modbus_rtu_set_rts(3)

1MODBUS_RTU_SET_RTS(3)          libmodbus Manual          MODBUS_RTU_SET_RTS(3)
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NAME

6       modbus_rtu_set_rts - set the RTS mode in RTU
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SYNOPSIS

9       int modbus_rtu_set_rts(modbus_t *ctx, int mode)
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DESCRIPTION

12       The modbus_rtu_set_rts() function shall set the Request To Send mode to
13       communicate on a RS485 serial bus. By default, the mode is set to
14       MODBUS_RTU_RTS_NONE and no signal is issued before writing data on the
15       wire.
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17       To enable the RTS mode, the values MODBUS_RTU_RTS_UP or
18       MODBUS_RTU_RTS_DOWN must be used, these modes enable the RTS mode and
19       set the polarity at the same time. When MODBUS_RTU_RTS_UP is used, an
20       ioctl call is made with RTS flag enabled then data is written on the
21       bus after a delay of 1 ms, then another ioctl call is made with the RTS
22       flag disabled and again a delay of 1 ms occurs. The MODBUS_RTU_RTS_DOWN
23       mode applies the same procedure but with an inverted RTS flag.
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25       This function can only be used with a context using a RTU backend.
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RETURN VALUE

28       The function shall return 0 if successful. Otherwise it shall return -1
29       and set errno to one of the values defined below.
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ERRORS

32       EINVAL
33           The libmodbus backend isn’t RTU or the mode given in argument is
34           invalid.
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EXAMPLE

37       Enable the RTS mode with positive polarity.
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39           modbus_t *ctx;
40           uint16_t tab_reg[10];
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42           ctx = modbus_new_rtu("/dev/ttyS0", 115200, 'N', 8, 1);
43           modbus_set_slave(ctx, 1);
44           modbus_rtu_set_serial_mode(ctx, MODBUS_RTU_RS485);
45           modbus_rtu_set_rts(ctx, MODBUS_RTU_RTS_UP);
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47           if (modbus_connect(ctx) == -1) {
48               fprintf(stderr, "Connexion failed: %s\n", modbus_strerror(errno));
49               modbus_free(ctx);
50               return -1;
51           }
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53           rc = modbus_read_registers(ctx, 0, 7, tab_reg);
54           if (rc == -1) {
55               fprintf(stderr, "%s\n", modbus_strerror(errno));
56               return -1;
57           }
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59           modbus_close(ctx);
60           modbus_free(ctx);
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AUTHORS

67       The libmodbus documentation was written by Stéphane Raimbault
68       <stephane.raimbault@gmail.com>
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72libmodbus v3.1.6                  02/04/2022             MODBUS_RTU_SET_RTS(3)