Bill’s Deep Dive: HART PSK Modem Expands Applications with 8 Times Speed Increase

By Bill Lydon, Editor, Automation.com       

The rapid technological growth of this era can vividly be seen in the enhanced speed and connectivity of solutions that enable greater communication and automation throughout a facility. Today’s deep dive will explore one of those solutions. The practical implementation of HART PSK (Phase Shift Keyed), by Smart Embedded Systems (SES), was designed to deliver 8 times faster HART  communications, meeting their users’ request for more device and measurement data.  The faster PSK communications are intended enhance the value of HART and support the goals of Industry 4.0 and Industrial Internet of Things.

The HART (Highway Addressable Remote Transducer) protocol, adopted worldwide, provides digital signal and measurement data from smart field instruments in a wide range of control, monitoring, and automation applications. The HART PSK implementation, specifically, communicates at 9600 bps (bits per second) vs HART FSK HART communicating at 1,200 bps and is part of the HART Specifications maintaining the use of the installed 4-20mA infrastructure.  These kind of edge field devices, implementing HART, have delivered a range of features including remote configuration & calibration, self-diagnostics, mass flow calculations, valve signatures, echo curves, alarms, and a wealth of asset management information.  These features contribute to more efficient operations plus provide data for analytics, optimization, and predictive maintenance. 

In order to get a better picture of the PSK, I discussed its development with Smart Embedded Systems’Baldev Krishan Ph.D., President/CEO and Pran Haran, VP Engineering & CTO.

What is the history or the origin of PSK?

PSK was defined in the HART 6 Specification released in 2001, but with the available technology at the time, it was not practical or cost-effective to deliver a 9600 baud solution.  Technology is now allowing cost-effective implementations that we are delivering.

Why is this important now?

Users are looking for faster speeds to support transferring more data, driven by digital transformation, to improve operations and become competitive manufacturers.  Ethernet is an option but it requires a different infrastructure, complex networking, and training personnel.  Using higher-speed HART, existing personnel can rely on an established technology they are familiar with to deliver greater results without infrastructure disruption.

What is FSK and PSK?

The short form of Frequency Shift Keying is referred as FSK. It is also digital modulation technique. In this technique, frequency of the carrier is varied in accordance with baseband digital input. The figure depicts the FSK modulation. As shown, binary 1 and 0 is represented by two different carrier frequencies. Figure 1 depicts that binary 1 is represented by frequency 'f1' (1200 HZ for HART modem) and binary 0 is represented by frequency 'f2'(2200 Hz for HART modem) . This technique delivers caller I.D. and voice on your phone’s land line.

Figure 1

The short form of Phase Shift Keying is referred as PSK. It is digital modulation technique where in the phase of the carrier is changed based on digital input. Figure depicts Binary Phase Shift Keying modulation type of PSK. As shown in Figure 2, Binary 1 is represented by 180 degree phase of the carrier and binary 0 is represented by 0 degree phase of the carrier.

Figure 2

Using PSK with multiple (4, 8, or more) phase changes, this method allows more bits to be transmitted over the same bandwidth as compared to FSK. 8PSK (8 Phase Shift Keying) is a phase modulation algorithm which is modulation scheme for C8PSK modem for HART 9600 bps modem. (Figure 3)

Figure 3

Phase modulation is a version of frequency modulation where the phase of the carrier wave is modulated to encode bits of digital information in each phase change.

The “PSK” in 8PSK refers to the use of Phased Shift Keying. Phased Shift Keying is a form of phase modulation which is accomplished by the use of a discrete number of states. 8PSK refers to PSK with 8 states. (Figure 4) With half that number of states, you will have QPSK. With twice the number of states as 8PSK, you will have 16PSK.

Because QPSK has 8 possible states 8PSK is able to encode three bits per symbol. 8PSK is less tolerant of link degradation than QPSK, but provides more data capacity.

Figure 4

Why are there no PSK modem chips from the FSK chip providers?

When the HART PSK specification was initiated, it was technically challenging to deliver 9600 bps with the limited available power of less than 4 mA at an economic price.   

There has been a big shift in technology with higher performance, low cost processor chips and we have leveraged this by creating and patenting firmware-based soft modem that runs on a microcontroller. In some cases, it runs on an existing controller chip found in most smart devices.

What are your different soft modem products?

We deliver a number of solutions for designers to add HART communications to control, monitoring, and automation products:

Single Microcontroller based HART modem

• 1200 BPS modem only - certified
• 9600/1200 BPS modem –submitted to FieldComm Group for certification
• HART Stack 7.0 available with the above modems

Single Microcontroller based HART modem and Stack.

• 1200 BPS modem - certified
• 9600/1200 BPS modem – dual speed
• HART Stack 7.0 – available with the above modems
• Reference design with hardware and software documentation

Do you have a soft modem solution for the host or I/O applications?

Yes, a patented Quad HART Soft Modem solution for FSK / 1200 bps eliminates the need for multiple modem chips with lower power consumption.  This can be ported to other microcontrollers and is presently available on Texas Instruments MSP 430 for the I/O controller side requiring multiple modems

What services does your company offer?

We offer the following services:

• HART Device Customization and Certification
• Product Development Services:
• Hardware and software development capabilities
• System architectures based on ARM, DSP, and others
• R&D prototyping with ARM,DSP, and FPGA’s
• BSP drivers, HAL Middleware Networking, Wireless, and Industrial Protocols
• I/O Controller side communication solutions

How does PSK differ from Ethernet-based Advanced Physical Layer (APL)?

The Advanced Physical Layer technology is Ethernet-based, requiring new infrastructure (Ethernet cable, switches, connectors, etc.), new HART devices, different training, new test tools, and the final specification is not expected to be released until 2022.

More information is available at www.smartembeddedsystems.com

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