By Dick Caro, CMC Associates
Two of the highlights of this years ISA EXPO, held in Houston last month, were the live demonstration of WirelessHART in the HART Communications Foundation booth and the demonstration of wireless coexistence in the ISA100 Wireless booth. The ISA100 standards committee also held a week-long meeting to finalize the definition of its standard, but not the standard itself, which remains only proposed, using a refined Principles of Operation document. The PoO is like the abstract to the proposed standard. Creating the protocol, data structures and state diagrams needed for the actual standard will follow and are tentatively scheduled for completion by the end of 2007.
While the long-anticipated collaboration meeting between the HART Communications Foundation and ISA100 was NOT a shoot-out, it was at times a little tense. Claims were made by some ISA100 members that on more than 30 points, the proposed ISA100.11a was superior to the released WirelessHART. HART Communications Foundation members present, representing many major vendors, insisted otherwise. Following the posturing, the group proceeded to examine network and device architectures in order to enable cooperation.
One of the options discussed was the possibility for a manufacturer to build a dual-stack device capable of operating in either an ISA100 or WirelessHART network. No modifications to either specification would likely be necessary to construct such a device, although no details were offered. During the discussion, it was illustrated that the protocol splitter could be located at any point above the media access control (MAC) layer present on the IEEE 802.15.4 chip radio used by both networks.
Another result of the discussion was the proposal of a more integrated solution using the options being proposed for ISA100 to be arrived at by configuring ISA100 field devices to route WirelessHART messages through the ISA100 mesh network between WirelessHART field devices and a gateway designed to separate these two messages.
Along with the low-level protocol options that would make this possible, a proposal was made to enable ISA100s Gateway/Application Layer to tunnel non-ISA100 protocols to and from the gateway. In this plan, the gateway would be responsible for separating the WirelessHART traffic from the ISA100 traffic, but the gateway would use only a single radio, since WirelessHART and the proposed ISA standard use the same radio.
Suppliers of ISA100 network equipment would be free to provide such support should they wish to. Much appears to be riding on the outcome of forthcoming technical meetings.
The outcome of the meeting between HART Communications Foundation and ISA100 leadership was an agreement by both groups to specify a dual-port gateway in which ISA100 and WirelessHART signals could be received (each with its own radio.) Messages would be channeled to a single host network, most likely using the host interface for Profibus and Foundation fieldbus announced previously by the HART Communications Foundation.
While this seems to be minimal cooperation, at least it is based on convincing evidence that WirelessHART and ISA100 networks will coexist without degrading each others performance. The other suggested solutions would be investigated during later releases of ISA100.
