Peter M. Groen / libmodbus

Commit e10ee2d58551c6fab08e91cced6368d8d7910bbb

Authored by Michael Heimpold
Committed by Stéphane Raimbault
1 parent e2e22503

Do not reply on broadcast requests (fixes #153) 

According to the Modbus specification
(http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf, section 2.1)
a Modbus RTU master can send a broadcast to all of it's slaves. This
broadcasts can only be write requests as otherwise collisions could
occur, eg. on a RS-485 bus.

When receiving such a broadcast, the slave should process the request as
usual, but must not reply anything, neither a normal response nor an
exception reply in case of an error.

Adjust the unit test for this case, too.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Inline Side-by-side
Showing 2 changed files with 4 additions and 3 deletions
src/modbus.c
  View file @e10ee2d
... ... @@ -1055,7 +1055,8 @@ int modbus_reply(modbus_t *ctx, const uint8_t *req,
1055 1055 break;
1056 1056 }
1057 1057  
1058   - return send_msg(ctx, rsp, rsp_length);
  1058 + /* Suppress any response when the request was a broadcast */
  1059 + return slave ? send_msg(ctx, rsp, rsp_length) : 0;
1059 1060 }
1060 1061  
1061 1062 int modbus_reply_exception(modbus_t *ctx, const uint8_t *req,
... ...
tests/unit-test-client.c
  View file @e10ee2d
... ... @@ -429,8 +429,8 @@ int main(int argc, char *argv[])
429 429  
430 430 rc = modbus_read_registers(ctx, UT_REGISTERS_ADDRESS,
431 431 UT_REGISTERS_NB, tab_rp_registers);
432   - printf("2/3 Reply after a broadcast query: ");
433   - ASSERT_TRUE(rc == UT_REGISTERS_NB, "");
  432 + printf("2/3 No reply after a broadcast query: ");
  433 + ASSERT_TRUE(rc == -1 && errno == ETIMEDOUT, "");
434 434  
435 435 /* Restore slave */
436 436 if (use_backend == RTU) {
... ...