Have to read some more about UART on arduino.

Propably MAX2323 is needed for TTL conversion.

ARDUINO_NANO_CONTROLL/.gitignore

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+.pio
+.vscode/.browse.c_cpp.db*
+.vscode/c_cpp_properties.json
+.vscode/launch.json
+.vscode/ipch
+.vscode

ARDUINO_NANO_CONTROLL/include/README

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+
+This directory is intended for project header files.
+
+A header file is a file containing C declarations and macro definitions
+to be shared between several project source files. You request the use of a
+header file in your project source file (C, C++, etc) located in `src` folder
+by including it, with the C preprocessing directive `#include'.
+
+```src/main.c
+
+#include "header.h"
+
+int main (void)
+{
+ ...
+}
+```
+
+Including a header file produces the same results as copying the header file
+into each source file that needs it. Such copying would be time-consuming
+and error-prone. With a header file, the related declarations appear
+in only one place. If they need to be changed, they can be changed in one
+place, and programs that include the header file will automatically use the
+new version when next recompiled. The header file eliminates the labor of
+finding and changing all the copies as well as the risk that a failure to
+find one copy will result in inconsistencies within a program.
+
+In C, the convention is to give header files names that end with `.h'.
+
+Read more about using header files in official GCC documentation:
+
+* Include Syntax
+* Include Operation
+* Once-Only Headers
+* Computed Includes
+
+https://gcc.gnu.org/onlinedocs/cpp/Header-Files.html

ARDUINO_NANO_CONTROLL/lib/README

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+
+This directory is intended for project specific (private) libraries.
+PlatformIO will compile them to static libraries and link into the executable file.
+
+The source code of each library should be placed in a separate directory
+("lib/your_library_name/[Code]").
+
+For example, see the structure of the following example libraries `Foo` and `Bar`:
+
+|--lib
+|  |
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |  |- library.json (optional. for custom build options, etc) https://docs.platformio.org/page/librarymanager/config.html
+|  |
+|  |--Foo
+|  |  |- Foo.c
+|  |  |- Foo.h
+|  |
+|  |- README --> THIS FILE
+|
+|- platformio.ini
+|--src
+   |- main.c
+
+Example contents of `src/main.c` using Foo and Bar:
+```
+#include <Foo.h>
+#include <Bar.h>
+
+int main (void)
+{
+  ...
+}
+
+```
+
+The PlatformIO Library Dependency Finder will find automatically dependent
+libraries by scanning project source files.
+
+More information about PlatformIO Library Dependency Finder
+- https://docs.platformio.org/page/librarymanager/ldf.html

ARDUINO_NANO_CONTROLL/platformio.ini

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+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[env:nanoatmega328]
+platform = atmelavr
+board = nanoatmega328
+framework = arduino
+lib_deps = 
+	adafruit/DHT sensor library@^1.4.6
+	featherfly/SoftwareSerial@^1.0
+board_build.mcu = atmega328p

