EBVchips - Hermes

Entirely New Standards in M-Bus Communication

For communications in the world of smart metering the new EBVchip will refresh the existing portfolio as an upgrade of the IC which has served the market for the past 20 years
EBV has teamed-up with ON Semiconductor and developed the Hermes M-Bus Transceiver as an EBVchip that reduces power consumption offering significant advantages. Hermes meets the M-Bus specifications in full. The new EBVchip offers space reduction, lower cost and additional functions that enable a range of attractive new designs.

This EBVchip employs a relatively complex mixed-signal design, which was developed by the former AMIS team within ON Semiconductor. As with all EBVchips, this is an official chip from the respective manufacturer designed to the corresponding quality, although it is only available worldwide from EBV Elektronik. Hermes is housed in a 4 x 4 mm QFN-20 package, thus saving a good 60% of the IC footprint compared with the previous solution, which was packaged in an SO-16.

In addition, the classic transceiver can only drive a maximum of two M-Bus loads, which means that the slaves can only draw up to 3 mA per node. With higher power consumption levels, it used to be the case that two conventional M-Bus chips had to be installed in parallel so that four M-Bus loads could be used. Hermes is designed such that up to four programmable M-Bus loads can be connected. If required, you can even connect up to six M-Bus loads to a component, although this feature is not covered in the official M-Bus standard. The M-Bus loads can be programmed with the aid of an external resistor. At 38.4 kbit/s, the useful data rate of Hermes is also significantly higher than that of the well-established M-Bus component. To produce the new EBVchip, ON Semiconductor uses a tried-and-tested automotive process to safeguard supply over a long period of time.
  • Number of M-Bus loads: this new EBVchip is allowing connection to up to four M-Bus loads (actually it is supporting up to six, but unfortunately six is no longer covered by official M-Bus standard)
  • Wireless M-Bus: allowing connection of external radio modules for smart grid RF and LPRF infrastructure applications with minimum drain on the battery
  • Data rate: up to 38.4 Kbits/s
  • Power consumption : ca.330 uA which is half compared to competitive IC
  • PCB area: Saving ca. 60% of the PCB area
  • Package: Hermes is available in the 4 x 4 mm QFN-20 package
  • Price is tailored to the metering market and infrastructure
  • Energy meters, Smart grid, high-end household E-meters and basically whole family of meters featuring M-Bus master in order that Electricity meter is serving as the central reading point for the other house energy meters like water, gas and heat.
  • Calorimetric meter, water meter and gas meter which use the M-Bus (wired or wireless) infrastructure to communicate with the Master Electricity meter or Data Concentrator, which forwards the consumption data.
  • Data concentrator
  • MUC (Multi Utility Communication) box
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Supplying power to the M-Bus interface of a slave (gas/water/ heat meter, etc.) from a radio battery involves a fairly high circuit outlay, because a relatively high level of electricity (around 20 mA) as well as a DC/DC booster would be necessary to transmit a small amount of data.
The booster would increase the battery voltage (usually 3 or 6 V, coming from two lithium cells) to the M-Bus voltage of 36 V. Since this method conflicts with the low-power approach and would require the batteries to be changed very frequently, this new application which played no role when the M-Bus was defined more than 20 years ago calls for alternative concepts.
Theoretically, the simplest solution is for additional contacts to the electricity supply of a radio module and contacts for the data connection to be integrated in the meter. To preclude manipulation, however, the meters are hermetically encapsulated, which means that every single contact made with the outside world entails disproportionately high costs of around one dollar per contact. For this reason, as part of a compromise, a maximum of three contacts are possible: the two M-Bus contacts and, as a concession to the low-power operation, a third contact, which is only used beyond the M-Bus specification.

The new EBVchip is compatible with the M-Bus standard, and thus with old M-Bus products, but also offers many new features. For example, it conserves the batteries in an external HF module to an exceptional degree.
The devices are therefore optimised for M-Buses, but can also operate the data link via radio if required. And this is precisely where the third contact of the Hermes comes in.

The three-wire interface is designed in such a way that when the system is switched on, it detects via the third contact whether any direct connection with the external battery module exists. In this case, communication does not take place via energy-intensive current modulation, but instead by means of a purely digital signal present on the contacts otherwise used by the M-Bus. This enables extremely energy-saving communication with the M-Bus master, which means that the battery in the slave module very rarely needs to be changed. This third contact is connected to the OD pin on Hermes. Under normal circumstances, the sensor does not receive a supply current from the radio module. When the system is switched on, the chip checks whether the OD is high-resistance or connected via a pull-up resistor (OD here stands for open drain).

The logical status 1 is VDD, while logical 0 is 0 V. Half a second after being switched on, the sensor supplies its data to the HF module via the OD pin and then switches itself off again. This variant is used only if the reading is to be requested and no communication is required from the measurement data receiver downwards (in the direction of the meter). As soon as the meter registers that it is being supplied with energy on the M-Bus interface, it sends its current reading automatically. If the energy meter connected via the radio module is also to receive data from the M-Bus master, however, bi-directional communication is required. Hermes also offers a suitable solution for this, enabling low-power operation. As with the M-Bus, the HF module signals the data downwards through voltage modulation by switching the supply voltage on and off. However, while the M-Bus has at least 12 V, this is purely on-off keying, in which the power supply (which can be a maximum of 9 V) is switched on and off.

The sensor must therefore also make sure that it has sufficient energy to send data and save enough energy in a capacitor for this purpose. During downwards communication, the sensor must also have sufficient energy, and buffer this if necessary. This low-power method could also be used for updating software although not with 36 V, but with the voltage from the radio module. The radio module power supply can be 4.75...9 V; two 3 V lithium batteries are normally used in a system like this. As in uni-directional low-power mode, communication from the slave to the radio module (which then transmits the data to the master) takes place with a purely digital signal via the OD output.

In the block diagram, all the elements that the EBV chip contains in addition to the well-established standard M-Bus chip are highlighted by green dashes. For example, Hermes features the aforementioned additional OD pin for the three-wire interface (shown in the top right). Three further pins are shown on the left side of the diagram. The PMOD E pin signals to the microcontroller - as purely a voltage monitor - which voltage is present. The MCU then switches to the relevant mode - either M-Bus mode or LP (low power) mode. This information is sent to the microcontroller and not used directly inside the EBVchip. With the aid of the PMOD E pin, effective hot plugging is therefore possible without the M-Bus chip hanging. Using the 2WLPM and 3WLPM pins (below left: 2/3-wire lowpower), the microcontroller signals to the new EBVchip the mode in which it is to be operated. EBV Elektronik has deliberately removed the entire logic from Hermes to make sure that this chip is particularly robust and contains as little 'intelligence' as possible. A firmware upgrade for the microcontroller thus enables as yet unknown situations to be tackled and any faults to be eradicated via software. If chatter, very slow voltage rises or other phenomena impair the system, the developers can respond immediately with a software update. With this basic idea in mind, EBV Elektronik has deliberately opted not to develop a state machine in Hermes, which could be prone to faults, when designing this component. Together with a leading customer, EBV will also draw up a reference esign for the new EBVchip that will be complemented by the ppropriate software.