Many standards can mean tough decisions for process control engineers.
contributor and SP100 Users Subcommittee co-chair, Dick Caro, reported in the July issue (www.controlglobal.com/articles/2007/204.html) that while the protocol specifications for WirelessHART are very close in many ways to the planned protocol for SP100.11a Release 1, they are not identical. The protocols used for the upper communications layers are not the same, making it impossible for either network to carry signals from the other.Control
The SP100 User Working Group noted that users have always stated a need for a single wireless data communications standard in their plants, and this situation would result in two non-interoperable protocols installed in the same plant areas and operating in the same radio frequency band.
There seem to be only three possible alternatives:
- Request WirelessHART to adopt the ISA100.11a protocol;
- Request ISA100.11a to adopt a protocol compatible with WirelessHART;
- Do nothing and allow both protocols to be installed and manage the co-existence issues, if any.
WirelessHART already has been adopted, and vendors already are prototyping products based on the technology used for this specification. While it would make technical sense to ask WirelessHART to use ISA100.11a protocol, there’s actually nothing to recommend, since these protocol layers of ISA100 aren’t yet completed. This would unnecessarily delay the adoption and release of WirelessHART.
It might be technically feasible for ISA100 to use the same middle-level protocols planned for WirelessHART, but the HART Communications Foundation has not released this intellectual property to ISA100, nor have any of the details of its protocol been revealed.
So what does this all mean to end users? Is it safe to use WirelessHART? Yes.
Caro says, “The situation in which both WirelessHART and ISA100.11a 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.”
Caro concludes, “Any interference means the ISA100 device will retry that message on a different channel—probably not the same one as the next channel used by WirelessHART. For these reasons, WirelessHART and ISA100 will most likely coexist without problems in most situations.”
One of the significant advantages of WirelessHART over other wireless protocols is an artifact of the dual nature of the original HART protocol: digital data superimposed on an analog 4-20 mA DC carrier. While the analog value reports the process variable, the digital data carries much more information, including additional process variables, diagnostics and calculated data. Because the digital data is being carried on the 4-20 mA signal wires, the digital data can be picked off anywhere between the field device itself and the I/O terminals to which the 4-20 loop is connected.
Several vendors have indicated they plan to produce devices that can inductively couple to the 4-20 loop from which they’re intended to transmit HART data, pulling both operating power and digital signal from that loop. If inexpensive enough, these may be the first of the “lick and stick” sensors Honeywell’s Kaufman is talking about.
ISA100: The Universal Network?
ISA100 is intended to be a single “universal” network for the wireless transport of information from all types of industrial wired protocols including:
- Foundation Fieldbus, both H1 and HSE,
- Profibus, including PA, DP and ProfiNet,
- Modbus, including RTU and TCP.
The proposed ISA100 Application Layer technology supports HART messages over the wireless network, along with data for other protocols providing this “universal” capability for the user. The Technical Steering Committee of the Fieldbus Foundation has voted to base its future wireless protocol on ISA100. A similar response is expected from Profibus International. It previously has stated its position to not develop its own wireless protocol, but to influence the design of ISA100 to meet its needs.
The One Big Network Option
In the meantime, big companies are trotting out their own, proprietary, “one big network” programs. Smaller companies, such as Banner Engineering and Turck, are producing new proprietary networks too. These companies, among others, are betting that Sensicasts’ Ambrosino is right.
Honeywell’s entry is called “OneWireless” and is described in this advertising videocast released at this year’s Honeywell User Group Americas in June. Honeywell wants to leverage the other wireless technologies from other divisions of the company, such as security, RFID, personnel location, hazardous gas and fire safety, as well as process information and control.
Invensys and Apprion have partnered around Apprion’s ION network. You can hear a recently recorded podcast interview with Invensys’ Harris (Hesh) Kagan, talking about the system, and about his chairmanship of WINA, the Wireless Industrial Networking Alliance (part of the Automation Federation) at www.controlglobal.com/wina.html.