competing for dominance as the enabler of smart instrumentation in Europe and North America. Complicating the issue for end users is the fact that these two standards don’t work together. In addition, a third standard, WIA-PA, exists in China and further complicates the task for those with Asian operations.
Most process facilities use a mix of wired networks along with their diverse array of instrumentation, but the inability to integrate the two main wireless standards makes that difficult in the wireless realm. Unlike with wired instrumentation, if you want to mix brands of wireless field devices to get an optimum mix of measurements, you can’t. You have to have two separate host systems to talk to two different types of field devices. And they have to come from different vendors.
In many ways, WirelessHART and ISA 100.11a are alike. They are designed to serve the same market in the same way. At an application level, they perform the same function and have the same benefits. Both ISA 100.11a and WirelessHART implement IEEE 802.15.4 radio hardware. Both protocols use DDL and Device Description files. Both can eliminate a lot of PLC I/O hardware, wiring and associated schematics.
The principal difference between the two protocols is in the specification of the protocols’ application layer. WirelessHART, for example, specifies HART as the application layer while ISA100.11a leaves that layer undefined. This means that data in the application layer of ISA100.11a can be transferred using Foundation Fieldbus, Profibus, Modbus, HART or other protocols. While this makes ISA100.11a highly flexible, the customer must decide which protocol to use. WirelessHART’s decision to specify only HART in the application layer was done to deliver simplicity via use of a single data communication specification through the network, meaning that data communication on the network is well-defined and understood.
Considering the potential for integrated use of the two wireless protocols, the obstacles preventing a convergence seem to be more commercial than technical. Though the two protocols are similar, investments have been made, vendors and early adopters are lined up on either side, and product certification processes have been established. The two protocols have been developed into products for sale. Marketing programs designed to win over additional customers and vendor partners are in high gear. Both sides believe their approach is “right” and others should come over to their way of thinking.
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