Modbus TCP Programming and Experimentation

This course does not support sensors that use the Modbus TCP protocol, so we will write two programs:

① modbus_server_tcp.c: Simulate a Modbus TCP sensor

② modbus_client_tcp.c: Operate the sensor

No special connections are required on the hardware.

The source code for this section is located in the following directory:

Next, we will explain the code using a scenario analysis method. Suppose the following commands are executed on the development board:

# ./modbus_server_tcp 127.0.0.1 &# ./modbus_client_tcp 127.0.0.1 led1 on

41 ctx = modbus_new_tcp(argv[1], 1502);42 if (ctx == NULL) {43 fprintf(stderr, "Unable to allocate libmodbus context\n");44 return -1;45 }4647 //modbus_set_slave(ctx, SERVER_ID);4849 mb_mapping = modbus_mapping_new_start_address(0,50 NB_BITS, /* 5 DO registers, corresponding to beep1, beep2, led1, led2, led3 */51 0,52 NB_INPUT_BITS,53 0,54 NB_REGISTERS,55 0,56 NB_INPUT_REGISTERS); /* 2 AI registers, corresponding to temperature and humidity */57 memset(mb_mapping->tab_bits, 0, NB_BITS);58 memset(mb_mapping->tab_input_registers, 0, NB_INPUT_REGISTERS*2);5960 memset(old_bits, 0, NB_BITS);61 memset(old_regs, 0, NB_INPUT_REGISTERS*2);6263 s = modbus_tcp_listen(ctx, 1);64 modbus_tcp_accept(ctx, &s);

In “modbus_client_tcp.c”, the code is as follows:

33 ctx = modbus_new_tcp(argv[1], 1502);34 if (ctx == NULL) {35 fprintf(stderr, "Unable to allocate libmodbus context\n");36 return -1;37 }3839 modbus_set_slave(ctx, SERVER_ID);4041 if (modbus_connect(ctx) == -1) {42 fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));43 modbus_free(ctx);44 return -1;45 }

• Line 33: Allocates a modbus_t structure, setting the IP and port.

• Line 39: Sets the Modbus sensor address to access.

• Line 41: Sends a connection request.

In “modbus_server_tcp.c”, the code is as follows:

66 while (1)67 {68 do {69 rc = modbus_receive(ctx, query);70 /* Filtered queries return 0 */71 } while (rc == 0);72

• Line 69: Waits for a client to send a request.

In “modbus_client_tcp.c”, the code is as follows:

65 if (!strcmp(argv[2], "beep1"))66 addr = 0;67 if (!strcmp(argv[2], "beep2"))68 addr = 1;69 if (!strcmp(argv[2], "led1"))70 addr = 2;71 if (!strcmp(argv[2], "led2"))72 addr = 3;73 if (!strcmp(argv[2], "led3"))74 addr = 4;7576 if (addr == -1)77 {78 usage(argv[0]);79 return -1;80 }8182 if (!strcmp(argv[3], "on"))83 status = 1;84 else85 status = 0;8687 rc = modbus_write_bit(ctx, addr, status);88 if (rc == 1)89 {90 printf("modbus_write_bit ok\r\n");91 }92 else93 {94 printf("modbus_write_bit err: %d, %s\r\n", rc, strerror(errno));95 }

In “modbus_server_tcp.c”, the code is as follows:

75 if (rc >= 0) {7677 printf("Get query for UID %d\r\n", query[6]);7879 /* Simulate temperature and humidity using random numbers */80 mb_mapping->tab_input_registers[0] = rand() % 1000; /* Temperature, unit: 0.1C */81 mb_mapping->tab_input_registers[1] = rand() % 1000; /* Humidity, unit: 0.1% */8283 rc = modbus_reply(ctx, query, rc, mb_mapping);84 }85 if (rc == -1) {86 printf("Connection closed!\r\n");87 modbus_close(ctx);88 modbus_tcp_accept(ctx, &s);89 }9091 /* Based on the values set by the client, pretend to operate the buzzer and LED */92 if (mb_mapping->tab_bits[0] != old_bits[0])93 {94 printf("set beep1 %s\r\n", mb_mapping->tab_bits[0] ? "on" : "off");95 old_bits[0] = mb_mapping->tab_bits[0];96 }9798 if (mb_mapping->tab_bits[1] != old_bits[1])99 {100 printf("set beep2 %s\r\n", mb_mapping->tab_bits[1] ? "on" : "off");101 old_bits[1] = mb_mapping->tab_bits[1];102 }103104 if (mb_mapping->tab_bits[2] != old_bits[2])105 {106 printf("set led1 %s\r\n", mb_mapping->tab_bits[2] ? "on" : "off");107 old_bits[2] = mb_mapping->tab_bits[2];108 }109110 if (mb_mapping->tab_bits[3] != old_bits[4])111 {112 printf("set led2 %s\r\n", mb_mapping->tab_bits[4] ? "on" : "off");113 old_bits[3] = mb_mapping->tab_bits[4];114 }115116 if (mb_mapping->tab_bits[4] != old_bits[4])117 {118 printf("set led3 %s\r\n", mb_mapping->tab_bits[4] ? "on" : "off");119 old_bits[4] = mb_mapping->tab_bits[4];120 }

Upload the code to Ubuntu.

Then, execute the following command in Ubuntu to compile:

$ source /opt/remi-sdk/environment-setup-aarch64-poky-linux$ make$ scp modbus_client_tcp [email protected]:/home/root$ scp modbus_server_tcp [email protected]:/home/root

Finally, execute the following commands on the development board (first execute modbus_server):

# cd /home/root# ./modbus_server_tcp 127.0.0.1 &# ./modbus_client_tcp 127.0.0.1 led1 onGet query for UID 4set led1 onmodbus_write_bit okConnection closed!# ./modbus_client_tcp 127.0.0.1 readGet query for UID 4Temperature = 38.6C, Humidity = 49.2%Get query for UID 4Temperature = 64.9C, Humidity = 42.1%Get query for UID 4Temperature = 36.2C, Humidity = 2.7%