Add example of controlling MOD-IO relays' state in C.

coupler/opc-ua-server/ic2_set_relays_example.c

+// Example controling MOD-IO's relays' state 
+// Based on https://www.kernel.org/doc/Documentation/i2c/dev-interface
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <linux/i2c-dev.h>
+#include <fcntl.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+int main()
+{
+   int file;
+   int adapter_nr = 1; /* probably dynamically determined */
+   char filename[20];
+
+   // step 1: open device
+   snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
+   file = open(filename, O_RDWR);
+   if (file < 0) {
+     /* ERROR HANDLING; you can check errno to see what went wrong */
+     printf("Error openind i2c device.");
+     exit(1);
+   }
+  
+   // step 2: address the slave by its address 
+   int addr = 0x58; /* The I2C address */
+   if (ioctl(file, I2C_SLAVE, addr) < 0) {
+    /* ERROR HANDLING; you can check errno to see what went wrong */
+    printf("Error addressing i2c slave.");
+    exit(1);
+   }
+
+   // step 3: write command over I2c
+   __u8 reg = 0x10; /* Device register to access */
+   __s32 res;
+   char buf[10];
+   buf[0] = reg;
+   buf[1] = 0x05; //0x00 -all off, 0x0F - all 4 on
+   buf[2] = 0x65; // seems irrelevant the value
+   if (write(file, buf, 3) != 3) {
+     /* ERROR HANDLING: i2c transaction failed */
+     printf("Error writing to i2c slave."); 
+   }
+
+   printf("OK\n");
+   return 0;
+}