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From [[http://www.gammon.com.au/spi|SPI - Serial Peripheral Interface - for Arduino]] by Nick Gammon. Modified to use the newish and preferred ''SPI.beginTransaction()'' and ''SPI.endTransaction()''.

<file cpp gammon-spi-data-controller.ino>
// Written by Nick Gammon
// April 2011
// With Mithat Mods -- July 2023

#include <SPI.h>

void setup (void)
{
  Serial.begin (115200);
  Serial.println ();
  
  digitalWrite(SS, HIGH);  // ensure SS stays high for now

  // Put SCK, MOSI, SS pins into output mode
  // also put SCK, MOSI into LOW state, and SS into HIGH state.
  // Then put SPI hardware into Master mode and turn SPI on
  SPI.begin ();

  // Slow down the master a bit
  // SPI.setClockDivider(SPI_CLOCK_DIV8);  // mithat: let's do this the preferred way
  
}  // end of setup

byte transferAndWait (const byte what)
{
  byte a = SPI.transfer (what);
  delayMicroseconds (20);
  return a;
} // end of transferAndWait

void loop (void)
{
  byte a, b, c, d;
  
  SPI.beginTransaction(SPISettings(400000, MSBFIRST, SPI_MODE0));  // mithat: this is the preferred way
  // enable Slave Select
  digitalWrite(SS, LOW);    

  transferAndWait ('a');  // add command
  transferAndWait (10);
  a = transferAndWait (17);
  b = transferAndWait (33);
  c = transferAndWait (42);
  d = transferAndWait (0);

  // disable Slave Select
  digitalWrite(SS, HIGH);
  SPI.endTransaction();      // mithat: finish preferred way

  Serial.println ("Adding results:");
  Serial.println (a, DEC);
  Serial.println (b, DEC);
  Serial.println (c, DEC);
  Serial.println (d, DEC);
  
  
  // enable Slave Select
  digitalWrite(SS, LOW);   

  transferAndWait ('s');  // subtract command
  transferAndWait (10);
  a = transferAndWait (17);
  b = transferAndWait (33);
  c = transferAndWait (42);
  d = transferAndWait (0);

  // disable Slave Select
  digitalWrite(SS, HIGH);

  Serial.println ("Subtracting results:");
  Serial.println (a, DEC);
  Serial.println (b, DEC);
  Serial.println (c, DEC);
  Serial.println (d, DEC);
  
  delay (1000);  // 1 second delay 
}  // end of loop
</file>