modbus_rtu_set_rts - Man Page
set the RTS mode in RTU
Synopsis
int modbus_rtu_set_rts(modbus_t *ctx, int mode)
Description
The modbus_rtu_set_rts() function shall set the Request To Send mode to communicate on a RS485 serial bus. By default, the mode is set to MODBUS_RTU_RTS_NONE and no signal is issued before writing data on the wire.
To enable the RTS mode, the values MODBUS_RTU_RTS_UP or MODBUS_RTU_RTS_DOWN must be used, these modes enable the RTS mode and set the polarity at the same time. When MODBUS_RTU_RTS_UP is used, an ioctl call is made with RTS flag enabled then data is written on the bus after a delay of 1 ms, then another ioctl call is made with the RTS flag disabled and again a delay of 1 ms occurs. The MODBUS_RTU_RTS_DOWN mode applies the same procedure but with an inverted RTS flag.
This function can only be used with a context using a RTU backend.
Return Value
The function shall return 0 if successful. Otherwise it shall return -1 and set errno to one of the values defined below.
Errors
- EINVAL
The libmodbus backend isn’t RTU or the mode given in argument is invalid.
Example
Enable the RTS mode with positive polarity.
modbus_t *ctx; uint16_t tab_reg[10]; ctx = modbus_new_rtu("/dev/ttyS0", 115200, 'N', 8, 1); modbus_set_slave(ctx, 1); modbus_rtu_set_serial_mode(ctx, MODBUS_RTU_RS485); modbus_rtu_set_rts(ctx, MODBUS_RTU_RTS_UP); if (modbus_connect(ctx) == -1) { fprintf(stderr, "Connexion failed: %s\n", modbus_strerror(errno)); modbus_free(ctx); return -1; } rc = modbus_read_registers(ctx, 0, 7, tab_reg); if (rc == -1) { fprintf(stderr, "%s\n", modbus_strerror(errno)); return -1; } modbus_close(ctx); modbus_free(ctx);
See Also
Authors
The libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com>
Referenced By
libmodbus(7), modbus_rtu_get_rts(3).