The network interface connects the microcontroller (often via a field bus) to the host system. The host can use the received information for logging (trending), display or to influence process parameters (control). Some field busses are also used to locally controls actuators (local control loops), like Foundation Fieldbus.
Depending on the type of field bus naming convention of the network interface can differ.
The media attachment unit (MAU) connects the microcontroller to the field bus. This name is most often use by Foundation Fieldbus and Profibus. The MAU generates the correct signals for the bus, filters incoming signals, but can also have a ‘jabber inhibit’ function which prevents erroneous transmitters to block the traffic on the bus. The MAU can sometimes also supply power from the bus to the instrument, in which case the instruments are called bus powered or 2-wire instruments.
For complex MAU’s as with Foundation Fieldbus and Profibus, special ASIC’s exist that implement all the necessary functionality. Availability of these ASIC’s can be a problem however, causing some to prefer a discrete solution.
The word Modem stems from Modulator and Demodulator. This is a technique in which the signal is moved from one frequency band to another. This is naturally the case in radio communication, but also when digital signals need to be transmitted over a telephone line. In the case of radio the purpose is to obtain multiple communication channels (transmitters or stations). In the case of telephone lines the purpose is tp move the signal to a frequency band where there is less noise and interference.
In the case of Hart communication a modulation technique from the telecom arena is used for field bus communication (the Bell 202 standard). This enables to superpose a digital signal on top of a 4 - 20 mA analog signal. Hart communication gives transmitter users a migration path from analog to digital instrumentation:
- The transmitter can be used as any other 4 - 20 mA transmitter (point-to-point).
- The transmitter can be digitally configured, through a Hart Configurator on de 4 - 20 mA line.
- The transmitter can be used completely digital, bus powered in multi drop mode. In this case the transmitters can also communicate with each other.
Over the years additional requirements have been placed on the mutual influence of the analog and digital signal. As a result, special Hart modems have appeared on the market. Availability can be a problem here as well.
The combination of a galvanic isolation with bus powering and modulation of the 4 - 20 mA signal is especially complex.
Exalon Delft has a platform available that combines Hart communication, galvanic isolation and bus powering in an elegant and efficient way.
Though other internet sources a VHDL description of a Hart modem is available, which can be implemented in a CPLD. Naturally this CPLD has only digital input and output signals, and some form of analog interface needs to be applied.
Exalon Delft is working on a software implementation of a Hart modem that will be marketed as an option to it’s Hart Protocol stack.
When available, this software Hart modem will achieve a significant saving in PCB area, components costs as well as solve the potential availability issues with dedicated ASIC’s.
The protocol stack implements the software needed to establish communication between the de software the microcontroller and the host, but not the application itself (the software needed to make the pressure transmitter etc. function).
Often the protocol stack is written for multiple microcontrollers families (portable), in some cases it it optimized for a certain type. Portable code is obviously in the interest of the software seller, because it broadens the application market. For the user this may mean that the software actually needs to be ported before it can be used.
Non-portable code uses the microcontroller hardware more efficiently, and can save current and components, but does tie the user to a certain microcontroller platform. The investments needed for these platforms can be significant in many cases.
Exalon Delft’s Hart Protocol Stack is fully optimized for the M16C family. For this family mini-emulators are available, as well as freeware compilers. The processor it self has many advantages for it’s use in intelligent transmitters.
Exalon Delft’s hardware platform is especially comprehensive and flexible. By leaving out unused parts or replacing them with others, we can quickly develop and produce demonstrators.
For instance by replacing the Hart modulator and the M16C processor by another from the same family with a USB (PC interface), you can turn your PC into an oscilloscope or connect various sensors to it.
This can also be useful to create demonstrators for your analog sensors. An external power supply is then not needed, the USB can supply up to 500 mA to the platform.
Naturally, be adding a Fieldbus MAU to the platform it is suited for Foundation Fieldbus and Profibus by using protocol stack from renowned stack suppliers..