The ISA100 technical committee has not yet taken HCF up on its offer of granting use rights to ISA for the wireless portions of the HART 7.0 specification, because ISA100 leadership did not adequately understand the need for a unified standard. Instead, a Joint Analysis Team (JAT) was formed with members from both the ISA100 standards committee, and members selected by the Board of Directors of the HCF to study the issues.
The JAT considered several alternatives and came to the conclusion that full integration of the two specifications would not be considered until a later release of the ISA100.11 standard, because no simple solution was offered, design of the integration could become very complicated, and would delay completion of the first release of ISA100.11a with no technical benefit. (Benefit of a unified standard to the end user was apparently not considered to be very important.) However, this committee did succeed in defining a host network interface to a dual-mode gateway that would be the same for both ISA100.11a and WirelessHART field networks, which is being written into the ISA100.11a draft standard.
This way, the DCS or other host system may be able to access information from a field device on either a WirelessHART or ISA100.11a network in basically the same way, from the dual-mode gateway, should the final ISA100.11a standard incorporate this feature. While the wireless field networks are separate, distinct, and not interoperable, they may coexist and might be joined at this common dual mode gateway.
Design of this common host level interface was made possible by incorporating the design for this interface already present in the HART 7.0 specification. So, while at the same time agreeing not to use WirelessHART, the ISA100.11a editorial team created the specification for a host level interface to the gateway in the ISA100.11a Release 1 draft standard that simply used the same core interface already in the HART 7.0 specification as the basis technology.
This host level interface should be very easy for the Wireless Cooperation Team (Fieldbus Foundation, HART Communication Foundation and Profibus Nutzerorganisation) to build upon as they define a common wireless gateway interface for WirelessHART onto Foundation Fieldbus, Profibus and Profinet. In so doing, they will have also created a common interface to ISA100.11a as well. These consortiums had previously agreed to specify a common interface to a WirelessHART gateway. The dual-mode gateway interface proposed by the JAT and embedded into the ISA100.11a Release 1 specification seems to accomplish most of this task with the added benefit that it allows that same interface to be used with ISA100.11a Release 1 networks.
Integration of the Standards
Now, let’s turn to the rest of the problem: integration of WirelessHART and ISA100.11a. There are several different ways in which these two standards may be integrated:
- ISA100 may adopt the entire wireless protocol specification of HART 7.0 replacing the current draft standards work of ISA100.11a,
- HCF may adopt the entire wireless protocol specification that is ISA100.11a replacing the current wireless protocol contained in the HART 7.0 specification, or
- Merge the WirelessHART protocol into ISA100.11a by inserting a protocol splitter just above the common MAC (Media Access Control) layer creating a dual-protocol stack. Then messages would be handled by the proper protocol stack for routing and servicing according to the network identity configured into the field device.
- Merge the two specifications by creation of a truly unified single specification with options for the complex features of ISA100.11a, but retaining the simpler features of WirelessHART as the set of defaults.
Option 1 questions the necessity for a totally new ISA100.11a protocol when the WirelessHART protocol is available. If all of the process control needs of industry could be fulfilled through the use of the WirelessHART protocol alone, then this suggestion would have merit. However, it is the opinion of many technical experts working on ISA100.11a and some end users directly involved with its development, that this is not the case.
- If the needs for wireless field communications were defined by the requirements to support only HART and Profibus-PA applications, then WirelessHART might be adequate. However, the HART 7.0 specification currently does not provide for direct peer-to-peer data paths that would be necessary for Foundation Fieldbus “control in field device” functionality. The draft ISA100.11a standard was designed to provide direct peer-to-peer data paths necessary to close control loops using only field instruments similar to Foundation Fieldbus H1 wired networks. Some observers question the value of control in wireless field devices, however.
The HART Communication Foundation expects that the next revision of the WirelessHART standard will also support Foundation Fieldbus, since there is a joint working group in which HART Communication Foundation and members of the Fieldbus Foundation are working to develop that capability. And like every other revision of the HART standard since version 1.0, it will be fully backward compatible with every other version of HART ever issued.
The other issue is the speed of a WirelessHART network when used to supply measurement data for control loops. In wired HART, the 4-20mA analog signal is usually used to supply process variable (PV) data for control purposes. Modifications to the current WirelessHART specification are currently being made to reduce the latencies to approximately those of ISA100.11a as indicated in the current draft specification. WirelessHART then will have a maximum latency of 100 ms, which appears to be quite fast enough for process control. However, at least at first, most wireless applications are expected to be asset management and process monitoring related and much slower update rates will be acceptable.