10BASE-T1L Makes Powering Intelligent Field Instruments Painless

by Michal Brychta and Michael Jackson

A previous blog in this series showed how using a DC-DC converter to power signal chain components maximizes the use of the limited current in 4-20mA loop-powered field instruments. However, there is a growing demand for devices with additional diagnostics, intelligence, and security features and this blog shows why these are pushing the boundaries of what is achievable with this legacy analog interface. It also suggests using a 10BASE-T1L industrial Ethernet MAC-PHY to overcome these restrictions and to massively increase the data rate at which a sensor can communicate process information to a controller.

Estimating the power budget in a 4-20mA loop-powered field instrument

Figure 1 shows the typical signal chain for a field instrument with a HART interface (enabling data communications at rates up to 1200 bps). For a maximum loop current of 20mA and assuming an industrial power supply (typically 24V), the total amount of power available to the instrument is theoretically 480mW.

Figure 1 Signal chain power consumption in a HART-enabled 4-20mA field instrument

However, accounting for the voltage drops shown (cables, protection circuitry) and allowing 5V for signaling at the input to the process controller, this leaves only ~10 volts at the instrument's terminals. With 16mA of the loop current required for sensor signaling, only 4mA remains to power signal chain components. This means the actual power budget is only 10 V * 4 mA = 40 mW, which (approximately) breaks down as shown in Table 1. 

Table 1 Power budget in a 4-20mA loop-powered field instrument

The table shows that the instrument has already pushed its power budget to its limit, making including extra features impossible.

10BASE-T1L is faster and supplies more power

The ability to provide power to a sensor and signal chain components is a 'must-have' feature for any communications interface proposed for use in field instruments. 10BASE-T1L delivers on this requirement while also enabling digital data communication at up to 10Mbps (several orders of magnitude faster than HART) over distances up to 1km. This standard allows for up to 500mW of power delivery with a class A minimum supply voltage of 9V in Zone 0 (intrinsically safe areas). Figure 2 shows an instrument connected to a 10BASE-T1L field switch and the associated power distribution.

Figure 2 Field instrument using a 10BASE-T1L field switch for power and data communication

With 500mW of power available from a 9V supply, there is approximately 55mA of current for the signal chain components, and Table 2 shows how this breaks down. With 250mW available to power the sensor and measurement circuitry alone, this leaves scope to include extra features and functionality.

 Table 2 Power budget for a 10BASE-T1L enabled field instrument

Integrated MAC-PHY brings even more power benefits

Typically, a physical layer transceiver (PHY) connects the microcontroller which manages the instrument's industrial communications protocol, to an Ethernet network. However, this approach requires a higher-end (and hence higher power) microcontroller with the ability to manage not only the protocol stack but also the medium access control (MAC) sublayer (ensuring a device follows the rules governing when it can transmit on a shared network medium). A cleverer approach uses a transceiver that integrates the MAC and PHY in a single package, reducing the burden on the microcontroller by removing the requirement for it to manage the MAC sublayer. This approach gives instrument designers more freedom to choose from a broader range of lower-power microcontrollers.

10BASE-T1L Ethernet-APL MAC-PHY transceiver enables zone partitioning

Analog Devices’ ADIN1110 (Figure 3) implements 10BASE-T1L with the advanced physical layer (APL), a two-wire standard based on IEEE and IEC standards for process automation and hazardous locations. This MAC-PHY application enables partitioning for low-power field devices in Zone 0, providing Ethernet connectivity via an SPI interface to a host processor while consuming only 42 mW of power. It also supports the Open Alliance 10BASE-T1x MAC-PHY serial Interface for full-duplex SPI communications at 25 MHz clock speed.

Figure 3 ADIN1110 Robust, Industrial, Low Power 10BASE-T1L Ethernet MAC-PHY

Outside of the industrial field instrument arena, in intelligent building applications (HVAC systems, fire safety systems, access control, IP cameras, elevator systems, and condition monitoring), a MAC-PHY allows lower-power devices to join an Ethernet network.

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