Use port 502 instead of 1502 in documentation (closes #771)

The default port number of Modbus is 502 but it requires privileges on
Linux.

Author: stephane

docs/modbus_new_tcp.md

@@ -41,7 +41,7 @@ defined below.
 ```c
 modbus_t *ctx;
 
-ctx = modbus_new_tcp("127.0.0.1", 1502);
+ctx = modbus_new_tcp("127.0.0.1", 502);
 if (ctx == NULL) {
     fprintf(stderr, "Unable to allocate libmodbus context\n");
     return -1;

docs/modbus_new_tcp_pi.md

@@ -42,7 +42,7 @@ defined below.
 ```c
 modbus_t *ctx;
 
-ctx = modbus_new_tcp_pi("::1", "1502");
+ctx = modbus_new_tcp_pi("::1", "502");
 if (ctx == NULL) {
     fprintf(stderr, "Unable to allocate libmodbus context\n");
     return -1;

docs/modbus_read_registers.md

@@ -38,7 +38,7 @@ uint16_t tab_reg[64];
 int rc;
 int i;
 
-ctx = modbus_new_tcp("127.0.0.1", 1502);
+ctx = modbus_new_tcp("127.0.0.1", 502);
 if (modbus_connect(ctx) == -1) {
     fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
     modbus_free(ctx);

docs/modbus_send_raw_request.md

@@ -38,7 +38,7 @@ uint8_t raw_req[] = { 0xFF, MODBUS_FC_READ_HOLDING_REGISTERS, 0x00, 0x01, 0x0, 0
 int req_length;
 uint8_t rsp[MODBUS_TCP_MAX_ADU_LENGTH];
 
-ctx = modbus_new_tcp("127.0.0.1", 1502);
+ctx = modbus_new_tcp("127.0.0.1", 502);
 if (modbus_connect(ctx) == -1) {
     fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
     modbus_free(ctx);

docs/modbus_send_raw_request_tid.md

@@ -40,7 +40,7 @@ uint8_t raw_req[] = { 0xFF, MODBUS_FC_READ_HOLDING_REGISTERS, 0x00, 0x01, 0x0, 0
 int req_length;
 uint8_t rsp[MODBUS_TCP_MAX_ADU_LENGTH];
 
-ctx = modbus_new_tcp("127.0.0.1", 1502);
+ctx = modbus_new_tcp("127.0.0.1", 502);
 if (modbus_connect(ctx) == -1) {
     fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
     modbus_free(ctx);

docs/modbus_set_socket.md

@@ -23,7 +23,7 @@ The function shall return 0 if successful. Otherwise it shall return -1 and set
 ## Example
 
 ```c
-ctx = modbus_new_tcp("127.0.0.1", 1502);
+ctx = modbus_new_tcp("127.0.0.1", 502);
 server_socket = modbus_tcp_listen(ctx, NB_CONNECTION);
 
 FD_ZERO(&rdset);