Undefined reference to `_ZN3SDMC1ER14HardwareSerialliib'

[Holger]

Hello,
I'm triing to get a Wemos D1 mini with a RS485 to TTL adapter and a SDM72D-M running.

The adapter is connected via hardware serial to the Wemos. The Wemos sends debug-Messages over Software serial to another Wemos, which is working fine.

My code is:

#define USE_HARDWARESERIAL

#include <SDM.h>

#include <SoftwareSerial.h>

SoftwareSerial SoftSerial(D6, D5);  // RX, TX

SDM sdm(Serial, SDM_UART_BAUD, NOT_A_PIN, SERIAL_8N1, false); 


void setup() {
  sdm.begin();
  SoftSerial.begin(9600);  //initialize serial
  SoftSerial.println("Starting");
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  
  float voltage = sdm.readVal(SDM_PHASE_1_VOLTAGE);

  SoftSerial.print("voltage: " );SoftSerial.println(voltage);

  delay(500);  
  digitalWrite(LED_BUILTIN,LOW);
  delay(500);
}

At compiletime, I got:
....ino:25: undefined reference to `_ZN3SDMC1ER14HardwareSerialliib'

It is the line: float voltage = sdm.readVal(SDM_PHASE_1_VOLTAGE);

In SDM_Config_User.h I have also #define USE_HARDWARESERIAL and SDM_RX_PIN to 3, and SDM_TX_PIN to 1

Any suggestions who I can get this working?

Thanks