Missing Modbus TCP Server Sample

[napperley]

Although there is a C source file showing how to do a Modbus server it covers far too much (it is too complex), and lacks focus on a specific area. To make matters worse the source file isn't extensively documented (especially on memory management, and how to send a response back to the client with register data set in the response), and lacks proper code structure (all logic is done in a enormous sized main function).

The libmodbus library would greatly benefit from having a small Modbus TCP server sample that only focuses on a single area, and shows the minimum required to get a Modbus TCP server implemented.

[watsocd]

I had the same issue recently. Below is a Hello World Modbus TCP server. Hope this helps. I have the following in my main() function. //Start Modbus slave/server thread pthread_t thread_id; pthread_create(&thread_id, NULL, modbus_main, NULL); /* Hello World: Modbus slave function. Address node 3, data in registers 40001, 40002, and 40003 /* void *modbus_main(void *x) { int i; int server_socket = -1; modbus_t *ctx; ctx = modbus_new_tcp(NULL, 502); // modbus_set_debug(ctx, TRUE); int header_length = modbus_get_header_length(ctx); server_socket = modbus_tcp_listen(ctx, 2); //Listen to max two connection //This next line will block program execution until there is a connection made modbus_tcp_accept(ctx, &server_socket); modbus_mapping_t *mb_map, *mb_mapping[100]; //Array of pointers for each device //this sets up 500 holding registers starting at 40001 mb_map = modbus_mapping_new(0, 0, 500, 0); mb_mapping[1] = mb_map; if (mb_mapping[1] == NULL) { fprintf(stderr, "Failed to allocate the mapping: %s\n", modbus_strerror(errno)); modbus_free(ctx); return; } for (;;) { uint8_t query[MODBUS_TCP_MAX_ADU_LENGTH]; int rc; do { rc = modbus_receive(ctx, query); /* Filtered queries return 0 */ } while (rc == 0); /* The connection is not closed on errors which require on reply such as bad CRC in RTU. */ if (rc == -1 && errno != EMBBADCRC) { /* Quit */ break; //the loop } //Set up some dummy data mb_mapping[1]->tab_registers[0] = rand() % 1000; mb_mapping[1]->tab_registers[1] = rand() % 100; mb_mapping[1]->tab_registers[2] = 333; //See here for a description of the query //http://www.simplymodbus.ca/FC03.htm printf("Node=%d function=%d num registers=%d start addr=%d\n", query[header_length - 1], query[header_length], MODBUS_GET_INT16_FROM_INT8(query, header_length + 3), MODBUS_GET_INT16_FROM_INT8(query, header_length + 1) + 40001); //query[6] = modbus node address //query[7] = modbus function if (query[6] != 3) { //If the node is NOT correct modbus_reply_exception(ctx, query, MODBUS_EXCEPTION_GATEWAY_TARGET); continue; } else { /* Return data */ /* rc is the query size */ modbus_reply(ctx, query, rc, mb_mapping[1]); } } printf("Quit the loop: %s\n", modbus_strerror(errno)); if (server_socket != -1) { close(server_socket); } modbus_mapping_free(mb_mapping[1]); modbus_close(ctx); modbus_free(ctx); } From: Nick Apperley <notifications@github.com> Sent: Wednesday, April 8, 2020 10:42 PM To: stephane/libmodbus <libmodbus@noreply.github.com> Cc: Subscribed <subscribed@noreply.github.com> Subject: [stephane/libmodbus] Missing Modbus TCP Server Sample (#530) Although there is a C source file <https://github.com/stephane/libmodbus/blob/master/tests/unit-test-server.c> showing how to do a Modbus server it covers far too much (it is too complex), and lacks focus on a specific area. To make matters worse the source file isn't extensively documented (especially on memory management, and how to send a response back to the client with register data set in the response), and lacks proper code structure (all logic is done in a enormous sized main function). The libmodbus library would greatly benefit from having a small Modbus TCP server sample that only focuses on a single area, and shows the minimum required to get a Modbus TCP server implemented. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#530> , or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAFDSAKLM6IJW6CVTBQER43RLUYXLANCNFSM4MEMNACQ> . <https://github.com/notifications/beacon/AAFDSAJVOLP543N3PSR2XQDRLUYXLA5CNFSM4MEMNAC2YY3PNVWWK3TUL52HS4DFUVEXG43VMWVGG33NNVSW45C7NFSM4I4VGNUQ.gif>

[napperley]

That example doesn't work. An error message appears after sending the reply on the server side:

Illegal data address 0x9C44 in read_input_registers

[karlp]

0x9c44 is simply not in range of the registers setup in @watsocd 's example, so it's behaving exactly as expected. They setup 500 holding registers, and no input registers.

[watsocd]

Code comment at the top:
Hello World: Modbus slave function. Address node 3, data in registers 40001, 40002, and 40003

Of course no/limited error checking. IT IS a Hello World minimal example for holding (40000) registers.

[alongL]

Caution: this demo server code only serve for one client.
If you want the server serving for multi clients at the same time.
You should see bandwidth-server-many-up.c in tests dir.
or you can try this https://github.com/alongL/modbusServer

[sunasrariyaz]

The code above gives me segmentation fault during runtime. So does the random-test-server when i access the mb_mapping->tab_registers[0] to print or to assign some value to it. Without accessing the mb_mapping->tab_registers[index] it runs fine. As a sever i want to assign some values in the tab_registers array. Any kind of help is greatly appreciated.

I am running it in rhel 7 with libmodbus-3.1.10 installed.