ARDUINO_NANO_CONTROLL/src/main.cpp

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+#include <Arduino.h>
+#include <Adafruit_Sensor.h>
+#include <DHT.h>
+#include <DHT_U.h>
+#include <SoftwareSerial.h>
+
+// -------------------------------------------------------------
+// DHT PreConfiguration
+#define DHTPIN 4 // Digital pin connected to the DHT sensor
+//#define DHTPIN PB0
+// Pin 15 can work but DHT must be disconnected during program upload.
+#define DHTTYPE DHT22 // DHT 22  (AM2302), AM2321
+// -------------------------------------------------------------
+// Initialize DHT sensor.
+// Note that older versions of this library took an optional third parameter to
+// tweak the timings for faster processors.  This parameter is no longer needed
+// as the current DHT reading algorithm adjusts itself to work on faster procs.
+DHT dht(DHTPIN, DHTTYPE);
+
+
+// VARS
+const int MD1200BAUDS = 38400;  // From what I've read it is always 38400
+//const int EPYSLEEPY = 600000;  / 10 minutes
+const long EPYSLEEPY = 300000;  // 5 minutes
+//const int EPYSLEEPY = 150000;  // 2,5 minutes
+
+#define RX_PIN 3
+#define TX_PIN 2
+SoftwareSerial MDSerial(RX_PIN, TX_PIN);  // Goes up to 115200 bauds
+
+// declarations
+int getTemp();
+int setFanTrsh(int);
+float dhtRead();
+
+void setup() {
+  // Setup connection to MD1200
+  // Serial because we're using RX/TX pins
+  MDSerial.begin(MD1200BAUDS);
+
+  // Just debug
+  Serial.begin(9600);
+
+  dht.begin();
+
+  Serial.println("setup end");
+}
+
+void loop() {
+
+  Serial.println("Loop start");
+
+  int fanPercnt = getTemp();
+  
+  if (fanPercnt < 10) {
+    setFanTrsh(fanPercnt);
+  }
+
+  /* 
+  check temperature and
+  set fan speed every X minutes
+  */
+  delay(EPYSLEEPY);
+
+}
+
+// Get current temperature 
+int getTemp() {
+  Serial.println("getTemp start");
+
+  int bp1 = 0;
+  int bp2 = 0;
+  // int exp0 = 0;
+  // int exp1 = 0;
+  // int simm0 = 0;
+  // int simm1 = 0;
+  String MD1200output;
+  
+  MDSerial.println("_temp_rd");
+
+  // wait for MD1200 to answer
+  delay(30);
+  Serial.println("start parsing temperature start");
+  
+  while (MDSerial.available()) {
+    MD1200output = MDSerial.readStringUntil('\n');
+
+    // Check backplane 1
+    if (MD1200output.startsWith("BP_1")) {
+        // check index number of =
+        int eq = MD1200output.indexOf('=');
+        // check index number of c
+        int c = MD1200output.indexOf('c');
+        // check if both exists
+        if (eq != -1 && c != -1) {
+          /* 
+          take value between "= " and "c". 
+          NOTICE that eq + 1 is there because in 
+          "BP_1[2] = 25c" there is a space between = and 25.
+          */
+          bp1 = MD1200output.substring(eq + 1, c).toInt();
+          Serial.println("Backplane 1 " + String(bp1));
+        }
+    }
+
+    // Check backplane 2
+    if (MD1200output.startsWith("BP_2")) {
+      int eq = MD1200output.indexOf('=');
+      int c = MD1200output.indexOf('c');
+      if (eq != -1 && c != -1) {
+        bp2 = MD1200output.substring(eq + 1, c).toInt();
+        Serial.println("Backplane 2 " + String(bp2));
+      }
+    }
+
+    // Uncomment if you want to also get temperature for expanders
+    /*
+
+    // Check expander 0
+    if (MD1200output.startsWith("EXP0")) {
+      int eq = MD1200output.indexOf('=');
+      int c = MD1200output.indexOf('c');
+      if (eq != -1 && c != -1) {
+        exp0 = MD1200output.substring(eq + 1, c).toInt();
+      }
+    }
+
+    // Check expander 1
+    if (MD1200output.startsWith("EXP1")) {
+      int eq = MD1200output.indexOf('=');
+      int c = MD1200output.indexOf('c');
+      if (eq != -1 && c != -1) {
+        exp1 = MD1200output.substring(eq + 1, c).toInt();
+      }
+    }
+
+    // Check controller 0
+    if (MD1200output.startsWith("SIM0")) {
+      int eq = MD1200output.indexOf('=');
+      int c = MD1200output.indexOf('c');
+      if (eq != -1 && c != -1) {
