After I had finished most of the development work on a
valve position sensor, my customer began integrating the
firmware and hardware into their valve positioner. They planned to
incorporate the HART communications protocol, too.
HART, the acronym for Highway Addressable Remote Transducer, is a
hardware and software protocol that allows industrial sensors,
transducers, controllers, and other devices to communicate in a
network over a 4-20 mA current loop connection. The physical layer
creates a 1200 baud FSK signal that modulates the current in the
loop.
My client hired a large firm that claimed significant expertise
in this area to develop the hardware and software for the valve
controller product. But they ran into a snag. After months of
promises and delays with this firm, my client asked me to look the
situation. I found that this firm had produced nothing of value, at
least in the hardware design, and in fact, had not even been working
from the correct version of the HART specification. After one phone
conference, they backed out of the hardware job, and my client asked
me to do it.
I designed a transformer-coupled interface to the HART circuitry
that met some very tricky terminal impedance requirements. I
researched and tested communication transformers and found an
excellent match in an existing commercial product. I designed the
schematic, obtained the printed circuit board layout, purchased the
parts, and obtained the assembly of the first batch of prototype
boards. The boards met all of the HART requirements, and my customer
was back on track.
Later, I manufactured the first 200 HART boards before my
customer took over the manufacturing.
