I have an Amprobe 38XR-A (sometimes seen as a meterman), these DMM's have a IR RS232 port on top and transmit a HEX code. Using the official cable I get a hex value of:


I am attempting to read the IR RS232 via Serial1 of a pro-micro and display the output via Serial, but am getting an incorrect HEX code of:


I have tried changing the resistor, in case it wasn't turning on/off fully with no change to the output (unless its a drastic change, then nothing is seen).

A buffer size of 15 appears to give repeatable results and is inline with the specification.

In an attempt for repeatability I have a test lead between the common and ohm port of the DMM.


Wiring diagram

Arduino Code:

const int BUFFER_SIZE = 15;
char buf[BUFFER_SIZE];

void setup() {

  while(!Serial){  }

void loop() {
    // read the incoming bytes:
    int rlen = Serial1.readBytesUntil("\r\n", buf, BUFFER_SIZE);

    // prints the received data
    Serial.print("HEX Data: ");
    for(int i = 0; i < rlen; i++)
      Serial.print(buf[i], HEX);

Additional links: Output Codes PDF DMM Manual IR photodiode


Original IR receiver circuit. Original IR

  • I find it hard to believe that the signal is not modulated as common IR TV remote control signals are. The only widely known exception I know of is the obsolete IRDA format used in Palm Pilots. I'm betting that the actual 83SW adapter contains one of these COTS ~48KHz all in one demodulator IR receiver. Do you have an oscilloscope?
    – st2000
    Commented Sep 8, 2023 at 12:16
  • Also, it is not clear what data you received and what data you were expecting. Can you edit your question to include both and clearly say which is which?
    – st2000
    Commented Sep 8, 2023 at 12:17
  • RS232 uses 10 bits, or more, for every byte ... you have to look at the binary data, not hex ... don't use serial input ... sample the raw data on the pin at regular intervals instead, then examine the bits
    – jsotola
    Commented Sep 8, 2023 at 17:22
  • hex 6C666196665665F6793D is binary 0110 1100 0110 0110 0110 0001 1001 0110 0110 0110 0101 0110 0110 0101 1111 0110 0111 1001 0011 1101 ... there are a lot of 11 and 00 ... that may be an indication that the signal baud rate is lower than you think
    – jsotola
    Commented Sep 8, 2023 at 17:38
  • @st2000 yes I have an oscilloscope and 83SW has very little inside, transistor, diode a couple of resistors and caps - will take a pic Monday. Also, I am after the 83SW output, so 08000400500A0
    – Ash_noob
    Commented Sep 9, 2023 at 7:08


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