Reply To: SDM120 meter ignores my hand-built Modbus frame from a Nano

CRC order is right, Modbus sends the low byte first and 0x71 0xCB is correct for that frame. The problem is print.

Serial.print(0x01) sends the text « 1 », the byte 0x31. Serial.print(0x71) sends « 113 », three bytes. So the meter receives something like « 1400021137… » in ASCII, which isn’t a Modbus frame at all, and it stays quiet. Serial.write sends the raw byte, which is what Modbus RTU needs.

Second problem: you drop DE the moment the loop ends, but at 2400 baud the bytes are still going out of the UART for another ~30 ms. Switching to receive cuts the end of the frame. Wait for the transmit to finish:

digitalWrite(2, HIGH);
Serial.write(frame, sizeof frame);
Serial.flush();            // returns when the last byte has left the UART
digitalWrite(2, LOW);

The answer is 9 bytes: address, 0x04, byte count 0x04, four data bytes, CRC. The SDM120 voltage is a 32-bit float, big-endian, high word first:

union { uint32_t u; float f; } v;
v.u = ((uint32_t)r[3] << 24) | ((uint32_t)r[4] << 16) | (r[5] << 8) | r[6];
// v.f is the voltage

Since the Nano’s only hardware UART is now on the RS485 side, debug through a SoftwareSerial pin or a blink, not the USB serial monitor. When you send it on, send {"voltage": 231.4} as a number so the chart works.