+        simm0 = MD1200output.substring(eq + 1, c).toInt();
+      }
+    }
+
+    // Check controller 1
+    if (MD1200output.startsWith("SIM1")) {
+      int eq = MD1200output.indexOf('=');
+      int c = MD1200output.indexOf('c');
+      if (eq != -1 && c != -1) {
+        simm1 = MD1200output.substring(eq + 1, c).toInt();
+      }
+    }
+
+    */
+
+    // Stop when prompt returns
+    if (MD1200output.endsWith(">")) {
+      Serial.println("stopping temp reading");
+      break;
+    }
+  }
+
+  // Do (BP_1 + BP_2) / 2 to get the average of backplane
+  if (bp1 != -1 && bp2 != -1) {
+    int bpAvg = (bp1 + bp2) / 2;
+
+    Serial.println("Average temp is " + String(bpAvg));
+
+    // define default
+    int outPrcntg = 21;
+
+    // check external sensor temp
+    int ownReadTemp = int(dhtRead());
+
+
+    Serial.println("Checking DHT temp");
+    if (ownReadTemp <= 35) {
+      return 40;
+    }
+
+    // a 
+    switch (bpAvg) {
+      case 23:
+        outPrcntg = 21;
+        break;
+        /*
+        Minimum is 21 (akhsually 20)
+        BUT
+        https://forums.servethehome.com/index.php?threads/fun-with-an-md1200-md1220-sc200-sc220.27487/
+        */
+      case 25:
+        outPrcntg = 23;
+        break;
+      case 27:
+        outPrcntg = 24;
+        break;
+      case 29:
+        outPrcntg = 26;
+        break;
+      case 31:
+        outPrcntg = 27;
+        break;
+      case 33:
+        outPrcntg = 30;
+        break;
+      case 35:
+        outPrcntg = 34;
+        break;
+      case 37:
+        outPrcntg = 38;
+        break;
+        /*
+          I don't wan't to become deaf so max is 40.
+        */ 
+      
+      default:
+        return -1;
+    
+    }
+
+    Serial.println("Returnning " + String(outPrcntg) + " %");
+    return outPrcntg;
+
+  } else {
+    return -1; // failed to read temp
+  }
+
+  /*
+  BP_1[2] - Back plane, maybe left 
+  BP_2[3] - Back plane, maybe right
+  SIM0[0] - Controller A 
+  SIM1[1] - Controller B
+  EXP0[4] - Expander 0
+  EXP1[5] - Expander 1
+
+  AVG - average of all sensors
+  */
+}
+
+// take percentage as int and set it.
+int setFanTrsh(int fanTrshInp) {
+
+  Serial.println("Setting fan speed");
+  String outputStatement = "set_speed " + String(fanTrshInp);
+
+  // MDSerial.println("Sending " + outputStatement + " to MD1200");
+
+  if (MDSerial.println(outputStatement)) {
+    Serial.println("Setting fan speed success");
+    return 1;
+  }
+  else {
+    Serial.println("Setting fan speed unsuccess");
+    return -1;
+  }
+
+}
+
+float dhtRead() {
+  // Reading temperature or humidity takes about 250 milliseconds!
+  // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
+  // float humidt = dht.readHumidity();
+  // Read temperature as Celsius (the default)
+  float tempr = dht.readTemperature();
+  // Read temperature as Fahrenheit (isFahrenheit = true)
+  // f = dht.readTemperature(true);
+
+  // Check if any reads failed and exit early (to try again).
+  // if (isnan(humidt) || isnan(tempr))
+  if (isnan(tempr))
+  {
+    // MDSerial.println("Failed to read from DHT sensor!");
+    return -1;
+  }
+  else {
+    return tempr;
+  }
+
+  // Compute heat index in Fahrenheit (the default)
+  // hif = dht.computeHeatIndex(f, h);
+  // Compute heat index in Celsius (isFahreheit = false)
+  // hicc = dht.computeHeatIndex(tempr, humidt, false);
+
+}

ARDUINO_NANO_CONTROLL/test/README

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+
+This directory is intended for PlatformIO Test Runner and project tests.
+
+Unit Testing is a software testing method by which individual units of
+source code, sets of one or more MCU program modules together with associated
+control data, usage procedures, and operating procedures, are tested to
+determine whether they are fit for use. Unit testing finds problems early
+in the development cycle.
+
+More information about PlatformIO Unit Testing:
+- https://docs.platformio.org/en/latest/advanced/unit-testing/index.html