1.Introduction to WM-Bus

WM-Bus (Wireless M-Bus) is a wireless communication protocol used for low-power remote automatic meter reading and monitoring applications. It is a wireless communication protocol based on European standards, mainly used for data collection and transmission of metering equipment such as water meters, electricity meters, and gas meters.

2.WM-Bus working mode

WM-Bus (Wireless M-Bus) is an extension based on the wired M-Bus standard. Therefore, the working mode of the WM-Bus protocol is still based on the master-slave mechanism. It's just that the working roles are redefined in WM-Bus. WM-Bus defines the meter-end device Meter (slave) and the collection-end device Other (master).

1) meter: meter end, which can be understood as the sending end. It represents various devices that can collect sensor data for reporting.

2) other: receiving end, which is defined in the WM-Bus protocol as mobile collection, base station type, and other data collection end devices.

3.WM-Bus transmission mode

The WM-Bus protocol uses different transmission modes, including one-way transmission, two-way transmission, and multi-point transmission. The protocol supports different frequencies and transmission rates, which can be selected according to the needs of the application scenario. At the same time, the WM-Bus protocol also supports data encryption and authentication functions to protect the confidentiality and integrity of the data.

The transmission modes are mainly divided into S (S1, S1-m, S2), T (T1, T2), R (R1, R2), C (C1, C2), N, F and other modes. The following table describes the characteristics of various transmission modes in detail.

4.WM-Bus protocol frame analysis

WM-Bus protocol frames are divided into two frame formats: A and B.

The following figure is Format A:

The following figure shows Format B:

1) L field: The L field of the two frame formats A and B has different interpretations.

ŸA: The total length of the data, excluding the 1 byte occupied by itself and the two bytes of each CRC check.

ŸB: The total length of the data, but excluding the 1 byte occupied by itself.

C field: as shown below.

ŸRES: reserved field

ŸPRM: indicates the message direction. 1 is from master to slave, 0 is from slave to master

ŸFCB: frame count bit. Used to eliminate information loss and duplication.

ŸFCV: frame count valid bit. FCB changes are only valid when FCV=1.

ŸACD: access request bit. ACD=0 means the slave has no data required by the master, ACD=1 means the slave has relevant messages that need to be accessed by the master

ŸFunction code: function code, as shown below.

3) M field: Manufacturer ID

4) A field: Refer to the following structure.

5) CI field: indicates the relevant protocol header carried by the application data.

5.WM-Bus communication process

WM-Bus communication is mainly divided into two modes: one-way and two-way communication. Based on these two communication modes, a star network topology is realized.

The communication process of one-way communication is relatively simple. The whole process consists of a sending device and a receiving device. It can be understood as a device connected to a sensor, which broadcasts its own data to the air a certain number of times every day, and another gateway collection device with receiving function stores the corresponding data after receiving the data.

In two-way communication, since the meter-end device only provides an access window for a very short time, it is normal for the meter-end device to continuously broadcast (the receiving window of the normal meter-end device is only opened for a few milliseconds after sending the broadcast). When the host device (collection end device) receives this type of information, the collection end device knows that the meter end can receive data in the next few ms, and then the collection end can send the corresponding command. Combined with the master-slave mechanism, the accurate arrival of data has been guaranteed. The following figure shows a communication process taking T2 mode as an example.