Fix some typos

Originally done in arduino-libraries/ArduinoModbus#12

Author: peternewman

src/modbus-rtu.c

@@ -842,7 +842,7 @@ static int _modbus_rtu_connect(modbus_t *ctx)
        ONCLR ant others needs OPOST to be enabled
     */
 
-    /* Raw ouput */
+    /* Raw output */
     tios.c_oflag &=~ OPOST;
 
     /* C_CC         Control characters

src/modbus-tcp-private.h

@@ -14,7 +14,7 @@
 #define _MODBUS_TCP_CHECKSUM_LENGTH    0
 
 /* In both structures, the transaction ID must be placed on first position
-   to have a quick access not dependant of the TCP backend */
+   to have a quick access not dependent of the TCP backend */
 typedef struct _modbus_tcp {
     /* Extract from MODBUS Messaging on TCP/IP Implementation Guide V1.0b
        (page 23/46):

src/modbus-tcp.c

@@ -154,7 +154,7 @@ static int _modbus_tcp_prepare_response_tid(const uint8_t *req, int *req_length)
 
 static int _modbus_tcp_send_msg_pre(uint8_t *req, int req_length)
 {
-    /* Substract the header length to the message length */
+    /* Subtract the header length to the message length */
     int mbap_length = req_length - 6;
 
     req[4] = mbap_length >> 8;
@@ -905,7 +905,7 @@ modbus_t* modbus_new_tcp_pi(const char *node, const char *service)
         dest_size = sizeof(char) * _MODBUS_TCP_PI_SERVICE_LENGTH;
         ret_size = strlcpy(ctx_tcp_pi->service, service, dest_size);
     } else {
-        /* Empty service is not allowed, error catched below. */
+        /* Empty service is not allowed, error caught below. */
         ret_size = 0;
     }
 

src/modbus.c

@@ -1406,7 +1406,7 @@ int modbus_mask_write_register(modbus_t *ctx, int addr, uint16_t and_mask, uint1
     int rc;
     int req_length;
     /* The request length can not exceed _MIN_REQ_LENGTH - 2 and 4 bytes to
-     * store the masks. The ugly substraction is there to remove the 'nb' value
+     * store the masks. The ugly subtraction is there to remove the 'nb' value
      * (2 bytes) which is not used. */
     uint8_t req[_MIN_REQ_LENGTH + 2];