TSS721ADR Wired M-Bus Module

Overview

The TSS721ADR is a Wired M-Bus (Meter-Bus) transceiver designed for reliable communication with wired M-Bus compliant utility meters, such as water, heat, gas, and electricity meters.

On the S0 Baseboard, the TSS721ADR enables direct physical connection to wired M-Bus meters and acts as the physical layer interface between the meters and the S0 board.

This module allows the S0 board to collect meter data from the Baseboard and forward it to Magistrala via protocols like MQTT, CoAP, HTTP, and others.

What is Wired M-Bus?

Wired M-Bus (EN 13757-2 / EN 13757-3) is a European standard for wired meter reading. It supports:

• Multi-drop bus topology
• Long cable lengths
• Low power consumption at the meter
• Reliable communication in industrial environments

Wired M-Bus is commonly used in:

• Apartment buildings
• Industrial facilities
• District heating systems
• Utility metering cabinets

Role of the TSS721ADR on the S0 Board

The TSS721ADR performs the following functions:

• Provides voltage level shifting and signal conditioning for the M-Bus line
• Handles current modulation and detection
• Protects the MCU from direct exposure to the M-Bus line
• Enables half-duplex communication over the M-Bus

S0 communicates with the TSS721ADR using the UART interface, while the TSS721ADR interfaces with the M-Bus lines (MBUS+ / MBUS−).

Key Features

• Fully compliant with Wired M-Bus standards
• Supports master mode operation
• Integrated current limiting and line protection
• Operates from a single supply voltage
• UART-based interface to the host MCU

Hardware Connections

MCU ↔ TSS721ADR

TSS721ADR ↔ Meter Bus

How to Test the TSS721ADR Module

Prerequisites

Before testing, ensure you have:

• S0 Baseboard powered
• S0, or a microcontroller of your choice powered and programmed
• A wired M-Bus compatible meter or a (12–42)V DC power supply connected at the MBUS lines with proper M-Bus wiring (polarity observed)
• Serial logging enabled on the S0 or microcontroller of your choice
• Correct UART configuration (typically 2400 or 9600 baud)

Electrical Verification

1. Measure voltage on the M-Bus lines:
   • Typical M-Bus idle voltage: (12–42) V
2. Verify the TSS721ADR is powered correctly

UART Communication Test

Configure the MCU UART connected to the TSS721ADR:

• Baud rate: 2400 / 9600
• Data bits: 8
• Parity: Even
• Stop bits: 1

Send a simple test frame from the MCU and observe UART activity.

#define MBUS_RX 18   // ESP32 receives from TSS721A TXD
#define MBUS_TX 19   // ESP32 transmits to TSS721A RXD

void setup() {
  Serial.begin(115200);
  delay(500);

  Serial.println("TSS721A ESP32 M-Bus Test Starting...");

  Serial1.begin(2400, SERIAL_8E1, MBUS_RX, MBUS_TX);

  Serial.println("Ready. Listening for M-Bus data...");
}

void loop() {
  while (Serial1.available()) {
    byte b = Serial1.read();
    Serial.print("RX: 0x");
    Serial.println(b, HEX);
  }

  static uint32_t last = 0;
  if (millis() - last > 20000) {
    last = millis();

     while (Serial1.available()) {
    byte b = Serial1.read();
    Serial.print("RX: 0x");
    Serial.println(b, HEX);
    }

    byte pingFrame[] = { 0x10, 0x40, 0x40, 0x16 }; // Standard M-Bus SND-NKE
    Serial1.write(pingFrame, sizeof(pingFrame));

    Serial.println("TX: Sent SND-NKE frame (reset command)");
  }
}